Family Togaviridae - Viruses And Human Disease

ORF 1

ORF 2

genome

ORF 1a

ORF 1b

mRNA

Serine Proteinase

Nucleocapsid protein

Polymerase (GDD)

Nonstructural proteins

Frameshift

FIGURE 3.15 Genome organization of a human astrovirus. ORF 1a is in a different reading frame than ORF 1b.

Ribosomal frameshifting (arrows) during translation is required to produce the ORF 1b protein. Adapted from Fauquet

et al. (2005) p. 860.

TABLE 3.9 Viruses Causing Acute Diarrhea

Virusa

Nucleic acid

Family

Host range

Single-stranded,

Caliciviridae

Sapporo, Norwalk feline

Humans, cattle, swine, chickens, dogs, cats

plus-sense RNA

calicivirus

Astroviridae

Numerous astroviruses

Humans, cattle, swine, cats, dogs, avian species

Swine, cattle, foals, mice, rabbits, dogs, cats turkeys, (humans?)b

Coronaviridae

PEDV, TGEV and others

Numerous toroviruses

Cattle, horses (goats, sheep, swine, rabbits, mice, humans?)

Flaviviridae

Pestivirus BVDV

Cattle

Picornaviridae

Aichi, human parechovirus 1

Humans

Single-stranded,

Paramyxoviridae

Canine distemper

Dogs

minus-sense RNA

Newcastle disease

Chickens, fowl

Double-stranded RNA

Reoviridae

Rotavirus A

Mammalian and avian species, humans

Rotavirus B

Swine, cattle, sheep, rodents, humans

Rotavirus C

Swine, ferrets, humans

Rotaviruses D, F, G

Avian species

Rotavirus E

Swine

Single-stranded DNA

Parvoviridae

Numerous parvoviruses

Cattle, cats, dogs, mink, (humans?)

Double-stranded DNA

Adenoviridae

Human Ad40,41

Humans

Abbreviations: PEDV, porcine epidemic diarrhea virus; TGEV, transmissible gastroentieritis virus; BVDV, bovine viral diarrhea virus.

The role of the listed viruses in causing diarrhea has not been proven for species listed in parentheses.

Source: Adapted from Granoff and Webster (1999), p. 442.

FAMILY TOGAVIRIDAE

be other small viruses that cause gastroenteritis in humans.

Virus particles have been seen in stools of humans suffer-

ing from gastroenteritis that have not as yet been charac-

The family Togaviridae contains two genera, genus

terized and are referred to simply as SRVs (small, round

Alphavirus and genus Rubivirus. The family name comes

viruses). Some of these may be members of families not

from the Latin word for cloak, and the name was given to

yet characterized.

them because they are enveloped. The 30 alphaviruses have

Alphavirus (Sindbis virus)

An genome RNA

CAP

nsP1

nsP2

nsP3

nsP4

An 26S mRNA

CAP

Rubivirus (rubella virus)

CAP

genome RNA

p150

p90

24S mRNA

CAP

Motifs

Proteolytic

Coding domains

Cleavages

Helicase

Nonstructural proteins

Capsid autoprotease

Nucleocapsid protein

Polymerase (GDD)

nsP2 or p150 protease

Opal codon

Glycoproteins

Papain protease

Signalase

Serine protease

Furin

Methyl transferase

FIGURE 3.16 Genome organizations of the Togaviridae. A number of protein motifs are indicated, as well as the enzymes

responsible for the proteolytic cleavages. The opal codon shown between nsP3 and nsP4 is leaky and is not present in all

alphavirus isolates; readthrough produces small amounts of nsP4. Adapted from Strauss and Strauss (1994) Figure 34.

a (+)RNA genome of about 12 kb, whereas rubella virus, the

selection of these viruses is presented in Table 3.10. Because

only member of the Rubivirus genus, has a genome of 10 kb.

of their importance as disease agents and aided by the fact that

The genomes of alphaviruses and of rubella virus are organ-

alphaviruses grow well in cultured cells and in animal mod-

ized in a similar fashion, as illustrated in Fig. 3.16. The viri-

els, this group of viruses has been well studied. The genomes

ons of the two groups are also roughly similar in size (70 nm

of many of them have been sequenced in their entirety. All

for alphaviruses, 50 nm for rubiviruses) and structure (icosa-

members of the genus are closely related and share extensive

hedral nucleocapsids surrounded by a lipoprotein envelope).

amino acid sequence identity, with the most distantly related

Structures of alphaviruses are illustrated in Figs. 2.5, 2.14,

alphaviruses sharing about 40% amino acid sequence identity

and 2.18. However, although the two genera exhibit simi-

on average and viruses belonging to the same lineage sharing

larities, they are only distantly related. As an historical foot-

higher sequence identity. A dendrogram that illustrates their

note, the flaviviruses, described after the togaviruses, were

relationships is shown in Fig. 3.17. A number of lineages or

once classified as a genus within the Togaviridae. When

clades are present, including a clade of aquatic viruses, a clade

sequences of alphaviruses and flaviviruses were determined,

of encephalitic viruses (EEE, VEE), the Sindbis clade (which

however, they were found to be unrelated and the flavivi-

includes Aura virus and many strains of Sindbis virus), the

ruses were placed into a new family.

SFV clade (which includes many viruses which cause arthri-

tis including polyarthritis), and a clade of recombinant viruses

(the WEE lineage). The dendrogram illustrates the interesting

Genus Alphavirus

fact that during evolution of alphaviruses, there was a singu-

The alphaviruses have a worldwide distribution. They get

lar recombination event between Eastern equine encephali-

their name from the Greek letter alpha because they were

tis virus and a Sindbis-like virus to produce Western equine

once known as the Group A arboviruses. Many cause impor-

encephalitis virus, which subsequently evolved into a number

tant illnesses in humans, and information for a representative

of different viruses (the WEE lineage). Recombination events

TABLE 3.10 Togaviridae

Virus name

World

Genus/members

abbreviation

Usual host(s)

Transmission

Disease

distribution

Alphavirus

Mammalsa, Birds

Sindbis

SINV

Mosquito-borne

Arthralgia, rash, fever

Old World

Mammalsa

Semliki Forest

SFV

Mosquito-borne

Arthralgia, fever

Africa

Mammalsa

Ross River,

RRV, BFV

Mosquito-borne

Polyarthritis, fever, rash

Australasia

Barmah Forest

Ft. Morgan

FMV

Birds

Vectored by

North America

swallow bug

Chikungunya,

CHIKV,

Humans

Mosquito-borne

Arthralgia, fever

Africa

O'Nyong-nyong

ONNV

Mammalsa

Mayaro

MAYV

Mosquito-borne

Arthralgia, fever

South America

Mammalsa, Birds

Eastern, Western,

EEEV, WEEV,

Mosquito-borne

Encephalitis

Americas

Venezuelan equine

VEEV

encephalitis

Salmon pancreas

SPDV

Fish

No arthropod vector

disease virus

Rubivirus

Rubella

RUBV

Humans

No arthropod vector

Rash, congenital

Americas, Europe

abnormalities

Humans can be infected by these viruses, but humans are not the primary mammalian reservoir.

