Biology Reference
In-Depth Information
Discovery of this form of influenza led public health authorities to order
the killing of more than 1 million chickens, perhaps preventing a human
pandemic.
The flaviviruses, which include dengue hemorrhagic fever virus,West
Nile virus, yellow fever viruses, and several encephalitis viruses, are a
highly variable group of about 68 viruses. Many are transmitted by insect
vectors.
The virus causing dengue hemorrhagic fever appears to be a relatively
recent introduction to human hosts (Twiddy et al. 2003).The dengue fla-
vivirus affects about 50 million people annually in tropical regions, mak-
ing it the most frequent vector-borne viral disease.The virus has a reser-
voir in wild primates, and some cases result from transmission of the virus
from these primates to humans by
Aedes
mosquitoes native to tropical for-
est habitats. More frequently, the virus is transmitted from human to
human by
Aedes aegypti
,a mosquito that has spread worldwide and typi-
cally lives in close association with humans.
Four major strains of the dengue hemorrhagic fever virus exist, desig-
nated DEN-1 to DEN-4. These viruses have recently spread widely in
tropical and subtropical regions, and the individual strains have become
more variable genetically. All four strains of the dengue virus occur in
Asia and the New World, but only one has been recorded in Africa (Fail-
loux et al. 2002). Significant evolutionary changes have influenced recent
patterns of human disease.The particular variety responsible for explosive
outbreaks of dengue in Cuba, Venezuela, and Brazil between 1981 and
1990, for example, appears to have been a newly evolved variant of
the DEN-2 virus (Monath 1995). As we noted in chapter 10, strains
of the dengue virus show differential association with different forms of the
mosquito
Aedes aegypti
.
Molecular genetic studies have now revealed much information about
the origin of the dengue virus and its major human disease strains
(Twiddy et al. 2003). Based on a molecular clock relationship, these stud-
ies suggest that the basic dengue virus originated about 1,115 yr ago. For
three of the four strains for which combined human and primate samples
were available, the estimated age of divergence ranged from 125 to 320
yr. This would also represent the earliest date for transmission from wild
primates to humans. Estimated divergence dates for human-only strains
were more recent, ranging from 35 to 215 yr. These dates are considered
to be the latest dates for introduction to human hosts from wild primate
reservoirs and the approximate dates of epidemic spread within the
human population. Thus, the genetic diversification of the dengue virus
