strict quarantines in order to prevent the virus from reap-
of beef from areas where vaccination is practiced because it
pearing in this country. In the United States, work with the
is not possible to rule out the presence of FMDV infection.
virus is allowed only on Plum Island in Long Island Sound,
Although FMDV was thought to have been eradicated by
in order to prevent its accidental release. Because of its
June 2001, and restrictions were about to be lifted, 16 new
agricultural importance, the molecular biology of the virus
cases of FMDV were then found in Britain that delayed lift-
has been intensively studied.
ing of the restrictions in that country. The last new case was
In 2001, a large epidemic of FMDV occurred in Western
found on September 30, 2001, and Britain was declared free
Europe. The epidemic began in February in British sheep
of FMDV on January 15, 2002.
and spread to cattle and pigs in Britain and on the conti-
nent. By June more than 2000 infected animals had been
FAMILY CALICIVIRIDAE
detected. The epidemic was controlled by restricting the
movement of sheep, cattle, and swine, and culling of herds
in which FMDV was found. Almost 4 million animals were
The caliciviruses are nonenveloped viruses possessing
destroyed in this process. The damage to the British cattle
icosahedral symmetry and having a diameter of about 30 nm.
industry was particularly distressful because this epidemic
The name comes from the Latin word for cup or goblet (source
occurred only a few years after widespread culling of cattle
of the English word chalice) because there are cuplike depres-
to control an epidemic of a prion disease called "mad cow
sions in the surface of the virion when viewed in the electron
disease" (Chapter 9). Beginning in March, many rural areas
microscope. The characteristics of a number of caliciviruses
often visited by tourists were closed to prevent the spread
are shown in Table 3.7. Four genera of caliciviruses are cur-
of the virus, and the U.S. Department of Agriculture was
rently recognized, two of which, genus Norovirus (whose
especially vigilant in examining travelers returning from
members were formerly called the Norwalk-like viruses) and
affected countries. British authorities considered vaccinat-
genus Sapovirus (formerly called the Sapporo-like viruses),
ing cattle with commercial vaccines to control the epidemic,
contain agents that cause human gastroenteritis. The genera
but vaccines have not been used by the British to date.
Vesivirus and Lagovirus contain viruses of cats, rabbits, pigs,
Vaccination for FMDV is used in some parts of the world,
and sea lions. Additional caliciviruses of cattle, dogs, mink,
but is controversial because it is then difficult to distinguish
walrus, and chickens remain to be classified as to genus.
between vaccinated animals and infected animals. Thus, for
Most or all caliciviruses are host specific, infecting only a
example, the United States does not allow the importation
single animal species.
TABLE 3.7 Caliciviridae
Virus name
Usual
World
Genus/members
abbreviation
host(s)
Transmission
Disease
distribution
Vesivirus
Vesicular exanthema of swine
VESV
Swine
Oral, contact
Fever, lesions on
California
(includes San Miguel sea lion virus)
snout and feet
Feline calicivirus
FCV
Cats
Contact
Rhinitis, pneumonia,
Worldwide
fever
Rabbit vesivirus
Rabbits
Lagovirus
Rabbit hemorrhagic disease
RHDV
Rabbits
Water-borne,
Hemorrhages
China, Europe,
Australiaa
oralfecal
European brown hare syndrome
EBHSV
Hares
Europe
Norovirus
Norwalk
NV
Humans
Water-borne,
Epidemic
Worldwide
oralfecal
gastroenteritis
Sapovirus
Sapporo
SV
Humans
Water-borne,
Epidemic
Worldwide
oralfecal
gastroenteritis
Unassigned members of the family include caliciviruses of cattle, dogs, fowl, mink, and walrus
a
RHDV was inadvertently introduced into Australia.
with the picornavirus 2CATPase, 3Dpol, and 3Cpro. Unlike the
The human caliciviruses have been difficult to study
because it has not been possible to grow them in cell cul-
picornaviruses, however, the nonstructural proteins are
5′ terminal and the structural protein(s) 3′ terminal (Fig. 3.11).
ture or to infect experimental animals. As a consequence,
we know less about them than we do about such well-studied
Translation of the genomic RNA produces a polyprotein
viruses as the picornaviruses. The first calicivirus described
that contains the sequences of the nonstructural proteins.