RUBV

Aquatic Virus

SPDV

Aquatic Virus

Clade

SDV

OCKV

AURAV

Sindbis

SINV

OCKV

Clade

WEEV

SINV

AURAV

Equine

VEEV

Encephalitis

EEEV

Clade

WEEV

BFV

MIDV

MAYV

SFV

Semliki

RRV

Forest

RRV

SAGV

Clade

SAGV

CHIKV

ONNV

SESV

0.1

Genetic distance

FIGURE 3.17 Phylogenetic trees of the alphaviruses, constructed using the neighbor-joining method from the distances

computed using Clustal X v 1.81 software, with the sequences from rubella virus as outgroup. The Old World viruses are

shown in blue, the New World viruses in red. The vertical distances are arbitrary, but the lengths of the horizontal branches

indicate the number of amino acid substitutions along the branch. (A) A tree derived from the amino acid sequences of

nonstructural proteins nsP1 through nsP4, comprising roughly 2475 amino acids (B) A tree derived from the sequences of

the virion structural proteins or roughly 1245 amino acids. The trees are very similar in architecture with the exception of

the position of WEE (boxed), whose nonstructural proteins are most closely related to EEE, but whose structural proteins

resemble SIN, indicating that a recombinational event has taken place to produce WEE. Note that Fort Morgan, Buggy Creek,

and Highlands J (not shown) are three recombinant New World viruses closely related to WEE. Most virus abbreviations are

found in Table 3.10; AURAV, Aura; MIDV, Middelburg; SDV, sleeping disease; OCKV, Ockelbo; SAGV, Sagiyama; SESV,

Southern elephant seal virus. These trees were adapted from Luers et al. (2005) with permission.

in which the recombinant virus persists and prospers as a

Replication of the viral RNA takes place in association

distinct virus appear to be uncommon to rare, but there is

with cellular membranes. Small spherical invaginations

much evidence that recombination has played an important

form in the membranes, induced by viral proteins, in which

role in the evolution of viruses. The dendrogram also illus-

replication occurs. Protein nsP1 has been shown to interact

trates the fact that there have only been a limited number of

with membranes by means of a specific domain within the

transfers of alphaviruses between the Americas and the Old

protein, and it is assumed that this association is important

World. In fact, three transfers between the Americas and the

for the membrane association of the replication machinery.

Old World are sufficient to explain the dendrogram, and the

Studies of the viral nonstructural protease have shown that

majority of members of the three major lineages are restricted

the cleavages that process the polyprotein control viral RNA

to either the Americas or to the Old World. This contrasts

replication. During or shortly after translation, the full-length

with many virus families, where individual viruses are often

polyprotein precursor (called P1234) cleaves itself in cis

worldwide in distribution and evidence is abundant for the

to produce P123 and nsP4. These form an RNA synthetase,

mixing of lineages between the two hemispheres.

probably together with cellular proteins, that can make com-

plementary (-)RNA from the genomic RNA template, but

which cannot make (+)RNA efficiently. Subsequent cleav-

Expression of the Genome

age of P123 in trans, between nsP1 and nsP2, gives rise

Alphaviruses enter a cell via endosomal uptake and fuse

to a synthetase that can make both (+)RNA and (-)RNA.

with the endosomal membrane upon exposure to a suita-

A second cleavage between nsP2 and nsP3 gives rise to a

bly low pH that differs somewhat from virus to virus. The

synthetase that can make only (+)RNA. Thus, (-)RNA tem-

capsid protein has an affinity for ribosomes and there is

plates are made early, but as infection proceeds and the con-

evidence that ribosomes help disassemble the nucleocap-

centration of protease builds up, trans cleavage occurs and

sid upon its entry into the cytoplasm by binding the capsid

(-)RNA synthesis is shut down (Fig. 3.18). After this time,

protein. Release of the genomic RNA, which is capped and

plus-strand RNA synthesis continues from the preformed

polyadenylated, is followed by its translation into a non-

minus-strand templates but no further increase in the rate

structural polyprotein that is cleaved into four polypep-

of RNA synthesis takes place. This control mechanism may

tides by a viral protease (Fig. 3.16). Activities present in

have evolved not only to make the infection process more

these proteins include a capping activity in the N-terminal

efficient, since genomic RNA for progeny virions and sub-

protein (nsP1); helicase, papain-like protease, and RNA

genomic RNAs for translation of structural proteins required

triphosphatase activities in nsP2; and RNA polymerase in

nsP4. A viral encoded capping activity is required to cap the

viral mRNAs (the genomic RNA and a subgenomic RNA)

because replication occurs in the cytoplasm and the virus

does not have access to cellular capping enzymes. The RNA

103

triphosphatase in nsP2 is required to remove the terminal

100

phosphate in the 5′ triphosphate on the RNA in order to

prepare the RNA for capping by the viral enzyme, the RNA

102

helicase is needed to unwind the RNAs during replication,

and the protease is required to process the precursor poly-

protein. The RNA polymerase is needed to synthesize viral

RNA. All four nonstructural proteins are required to synthe-

size the viral RNA. Replication of the RNA and synthesis of

a subgenomic RNA follow the pattern illustrated schemati-

cally in Figure 1.9B.

The function of the phosphoprotein nsP3 in RNA replica-

tion is unknown. It is phosphorylated on several threonines

and serines in a nonconserved domain in the C-terminal

Hours post infection

region of the protein. The N-terminal domain of nsP3 is a

FIGURE 3.18 Growth curve of Sindbis virus infection in chicken cells at

conserved domain (often referred to as the X domain) that is

30°C. At the times shown, the medium was harvested and replaced with fresh

medium, and the titer was determined. The red line shows release of infectious

also present in a number of other viruses (rubella virus, hep-

virus into the culture fluid. For determining the rate of RNA synthesis, cells

atitis E virus, coronaviruses) as well as being widely distrib-

infected as for virus assay were pulsed with radioactive uridine for 1 hour

uted in bacteria, archae, and eukaryotes. The virus domain

at the times shown. Monolayers were harvested and incorporation into

shares up to 3540% amino acid sequence identity with the

total RNA (green line) and minus-strand RNA (blue line) were determined.

cellular homologues, whose function is unknown.

Adapted from Strauss and Strauss (1994) Figure 5.

for assembly of progeny virions are needed in much larger

P123 and its cleavage products may serve to accelerate

quantities than minus-strand templates, but also to control

the rate of RNA synthesis. There is evidence from genetic

the virulence of the virus. In the case of alphaviruses, it is

studies that domains in nsP1 and nsP2, among others, are

particularly important to control the virulence of the virus

required for the recognition of viral promoters and the initia-

in the mosquito vector. As discussed in more detail later,

tion of RNA synthesis, and additional helicase activity could

it is important that the infection process in the mosquito be

also speed up the rate of RNA synthesis.

tightly controlled, and shutting down minus-strand RNA

During infection by alphaviruses, a subgenomic mRNA

synthesis not only prevents further exponential increase in

is produced that serves as the message for the production of

virus replication but also allows downregulation of virus

the structural proteins of the virus, which consist of a cap-

replication as minus-strand templates decay. It also has the

sid protein and two glycoproteins. The 4.1-kb subgenomic

effect that the infected cell becomes resistant to superinfec-

RNA is synthesized by the viral replicase from the (-)RNA

tion by the same or a related virus because no (-)RNA tem-

template using an internal promoter of 24 nucleotides. The

plates can be made, which could be especially important in

activity of this core 24-nucleotide promoter is increased by

the mosquito vector. The resistance to superinfection by the

enhancer sequences present in the 100 nucleotides upstream

same or related viruses is called superinfection exclusion or

of the core promoter. The structural proteins are translated

homologous interference.

as a polyprotein and cleaved by a combination of viral

The rate of cleavage of the early synthetase that makes

and cellular enzymes. The capsid protein is itself a serine

(-)RNA to convert it into one that can make (+)RNA is

autoprotease that cleaves itself from the N terminus of the

thought to be controlled in part by a leaky stop codon

nascent polyprotein. It has a fold that is similar to that of

between nsP3 and nsP4 that is present in most, but not all,

chymotrypsin (Fig. 2.14B), suggesting that it was derived

alphaviruses (Fig. 3.16). Termination at this codon produces

from a cellular serine protease during evolution of the

P123, which can act in trans as a protease but cannot act as

virus. After release of the capsid protein, N-terminal signal

a synthetase because it lacks the nsP4 RNA polymerase. In

sequences and internal signal sequences in the glycoprotein

addition to a more rapid buildup of protease that accelerates

polyprotein precursor lead to its insertion into the endo-

the rate of conversion to (+)RNA synthesis, this additional

plasmic reticulum (Figure 3.19). This precursor is cleaved

PE2

CHO

NH3+

COO-

Lumen

Acylation

Phosphorylation?

ER membrane

Heterodimerization

+++

PE2 cleavage

COO-

Cytoplasm

+++

Sites of fatty acid acylation

Signalase

Stop transfer signal

Fatty acid chain

Golgi protease (Furin?)

Signal sequence

CHO

Carbohydrate chain

Phosphorylation

FIGURE 3.19 Schematic diagram of the configurations of glycoproteins PE2 and E1 in membranes. The left panel shows

the configuration of PE2 immediately after signalase cleavage from 6K, with the C terminus in the lumen of the ER. The

right panel shows the configuration of the E2-E1 heterodimer after phosphorylationdephosphorylation with the C terminus

in the cytoplasm. Adapted from Strauss and Strauss (1994), Figures 7 and 9.

by signalase, a cellular enzyme that resides in the lumen of

the genomic RNA and for genomic RNA synthesis from the

the endoplasmic reticulum, to produce glycoprotein PE2

antigenomic template. The genomic RNA (and presumably

(a precursor to glycoprotein E2), 6K (a small hydrophobic

the antigenomic RNA as well) is known to cyclize in the

peptide located between E2 and E1), and glycoprotein E1.

absence of protein (Fig. 3.21), thus bringing the sequence

PE2 and E1 quickly form a heterodimer. At some time, the C

elements at the two ends of the molecule into close proxim-

terminus of PE2/E2 is withdrawn from the lipid bilayer into

ity, allowing the viral replicase to interact with both ends of

the cytoplasm, where it can interact with the capsid during

the RNA at once when initiating synthesis of new RNA. It

virus assembly. During transport of the heterodimer to the

seems likely that this mechanism evolved so that only full-

cell plasma membrane, PE2 is cleaved by another cellular

length RNA molecules can be replicated. This eliminates

enzyme, furin or a furin-like enzyme, to form E2 and E3. E3

replication not only of the subgenomic RNA but also of any

is a small glycoprotein that in most alphaviruses is lost into

broken RNA molecules.

the culture fluid.

It is noteworthy that the RNAs of many, perhaps most,

RNA viruses, both plus stranded and minus stranded, cyclize

at some stage of RNA replication or translation. In some

Viral Promoters

cases, as in the alphaviruses, cyclization occurs by means

The replication of alphaviral RNA requires the recognition

of complementary sequences near the ends of the RNA and

of specific promoters in the viral RNA by the viral RNA

requires no protein to stabilize the interaction. In other cases,

synthetase. These promoters act in cis, that is, they must

the interactions of the complementary sequences near the

be present in the RNA to be used as a template, and both

ends of the RNA are stabilized by the binding of viral or cel-

viral and cellular proteins may be involved in the recognition

lular proteins. And in still other cases, cyclization is effected

of these promoters. Four alphavirus promoters, or compo-

by cellular proteins. In one scenario, a cellular protein such

as the poly(A)-binding protein binds to the 3′ poly(A) tract

nents of promoters, are illustrated in Figure 3.20. The best

understood of these promoters is that for the production of the

and another cellular protein, such as one that binds to transla-

tion initiation signals, binds near the 5′ end of the molecule.

subgenomic mRNA for the structural proteins. The basal pro-

moter consists of 24 nucleotides, of which 19 are upstream

These two cellular proteins then interact with each other to

of the transcription start site and 5 are copied into the sub-

hold the two ends of the RNA near one another.

genomic RNA. This subgenomic promoter can be placed in

front of any RNA sequence and the viral synthetase will use

Assembly of Progeny Virions

it to synthesize a subgenomic mRNA. This property of the

promoter has made alphaviruses useful as expression vectors

The structure of alphaviruses has been described in

(Chapter 11).

Chapter 2. PE2-E1 heterodimers form in the endoplasmic

The promoters for synthesis of full-length genomic RNA

reticulum and are transported to the cell surface. During

and the antigenomic (-)RNA template are less well under-

transport, they are cleaved to form E2-E1 heterodimers. The

stood. A linear sequence element at the 3′ end of the (+)RNA

PE2-E1 heterodimer is more stable to acidic pH than is the

and two elements at or near the 5′ end of the genomic RNA

E2-E1 heterodimer and survives the mildly acidic pH of

that can form stem-loop structures (Fig. 3.20) are required

transport vesicles. Cleavage is necessary to prime the E2-E1

for the efficient synthesis of both the antigenomic RNA from

heterodimer for disassembly upon entry into a susceptible

7.6 kb

4.1 kb

CAP

26S RNA

AUUUUGUUUUUAAUAUUUC

170

190

160

[4]

200

ACCUCUACGGCGGUCCUAA AUAGG

19 NT CONSERVED

[3]

SEQUENCE

CAP

CONSENSUS JUNCTION

[1]

[2]

SEQUENCE

CONSERVED STRUCTURES

FIGURE 3.20 Promoters in the alphavirus genome. Adapted from Strauss and Strauss (1994), Figure 18.

FIGURE 3.21 Electron micrographs of Sindbis virus 49S genomic RNA. The RNA was treated with 0.5 M glyoxal in

0.1 M phosphate buffer for either 30 min (A) or 40 min (B) at 35°C before spreading. Scale bar is 100 nm. Adapted from

Frey et al. (1979) Figure 3.

cell, which is required for formation of E1 homotrimers that

interactions. The diameter of the assembled virion is 70 nm

are responsible for fusion. During transport or virus assem-

(Figs. 2.5 and 2.14A).

bly, three E2-E1 heterodimers trimerize to form the spikes

The 6K protein is required for efficient assembly of

found in the virion. Virions normally mature when a preas-

virions. Virions that appear to be normal in every way will

sembled nucleocapsid consisting of the genomic RNA and

assemble in the complete absence of 6K but only inefficiently

240 copies of capsid protein buds through the cell plasma

and virus that lacks the 6K gene produces only low yields.

membrane to acquire a lipoprotein envelope containing 240

It has been shown that 6K expressed alone will form ion

copies of the E1-E2 heterodimer (Fig. 2.25C). The free energy

channels, but whether this is important for virus assembly

for budding is provided by lateral interactions between the

is not known.

viral glycoproteins as they assemble around the nucleocap-

sid and by the interaction of the C-terminal domain of glyco-

Alphaviruses Are Arboviruses

protein E2 with a docking site on each nucleocapsid protein

molecule. The assembled virion has icosahedral symmetry

All alphaviruses are arboviruses (arthropod-borne

in which the symmetry axes of the nucleocapsid are coordi-

animal viruses), with the probable exception of the

nated with those of the glycoprotein shell by the E2capsid

newly described fish viruses, and were once referred to as

Mesenteron

Salivary Glands

A. A female mosquito takes a blood meal

B. Cutaway view of the mosquito showing

steps in the replication and transmission

of an arbovirus

FIGURE 3.22 Sequential steps necessary for a mosquito to transmit an arbovirus. (1) A female mosquito ingests an

infectious blood meal and virus enters the mesenteron. (2) Virus infects and multiplies in mesenteronal epithelial cells. (3)

Virus is released across the basal membrane of the epithelial cells and replicates in other tissues. (4) Virus infects salivary

glands. (5) Virus is released from the epithelial cells of the salivary glands and is transmitted in the saliva during feeding.