Cleavage of the polyproteins is presumably effected by 3Cpro
was a virus of sea lions (San Miguel sea lion virus, genus
(also called 3Cpro-like or 3CLpro). In those caliciviruses that
Vesivirus), and most of what we know of the molecular biol-
ogy of caliciviruses comes from studies of this virus and of
have been studied in cell culture, a subgenomic RNA is pro-
other nonhuman viruses such as feline calicivirus, which
duced that is assumed to be an mRNA for the major capsid
will replicate in cultured cells. The complete sequences of
protein and, at least in one case, a minor capsid protein as
a number of the human viruses have now been obtained,
well, and it is assumed that all caliciviruses produce a sub-
which has greatly expanded our knowledge of these viruses,
genomic RNA in order to produce the structural protein(s).
but details of the molecular biology of their replication are
There is evidence that this subgenomic RNA, once pro-
still lacking.
duced, can replicate independently of the viral genomic RNA.
The caliciviruses are distant relatives of the picornavi-
The organization of the calicivirus genome differs slightly
ruses. Where known, the (+)RNA genome of about 8 kb has
among the genera as shown in Fig. 3.11. In the lagoviruses
a 5′-terminal VPg and a 3′-terminal poly(A), as do the picor-
and sapoviruses, the major capsid protein is in frame with
naviruses, and calicivirus proteins share sequence identity
the nonstructural proteins and contiguous with them. Thus,
Vesivirus (FCV)
ORF 1
ORF3
ORF 2
An
genome
Vpg
COAT
CP + 3 Terminal ORF
An
mRNA
Vpg
Lagovirus (RHDV)
ORF 1
An
genome
Vpg
ORF3
An
Vpg CP + 3 Terminal ORF
mRNA
Sapovirus
ORF 1
An
genome
?
COAT
ORF2
?
An
mRNA
Norovirus
ORF 1
An
ORF3
ORF 2
genome
?
COAT
?
An
mRNA
Motifs
Coding Domains
Polymerase (GDD)
Cysteine Protease
Coat protein
Vpg
Helicase
Nonstructural proteins
FIGURE 3.11 Diagrammatic representation of the genome organization of the four genera of Caliciviridae. Notice that
the coat protein is encoded in ORF2 of vesiviruses and noroviruses but as the C-terminal portion of ORF1 in lagoviruses
and sapoviruses. Subgenomic mRNAs have been identified for vesiviruses and lagoviruses. FCV, feline calicivirus; RHDV,
rabbit hemorrhagic disease virus. Adapted from Fauquet et al. (2005), p. 847.
it is possible that the nonstructural polyprotein contains the
system in which the viral RNA is introduced by transcription
sequences of the structural proteins as well, but is assumed
from cDNA clones.
that the major source of the structural proteins is transla-
Several isolates of the Norwalk virus have been studied,
tion of a subgenomic mRNA, although details of the viral
all of which share more than 50% sequence identity and
expression strategy are still lacking. In the noroviruses and
which are named after the location where they were first
vesiviruses, the major capsid protein is in a different reading
isolated. These include Norwalk virus, Hawaii virus, Snow
frame.
Mountain virus, Southampton virus, Lordsdale virus, and
Another, fairly short, ORF is found in the 3′-terminal
Desert Shield virus. These viruses are extraordinarily
region of each calicivirus genome. The translation product
infectious. In one epidemic investigated by the Centers for
of this ORF is a minor structural protein.