Adapted from Monath (1988).

the Group A arboviruses. In nature, they alternate between

transmission cycle is illustrated, such as that of urban yellow

replication in arthropod vectors, usually mosquitoes, and

fever infection of humans (see the section on flaviviruses

higher vertebrates. A mosquito may become infected on

later). In this cycle, humans are the only vertebrate hosts and

taking a blood meal from a viremic vertebrate, which can

the virus alternates between infection of a human and infec-

have 108 or more infectious particles per milliliter of blood.

tion of the mosquito vector Aedes aegypti. Figure 3.23B

The infection in the mosquito, which is almost asymptom-

illustrates a complex transmission pattern, using as an exam-

atic and lifelong, begins in the midgut and spreads to the

ple the transmission of Eastern equine encephalitis virus in

salivary glands, as illustrated in Fig. 3.22. After infection

North America. This virus has a vertebrate reservoir con-

of the salivary glands, the mosquito can transmit the virus

sisting primarily of migratory songbirds and is transmitted

to a new vertebrate host when it next takes a blood meal,

by the mosquito, Culiseta melanura, a common inhabitant

injecting saliva in the process. Infection in the vertebrate

of freshwater swamps in eastern North America. However,

begins in the tissues surrounding the bite or in regional

the virus is capable of infecting other mosquitoes and has

lymph nodes, but then spreads to other organs. The infec-

even been isolated from naturally infected chicken mites. It

tion is usually self-limited and the vertebrate is capable of

also infects mammals, including humans. On occasion, the

infecting mosquitoes for only a brief time after viremia is

virus breaks out of its enzootic cycle to cause epidemics of

established but before an immune response limits circu-

disease in pheasants, transmitted and maintained by an epi-

lating virus. The necessity to alternate between two such

zootic vector mosquito. Either enzootic or epizootic vectors

different hosts has constrained the evolution of arbovi-

are capable of infecting humans or domestic animals if they

ruses--changes that adapt the virus to one host or that are

invade the areas in which these mosquitoes are present, but

neutral in one host are often deleterious in the alternate

these hosts are usually dead-end hosts and do not further

host. Thus, the evolutionary pressures on arboviruses are

spread the virus.

different from those on viruses such as poliovirus, which

Most alphaviruses are capable of infecting both mam-

infects only primates.

mals and birds, and the nature of the vertebrate reservoir

Different alphaviruses infect different spectra of mosquitoes

depends on the virus or even the strain of virus, which may

and vertebrates in nature. It is useful to distinguish between

differ by geographic location. Thus, for example, Sindbis

reservoir hosts in which the virus is maintained in nature and

virus in nature is normally maintained in birds, which are

dead-end hosts in which infection normally does not lead to

its usual vertebrate reservoir. However, the virus is capable

continuity of the infection cycle. We can also distinguish

of infecting mammals, including humans, and has also been

between enzootic cycles, in which the virus is continuously

isolated from amphibians and reptiles. Numerous species

maintained in nature and which may or may not result in

of mosquitoes form its insect reservoir, but it has also been

disease in the enzootic host, and epizootic cycles, in which

isolated from other hematophagous arthropods, including

the virus breaks out and causes epidemics of disease that

mites. In contrast, Ross River virus is maintained in small

may die out with time. Two types of natural transmission

marsupial mammals in Australia and does not appear to use

cycles are illustrated in Fig. 3.23. In Fig. 3.23A, a simple

birds as hosts.

A. Simple Transmission Cycle

Domestic Vector

Primary Vertebrate Host

Complex Transmission Cycle

Epizootic Host

Enzootic Host

Pheasant

Passerine Bird

Enzootic Vector

Epizootic Vector

Enzootic Vector

Epizootic Vector

Dead-End Hosts

Man and Horses

Enzootic Host

Epizootic Host

Passerine Bird

Pheasant

FIGURE 3.23 Two generalized transmission cycles of arboviruses. (A) Simple cycle, such as urban yellow fever,

involving a single vector (Aedes aegypti mosquitoes) and a single vertebrate host (man). (B) Complex cycle, such as that

for Eastern equine encephalitis, where the virus is maintained in an enzootic host (passerine birds) with an enzootic vector

(Culiseta melanura), but can enter an epizootic vector (another insect) and be transmitted to epizootic hosts, and tangentially

to dead-end hosts like man. An intermediate type of cycle is illustrated in Figure 5.12 for Colorado tick fever. Adapted from

Monath (1988) p. 129.

Arboviruses that are transmitted by mosquitoes,

that the female mosquito survive long enough and be healthy

ticks, sand flies, or other bloodsucking arthropods are

enough to take another blood meal, which is required for egg

known from several families of viruses. A selection of

development. The time between infection of the female and

arboviruses from three virus families, most of which cause

the time at which the mosquito is capable of transmitting the

human disease, is listed in Table 3.11, together with the

virus is called the extrinsic incubation period. This period

diseases they cause.

varies widely with temperature, humidity, the virus, and the

mosquito. It can be as short as 2 days or as long as 2 weeks,

Pathology in the Mosquito Vector

although in alphaviruses it is seldom longer than 1 week.

The mosquito vector suffers little pathology upon infec-

Although infection of the mosquito is relatively benign,

tion by an arbovirus. This must be so if the virus is to persist,

recent studies have shown that there is some pathology asso-

because transmission of the virus to a new vertebrate requires

ciated with arboviral infection. There is limited pathology

TABLE 3.11 Representative arboviruses that cause human disease

Predominant disease manifestationsa

Nonfatal

Encephalitis

Hemorrhagic Fever (HF)

systemic

Frequencyb

%Mortalityd

%Mortalityc

Family/virus

febrile illness

Togaviridae

Chikungunya

Most cases, Ep

Rare, Ep

rare

Mayaro

Most cases, Ep

O'nyong-nyong

Most cases, Ep

Ross River

Most cases, Ep

Sindbis

Most cases, Ep

EEE

Most cases

Rare

5070

0.120 e

VEE

Most cases, Ep

Rare

WEE

Most cases

Rare

510

Flaviviridae

Dengue (14)

Most cases, Ep

Rare, Ep

312

∼9%f

West Nile

3040 g

Japanese encephalitis

<1%

Kyasanur Forest

Murray Valley

2070

Rocio

St. Louis encephalitis

420

Tick-borne encephalitis

Eastern

Rare

Central European

Rare

110

Omsk hemorrhagic fever

Yellow fever

Most cases

520

Bunyaviridae

Bunyamwera

Germiston

Sand fly fever

Rift Valley fever

California encephalitis

Crimean hemorrhagic fever

1520

In addition to the disease manifestations listed, most viruses in this table can cause mild febrile illness; some viruses are endemic (En) but

others cause occasional outbreaks or epidemics (Ep).

Frequency relates to the relative number of cases exhibiting encephalitis or HF relative to the total number of infections. This number can be

difficult to estimate, since only the most seriously ill (e.g., hospital patients in an epidemic) may be reported as infections. Mortality ≥ 10%

is highlighted in red.

Percent mortality is the percent of those with encephalitis who succumb.

Percent mortality is the percent of those with HF who succumb.

Mortality in children is at the high end of the range given.

Mortality from recent epidemics in the United States.

Mortality generally lower in children.

in the gut, which might be important for virus spread from

results in decreased survival and reproductive capacity.

the gut into the hemolymph, from which it spreads to the

Thus, the persistence of an arbovirus requires a delicate

salivary glands, in some cases after an increase in titer

balance. The virus must replicate to sufficiently high titer

following replication in fat body cells. More importantly,

in the vertebrate to cause sufficient viremia to infect a

however, are studies showing that arbovirus infection

mosquito, which usually results in symptomatic disease.

But in the mosquito the infection must be controlled so

Seasonality of Disease

as to produce sufficiently high titers of virus in salivary

It is obvious from the preceding discussion that mos-

fluid without damaging too extensively the ability of the

quito-borne diseases are seasonal. In temperate areas, dis-

mosquito to survive and reproduce.

ease is absent in the winter. In spring when mosquitoes

first arise, disease is also absent or rare. It is only with

Overwintering by Arboviruses

time that the intensity of virus transmission builds up to

In humid tropical or subtropical areas in which

the point that humans become infected with some fre-

mosquitoes are active throughout the year, an arbovirus

quency. There have also been suggestions that the arbo-

can be maintained by continuous transmission between

viruses that first appear may not be as virulent as viruses

invertebrate and vertebrate hosts. Virus activity may vary

that appear later in the season, perhaps because they are

during the year, for example it may be much greater dur-

adapted to the mosquito vector and must readapt to the

ing a rainy season when the number of mosquito vec-

vertebrate host. In any event, arboviral epidemics char-

tors is higher, but the virus is active throughout the year

acteristically occur in mid to late summer and early fall

and human infection can occur at any time. However, in

in temperate regions. In other regions, epidemics may be

temperate zones in which adult mosquitoes die off in the

associated with heavy rainfall that results in an increase

winter, or in very dry areas in which mosquitoes are only

in the mosquito population.

active after sporadic rains, the virus must have a mecha-

nism by which it overwinters. Mosquitoes survive win-

Encephalitic Alphaviruses

ters (or extended droughts) by suspending development

of the young at some stage. In some mosquitoes, eggs are

Most, perhaps all, alphaviruses are neurotropic. They

laid but do not develop until conditions are favorable. In

readily infect neurons in culture or in experimental ani-

others, developing young diapause, suspending develop-

mals. Infection of neurons is dependent upon the age of

ment at some stage of embryonic development or larval

the animal as well as dependent upon the strain of virus.

development, until conditions are favorable, such as the

Sindbis virus infection of the mouse has been used as

return of spring. Thus, one mechanism for overwintering

an experimental model to study virus induced encepha-

by some arboviruses is transovarial transmission, in which

litis (inflammation of the brain, from enceph = brain,

the virus infects oocytes in the infected female at an early

itis = inflammation) and encephalomyelitis (inflamma-

stage of development; the replication cycle of the virus is

tion of the brain and spinal cord, from the preceding plus

suspended during diapause and the animal develops nor-

myel = medulla or marrow). Some strains of the virus will

mally. When the newly hatched mosquito begins to fly, it is

invade the central nervous system after peripheral inocu-

already infected. One way to search for transovarial trans-

lation and cause encephalitis, whereas other strains of the

mission in the field is to look for virus in male mosquitoes.

virus do not invade the CNS following peripheral inocula-

Since these do not feed on blood, the only way for them

tion but will cause encephalitis upon direct inoculation of

to become infected is by transovarial transmission. Some

the virus into the brain, and still other strains do not cause

alphaviruses are known to use this mechanism whereas

overt encephalitis even though neurons may be infected.

other alphaviruses do not. A second mechanism used by

In all cases, the infection of neurons to cause encephalitis

some arboviruses is persistent infection of a vertebrate host

is age related. Very young mice are susceptible to most

so that infected vertebrates are present when the mosqui-

strains of the virus, whereas infection of older mice does

toes emerge once again. Some alphaviruses are known to

not result in encephalitis for many strains of the virus.

persist in humans or other vertebrates for extended periods,

Manipulation of the viral genome in the laboratory has

but such persistence is rare and it is not known whether this

resulted in the identification of specific genes that are

could be a means of overwintering. In some arboviruses,

important for neurovirulence.

however, such as the coltiviruses (see Chapter 5), persis-

It is important to distinguish neurotropism, the ability to

tence is known to be essential to the maintenance of the

infect neurons or other cells of the CNS, from neuroinvasion,

virus in nature and the virus has evolved specific mecha-

the ability of the virus to cross the bloodbrain barrier and

nisms to persist. A third mechanism is the reintroduction of

invade the CNS, from neurovirulence, the ability to cause

the virus into an area from regions where it persists year-

brain disease once the CNS has been invaded. Although

round, for example, by the return of migratory birds or by

most alphaviruses studied are neurotropic, only three viruses

infected mosquitoes being blown over large distances by

regularly cause encephalitis in humans or domestic animals.

storms. The whole subject of overwintering is an interest-

These three viruses, Eastern (EEE), Western (WEE), and

ing evolutionary and ecological study, and in the case of

Venezuelan equine encephalitis (VEE) viruses, are New

many arboviruses, including most alphaviruses, overwin-

World alphaviruses that cause fatal encephalitis in horses.

tering is only poorly understood.

WEE and EEE regularly cause encephalitis in humans as

Western Equine Encephalitis

17 3

Rate per 100,000 population

none

<0.01

or equine only

0.01 0.10

0.11 0.59

0.60 1.00

>1.0

Eastern Equine Encephalitis

FIGURE 3.24 Geographic distribution of reported human cases of alphavirus encephalitis in the United States from 1964

to 2003. Colors indicate the rate in cases per 100,000 population by state and the actual numbers of cases are shown. Note that

there were two human cases of WEE in 1994 and one in 1999, but none have been reported since then. Reported cases of EEE

from 1994 through 1998, for which no state locations were given, were 1 in 1994, 4 in 1995, 5 in 1996, 14 in 1997, and 4 in

1998. There were 5 cases of EEE in 1999, 3 in 2000, 9 in 2001, 10 in 2002, and 14 in 2003. Since 1998 EEE cases are reported

by state and the cases for 19992003 have been added to the cumulative state totals. Adapted from Fields et al. (1996) p. 875

and additional data from MMWR, Summary of Notifiable Diseases, 1998, 1999, 2000, 2001, 2002, 2003.