Disease Control and Prevention and local health authori-
Caliciviruses have been shown to interfere with host
ties, a baker preparing food for a wedding was ill with
protein synthesis. The mechanism used is the same as one
gastroenteritis. After using the toilet, he washed his hands
of the mechanisms used by poliovirus, cleavage of poly(A)-
thoroughly before handling food, but his hands were still
binding protein by 3Cpro. The norovirus protease cleaves this
contaminated with virus, perhaps under the fingernails. He
host protein at the same site as does the poliovirus 3Cpro,
used his hands to stir a very large pot of icing used to glaze
whereas the feline calicivirus protease cleaves it at a site 24
cakes and doughnuts that were distributed at the wedding
residues downstream of the poliovirus site.
reception, and managed to contaminate the icing with virus.
Every guest at the reception who ate as much as a single
doughnut contracted gastroenteritis.
Noroviruses
The Norwalk viruses regularly cause epidemics of gastro-
Norwalk virus is a human virus that causes gastroenteri-
enteritis. The incubation period is short (24 hours on average)
tis. As described, there is no cultured cell line in which to
and the course of disease is also short (12 days). Following
propagate the viruses, nor even an animal model in which
recovery, immunity is established to the virus, but the dura-
to grow it, and studies have relied on human volunteers for
tion of immunity appears to be fairly short (perhaps one or
virus propagation. This has severely limited the amount of
a few years). Studies of immunity are made difficult by the
information (and material) that can be obtained. However,
finding that some fraction of the human population seems to
because of the power of modern gene cloning technology, the
be resistant to any particular Norwalk virus studied, perhaps
entire genome of Norwalk has been cloned and sequenced,
because of a lack of receptors for the virus, and by the fact
starting from stools of experimentally infected human vol-
that there are so many viruses that cause gastroenteritis.
unteers. These genome sequences can be aligned with the
The Norwalk group of viruses is worldwide in its dis-
genomes of other caliciviruses (Fig. 3.11), and the mecha-
tribution but epidemiological studies of these viruses have
nisms of the expression of the genome and the functions of
been difficult in the absence of a cell culture system or an
the encoded proteins can be predicted from studies of the
experimental animal that can be infected by the virus. Food-
animal caliciviruses. Thus, for example, it is assumed that
borne infections are estimated to cause 76 million cases
the Norwalk viruses possess a 5′-terminal VPg as does San
of human illness in the United States each year, of which
Miguel sea lion virus, but to date it has not been possible
300,000 lead to hospitalization and 5000 are fatal. A signifi-
to prove this. However, the genomic sequence of Norwalk
cant fraction of these illnesses, perhaps one-third, are caused
virus contains a sequence related to the VPg of other cali-
by bacteria, for which ready tests are available for use by
civiruses, providing evidence that Norwalk virus RNA does
health authorities and these epidemics have received wide
in fact carry a 5′-terminal VPg. Clones of various regions of
attention. Until recently, however, little testing for Norwalk
the viral RNA can be expressed in cultured cells to study the
virus was performed. With the determination of the sequence
expression and function of various domains. For example,
of Norwalk RNA followed by the development of RTPCR
the capsid protein of Norwalk virus has been expressed in
techniques to rapidly screen for Norwalk virus, the CDC has
insect cells and these proteins spontaneously assemble into
now attempted to assess the importance of Norwalk virus in
T=3 virus-like particles of 38-nm diameter whose structure
food-borne outbreaks of gastroenteritis. Analysis of avail-
has been determined to atomic resolution by X-ray dif-
able data suggests that perhaps 50% of food-borne outbreaks
fraction techniques. Of interest is the recent discovery of a
in the United States are attributable to Norwalk virus, and
mouse norovirus. The virus causes only silent infections in
these viruses are therefore responsible for the majority of
immunocompetent mice but, importantly, it will replicate in
outbreaks of nonbacterial gastroenteritis in the United
cultured cells, and thus has the potential to serve as a sur-
States. Further, the Norwalk outbreaks are larger than those
rogate virus to work out details of the molecular biology of
associated with bacteria, a median of 25 cases per outbreak
replication of noroviruses. Another recent development is
versus 15 cases in bacterial outbreaks and 7 cases in out-
the finding that human noroviruses will replicate in a cell
breaks of unknown etiology. More than half of the Norwalk
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