well, although the number of cases is small (Table 3.11 and

may be important for the invasion of the CNS by poliovirus

Fig. 3.24). The mechanisms by which the CNS is invaded

(see earlier). Infection of the nasal neuroepithelium, whose

by alphaviruses or by viruses belonging to other families

neurons project directly to the CNS, may also lead in infec-

that cause encephalitis (poliovirus, certain flaviviruses, and

tion of the CNS. Sindbis virus appears to use this mechanism

others) are imperfectly known. Model studies in mice have

as well as transport from the peripheral sites, and transport

indicated that Sindbis virus invades the CNS by retrograde

from the neural epithelium may be particularly important

axonal transport from the peripheral site of infection where

in VEE infection. A third possible mechanism is the estab-

primary replication occurs, one of the mechanisms that also

lishment of viremia followed by infection of choroid plexus

epithelial cells such that the virus replicates across the

reservoir for the virus. Perhaps because of this, the virus has

bloodbrain barrier. As described, this mechanism appears to

evolved into a number of strains that differ by up to 25% in

be particularly important in poliovirus invasion of the CNS,

nucleotide sequence.

but does not appear to be used during invasion by Sindbis

Western Equine Encephalitis Virus

virus. Thus, the primary mechanism used appears to differ

among different viruses, and more than one mechanism may

WEE is less virulent for humans than is EEE. Encephalitis

be important for any individual virus.

occurs in only 1 of 1000 adults infected by WEE, but in

Alphaviruses have been used as a model system to study

about 2% of children younger than 5, and the encephalitis

recovery of mice from virally induced encephalitis. The CNS

produced by WEE is less severe, with an overall case fatality

is immunologically privileged (see Chapter 10) because

rate of about 3%, but about 8% in persons older than 50. The

neurons are not replaceable once destroyed, and the mecha-

virus is also less virulent in horses, in which the case fatal-

nisms used to cure the CNS of viruses differ from those used

ity rate is about 40%. The virus is a recombinant between

in other organs such as the liver or intestinal tract. It has been

EEE and a Sindbis-like virus, probably Aura virus present

shown that humeral antibodies are required for clearance

in South America (Fig. 3.17). Because the glycoproteins

from the CNS but the mechanism is unknown. Cytotoxic T

were derived from the Aura parent, the virus is serologically

cells are also required for recovery from virally caused CNS

related to the Sindbis lineage. The RNA replication proteins

disease, but by means of secretion of interferon-γ rather than

are derived from EEE, as is the encephalitic potential of the

by killing of infected cells.

virus. The fact that the virus is less virulent than EEE is con-

sistent with laboratory studies that chimerization of a virus, a

Eastern Equine Encephalitis Virus

technique being used to produce live virus vaccines, usually

The majority of human infections by EEE are inap-

results in lowered virulence (see, e.g., Chapter 11).

parent or result in febrile illness that is usually mild.

WEE is endemic from western Canada discontinuously

Encephalitis results in only about 4% of infected adults

to southern South America. In the western United States,

but in more than 10% of children less than 10 years old,

the primary vector is Culex tarsalis and the vertebrate reser-

demonstrating the age-related aspects of the disease.

voir is again birds, although jackrabbits and possibly other

EEE-caused encephalitis is fatal about half the time, with

mammals may be important in some areas. In the past,

the highest fatality rates in the very young and the very

infection by the virus was common. An epizootic of WEE

old. Survivors usually have neurological deficits. There

in 1912 killed an estimated 25,000 horses in the western

have been an average of seven cases of EEE encephalitis

United States. In 1960, 34% of humans tested in rural areas

per year in the United States over the last 50 years. Horses

of California were found to be positive for antibodies to

are more sensitive to the virus and more likely to become

WEE, indicating past infection by the virus. Infection is

infected, and the virus is of major concern to horse breed-

now less common in the United States, and only about 2%

ers in the eastern United States. The case fatality rate is

of humans in similar areas of California were found to be

8090%, and in years past there have been epidemics

seropositive for WEE in the mid 1990s. Confirming this

involving thousands of horses; the largest such epidemic

trend, an average of 34 cases of human WEE encephalitis

on record killed more than 11,000 horses in Louisiana and

occurred per year in the United States from 1955 to 1984,

Texas in 1947.

but this number has declined thereafter and there has been

As described before, in North America EEE is main-

only one case in the United States since 1994 (Fig. 3.24).

tained in an enzootic cycle in birds that inhabit freshwater

This dramatic decline may have resulted from mosquito

swamps, vectored by Culiseta melanura. Perhaps because

control measures, the widespread use of insect repellents,

many of these birds are migratory and therefore capable of

the adoption of air-conditioning and window screens that

spreading the virus over large distances, the virus is fairly

has resulted in fewer bites by mosquitoes, especially night

uniform throughout its North American range. Viruses iso-

flying mosquitoes, and because fewer horses, which are

lated over a period of more than 60 years from many differ-

amplifying hosts for WEE, are used in farming. No vaccines

ent areas of North America show less than 2% nucleotide

for this virus are available for widespread use, although

sequence divergence. As illustrated in Fig. 3.23, epizootic

experimental vaccines exist that are given to laboratory per-

cycles in other birds can occur. Mosquitoes vectoring either

sonnel who work with the virus.

the enzootic cycle or an epizootic cycle are capable of trans-

In Central and South America, WEE remains a widespread

mitting the disease to horses or humans.

problem because the horse is still in widespread use as a farm

EEE in South America is a distinct virus. It is vectored

animal. However, the virus is not associated with significant

primarily by Culex species and is not associated with severe

human disease in South America, for reasons unknown. In

disease in humans. In addition to birds, small mammals,

South America, small mammals appear to be an important

especially rodents, which are sedentary, are an important

vertebrate reservoir.

As described, WEE arose by recombination, an event that

in Fig. 3.25) is also a South American virus but is only dis-

occurred at some unknown time but probably more that 1000

tantly related to other subtype I viruses, and is now consid-

years ago. Since its origin, it has diverged into a number

ered a separate species, named to the present by its isolate

of different viruses, including Fort Morgan virus found

designation 78V353I virus. These eight virus species are

in Colorado and Oklahoma and Highlands J virus present

maintained in an endemic cycle in which the principal mos-

along the east coast of the United States. Fort Morgan virus

quito vectors are species belonging to the genus Culex, sub-

is transmitted by swallow bugs present in the nests of swal-

genus Melanoconin, and the major vertebrate reservoirs are

lows to the young birds of the year, an interesting adaptation.

small mammals, primarily cotton rats and opossums. These

Highlands J virus is maintained in a cycle similar to that

endemic viruses are able to infect horses but replicate to only

for EEE. Neither of these viruses has been associated with

low titers in them, causing no disease and not establishing

human disease.

an epidemic cycle. Similarly, these endemic viruses are not

associated with significant human disease.

Venezuelan Equine Encephalitis Virus Complex

The endemic VEE ID gives rise to epizootic viruses IAB

The VEE complex consists of a number of closely related

and IC by a small number of mutations in envelope glycopro-

viruses that are endemic to tropical and subtropical areas of

tein E2 that allow the virus to grow to high titers in horses.

the Americas. These viruses were first classified by serology

The horse serves as an important amplifying host that allows

into six subtypes of VEE, numbered with Roman numer-

the virus to become epidemic and spread over wide areas.

als from I to VI. Some of these subtypes were further sub-

Aedes taeniorhynchus and Psorophora confinnis are impor-

divided; in particular, subtype I was subdivided into IAB,

tant mosquito vectors in these outbreaks. In these epidem-

IC, ID, IE, and IF. Partial or complete sequences of most of

ics, which occur at intervals of 1020 years in Venezuela,

these viruses are now available and these subtypes are now

Columbia, Peru, and Ecuador, large numbers of horses die of

considered to be full species (Fig. 3.25). As a species, VEE

encephalitis and significant episodes of human illness occur.

virus consists of IAB, IC, and ID; IE, also considered a strain

For example, an epidemic in Venezuela and Columbia in

of VEE, will be considered in more detail later. Subtype II

1995 resulted in disease in an estimated 75,000 to 100,000

is now named Everglades virus and is endemic to Florida.

people, including 3000 cases of neurological disease and 300

Subtype IIIA is called Mucambo virus, IIIB is Tonate virus

deaths. As with the other encephalitic alphaviruses, children

(of which Bijou Bridge virus is another isolate), IV is Pixuna

and older people are more at risk for serious illness.

virus, V is Cabassou virus, and VI is Rio Negro virus; most

Because of the importance of VEE epidemics, an inac-

of these are South American viruses. Subtype IF (not shown

tivated virus vaccine was produced many years ago from a

Serological

Subtype

Rio Negro

Pixuna

Cabassou

VEE-71D1252

IIIC

Mucambo

IIIA

Tonate

IIIB

Bijou Bridge

VEE

Everglades

VEE

IAB

0.05

VEE

Substitutions/site

FIGURE 3.25 Phylogenetic tree of the various strains of Venezuelan equine encephalitis virus (VEE) and species of

viruses formerly considered to be serological subtypes of VEE. This tree was generated from partial E1 envelope gene

sequences using a neighbor-joining algorithm. Adapted from Fauquet et al. (2005), Figure 4 on p. 1007.

strain of IAB virus called Trinidad Donkey for use in horses

annulirostris and Aedes notoscriptus in other areas, which

and for laboratory personnel who work with the virus. This

breed in freshwater. The vertebrate reservoir consists of var-

vaccine suffered from poor immunogenicity and insuffi-

ious macropods. Because mammals constitute the vertebrate

cient inactivation. Use of the vaccine, for example, led to

reservoir, different strains of the virus have evolved in differ-

a wide-ranging epidemic of VEE in 19691973 that spread

ent geographic regions for reasons discussed earlier. In some

from South America up through Central America to south-

areas of Australia in which the virus is endemic, a majority

ern Texas, causing many thousands of cases of disease in

of the population may be seropositive for RRV, indicating

horses and humans. Because of these problems, use of this

a high attack rate. It has been estimated that 2 to 30% of

vaccine was abandoned and a new attenuated vaccine strain

infected humans develop clinical symptoms following infec-

called TC-83, derived from the Trinidad Donkey virus by

tion. As noted, recovery may be prolonged, with arthritic

passage in culture, was developed. This vaccine is effective

symptoms recurring over a period of a year or more.

but is reactogenic and causes mild illness in many recipients.

Of interest was a wide-ranging epidemic of Ross River

An inactivated version of this attenuated virus has also been

polyarthritis that swept through the South Pacific in 19791980.

produced as a vaccine but is not as effective as the live virus

The epidemic began when a single viremic traveler from

vaccine.

Australia landed in Fiji. The epidemic began near the airport

VEE virus can spread by aerosols as well as by mosquito

and eventually spread throughout the island. From there it

transmission. Many cases of laboratory acquired infection

jumped to other islands having air contact with Fiji. During

have resulted from aerosols. Because of this property and the

this epidemic, it is believed that humans were the primary

incapacitating disease caused by VEE, this virus was weap-

or only vertebrate host, with the disease being transmitted

onized by the Russians in the past as a potential biowarfare

from mosquito to human to mosquito without the interven-

agent.

tion of another animal reservoir (the cycle illustrated in Fig.

Virtually all epidemics of epizootic VEE have been due

3.23A). Sequencing studies have suggested that a mutation

to IAB and IC viruses. However, a recent epizootic of VEE

in glycoprotein E2 may have allowed this cycle to become

in Mexico resulted from an IE strain that had a mutation in

established. The mosquito vector during this epidemic was

envelope glycoprotein E2. Unlike the IAB and IC strains,

Aedes polynesiensis. This epidemic was explosive with most

the epizootic IE virus grows to only low titers in horses. It is

of the people on the affected islands becoming infected by

believed that the mutation in IE virus allowed a more effi-

the virus and about 10% of them becoming ill. The epidemic

cient interaction with mosquito vectors that are widespread

eventually burned itself out because all the humans had

and numerous, allowing more efficient transmission of the

become immune and the virus failed to establish an endemic

virus. As shown in Fig. 3.25, IE virus is more closely related

cycle in other animals in the region. After 20 years without

to Everglades virus than it is to ID virus, and the virus may

RRV in Fiji, three recent cases of RRV disease have been

be reclassified in the future.

reported in travelers to Fiji. It is thought that RRV has been

reintroduced into the island on these occasions and sus-

ceptible tourists infected, but that the native population is

Alphaviruses That Cause Arthritides

now largely immune to the virus and no new epidemic has

A number of alphaviruses cause disease in humans char-

occurred.

acterized by fever, rash, and joint involvement. The pain

Sindbis Virus

from arthritis (joint inflammation, from arth = joint and

itis = inflammation) or arthralgia (joint pain, from arth and

The prototype alphavirus is Sindbis virus, named after

algia = pain) following infection by these viruses can be so

the town of Sindbis, Egypt, where it was first isolated in

severe as to be disabling and can last for a year or more with

1953 from mosquitoes. It has the widest distribution of

relapses of severe arthritis being common during this period.

any alphavirus, endemic from northern Europe through

The names of some of these viruses come from the crippling

the Middle East and India to South Africa, Southeast Asia,

pain caused by the disease resulting from viral infection.

Indonesia, the Philippines, New Guinea, and Australia. As

might be expected from this broad range, strains of virus

Ross River Virus

isolated from different regions may differ by 20% or so in

Ross River virus is widespread in Australia, New Guinea,

nucleotide sequence and differ in their epidemiology and

and the Solomon Islands. The disease induced by virus infec-

disease potential. As far as known, birds are the vertebrate

tion is known as epidemic polyarthritis and is characterized

reservoir for the virus over its entire range with different

by pain, often accompanied by frank swelling, in the small

mosquitoes serving as vectors in different areas. Over most

joints of the hands and feet and in the knees. The princi-

of its range no human illness is associated with infection or

pal vectors are Aedes camptorhynchus and Aedes vigilax

illness is very rare, even though in regions such as the Nile

in coastal regions, which breed in salt marshes, and Culex

Delta seroprevalence rates may be fairly high. However,

strains of the virus in northern Europe, South Africa, and

In 2006, a hugh epidemic of CHIK, accompanied in many

Australia cause significant episodes of arthritic illness.

locations by infections of dengue virus as well (see later)

In northern Europe the strains of Sindbis virus that cause

whose disease symptoms can be similar to those caused by

arthritic disease are called Ockelbo virus in Sweden (present

CHIK, occurred throughout the Indian Ocean region. The

between the 60th and 64th parallels), Pogosta virus, wide-

virus appears to have been imported from East Africa, and

spread in Finland, and Karelian Fever virus, present in far

many thousands of cases occurred in the region, affecting

Western Russia. The virus is maintained in migratory birds

the islands of Reunion, Mauritius, Madagascar, Mayotte, the

or in game birds and the mosquito vectors are various spe-

Seychelles, and the Maldives as well as the Indian subconti-

cies of Culex and Culiseta. Aedes species may spread the

nent. In Reunion there occurred more than 200,000 cases in a

virus to humans. The disease in humans is typical of alphavi-

population of under 800,000. Many of these islands are pop-

rus arthritic disease with fever, rash, and joint inflammation

ular tourist destinations, and many cases were imported into

and the number of cases can be large during epidemic years.

Europe. The primary mosquito vector was Aedes albopic-

Children are less likely to develop disease upon infection.

tus or Aedes aegypti, depending upon the location, both of

In South Africa strains of Sindbis virus also cause human

which are also efficient vectors for dengue virus.

disease, but the number of cases appears to be small and the

O'nyong-nyong (ONN) virus is a close relative of CHIK

virus has been less well studied. Strains of Sindbis virus that

that has caused very large epidemics of disease in East

are present in Australia also cause human disease, primarily

Africa similar to that caused by CHIK. The name also comes

in northeastern Australia. The virus appears to be fairly uniform

from the very painful arthralgia accompanying the disease.

throughout Australia but to be replaced every so often by

Epidemics affecting up to two million people have occurred

new strains that presumably invade from the north.

followed by the disappearance of the virus for long periods.

In these epidemics the virus is transmitted by Anopheles

Mayaro Virus

funestrus and Anopheles gambiae, mosquitoes that are

Mayaro virus is present in the northern half of South

major vectors of malaria, and these epidemics represent the

America (Trinidad, Surinam, Brazil, Columbia, Bolivia). It

only known cases of epidemic transmission of an alphavi-

is maintained by Haemagogus mosquitoes and humans usu-

rus by anopheline mosquitoes. An endemic cycle presum-

ally contract the virus while in humid tropical forests. Rubber

ably maintains the virus during interepidemic periods but, if

workers are at risk of infection and the polyarthritis caused

so, this cycle is unknown. A strain of ONN called Igbo-Ora

by the disease can be debilitating, preventing the workers

virus, from the name of the Nigerian village in which the

from gainful employment. Mayaro belongs to the Semliki

virus was first isolated, is present in West Africa.

Forest virus clade (Fig. 3.17) and causes a disease that is

Barmah Forest Virus

similar to that caused by the related Ross River virus. It is

the only known representative of this clade in the Americas

Barmah Forest virus is an Australian virus that is an out-

and represents one of the very few transfers of alphaviruses

lier in the Semliki Forest clade (Fig. 3.17). It also causes

between the Old and the New World.

polyarthritis in humans. It is probably maintained in a cycle

similar to that for Ross River virus.

Chikungunya Virus and Related Viruses

Chikungunya (CHIK) virus, a member of the Semliki

Other Alphaviruses

Forest virus clade, is endemic or epidemic from sub-Saharan

Africa through India and Southeast Asia to the Philippines.

Other alphaviruses are known that infect higher verte-

The name comes from Swahili meaning "that which bends

brates including humans, but most are not associated with

up," from the intense arthralgia that causes patients to lie with

disease in humans. Semliki Forest virus, named after the

joints flexed. In Africa the virus is maintained in an endemic

Semliki Forest in Uganda, has been extensively charac-

cycle that is similar to that for yellow fever virus (see later).

terized as a model system to study the molecular biology

The mosquito vectors are Aedes africanus and Aedes furcifer

of alphaviruses. Most strains cause no human illness, but

and subhuman primates are the vertebrate reservoir. During

strains from central Africa cause a disease characterized by

explosive epidemics in urban areas of Africa and Asia, Aedes

exceptionally severe headache, fever, and rash. One case of

aegypti is the vector and humanmosquitohuman cycles

fatal human encephalitis caused by this virus occurred in a

maintain the virus. During an epidemic, a large fraction of

laboratory worker, who is believed to have contracted the

the susceptible human population may contract the disease.

virus via aerosols. Getah virus, widespread in Asia, causes a

The epidemic then dies out, to return when reintroduced

mild febrile illness in humans.

after a period of time that allows a susceptible population

Recent isolates of new alphaviruses include Southern

to build up. Epidemics often occur during the rainy season

elephant seal virus, currently unclassified. It is spread by

when the population of Aedes aegypti is highest.

a louse, Lepidophthirus macrorhini, that infests the seals.

Two fish alphaviruses have been recently isolated, salmon

synthesis is controlled in the same way as in alphaviruses.

pancreas disease virus and sleeping disease virus, which

The uncleaved nonstructural polyprotein synthesizes minus-

are now considered to be strains of the same species, to

strand RNA, whereas the cleaved products synthesize only

be called salmonid alphavirus. A third strain of the virus,

plus-strand RNA.

Norwegian salmonid alphavirus, has been found in west-

Rubella virus infects only humans and there is no other

ern Norway. Salmonid alphavirus is an important pathogen

reservoir. Infection is by person-to-person contact, primarily

of Atlantic salmon and rainbow trout in Norway, Britain,

through aerosols. It causes a relatively benign illness, some-

Ireland, and France. It causes significant economic losses

times called German measles, with a characteristic rash and

in the farmed fish industry and is the leading cause of eco-

is (or was) one of the typical childhood diseases. However,

nomic loss in Ireland. The viruses can be spread by direct

infection of a pregnant woman in the first trimester of preg-

fish-to-fish infection and are not known to have an arthro-

nancy can have devastating effects on the developing fetus.

pod vector, but since all other alphaviruses are vectored

The virus sets up a long-lived infection in the fetus that often

by arthropods there has been speculation that a sea louse

causes developmental abnormalities resulting in children

Lepeophtheirus salmonis might be involved in the transmis-

with severe handicaps (congenital rubella syndrome). An

sion of the virus. It is not known if the primary location of

attenuated virus vaccine against rubella has been developed

infection is saltwater or freshwater.

that is now routinely given to children as part of mumps

measlesrubella vaccination (MMR). Since the vaccine was

introduced, there has been a drastic reduction in the number

Rubella Virus

of cases of rubella in the United States (Fig. 3.26) and other

Rubella virus is less well understood than alphaviruses

developed countries.

because it grows very poorly in cultured cells and its genome

Because the postnatal disease caused by rubella virus is

possesses an extraordinarily high GC content (70%), which

trivial, the purpose of the rubella vaccine is to protect against

retarded efforts to sequence and express the viral genome.

future birth defects rather than to protect the individual vac-

The complete sequences of several strains are now known

cinated. This raises interesting ethical questions about its

and detailed molecular studies are under way. The genome

use. In some societies, only females were vaccinated, since

is approximately 10 kb in size and is expressed similarly to

they would want to protect their future children from the

the alphavirus genome: The genomic RNA is translated into

effects of rubella. However, because males remained suscep-

a polyprotein cleaved by a papain-like protease into two

tible to the virus, it continued to circulate in the population

pieces, and a subgenomic mRNA is translated into structural

and rubella-caused birth defects continued to occur. To pro-

proteins consisting of a capsid protein and two envelope

tect against this, the only solution is to vaccinate the entire

glycoproteins (Fig. 3.16). Interestingly, minus-strand RNA

population so as to eliminate the virus from the society.

Vaccine Licensed

1.0

0.8

0.6

0.4

0.2

0.0

1982

1987

1992

1997

2002

Year

1967

1972

1977

1982

1987

1992

1997

2002

Year

FIGURE 3.26 Incidence of rubella virus, by year, in the United States. From MMWR Summary of Notifiable Diseases in

the United States for 1997, 1998, 1999, 2000, 2001, 2002, 2003.

Search WWH :

Custom Search

Viruses And Human Disease Topic Index

Viruses And Human Disease Bookmarks

Home