[TN-Bird] West Nile Virus and Birds

Since there has been some interest here in the link between West Nile Virus
and birds, I am forwarding the summary of a very interesting article
recently published in the Public Library of Science-Biology, a peer-reviewed
free access science journal on the web.  The full article is at
<http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/
journal.pbio.0040082>
http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/j
ournal.pbio.0040082.  Birds are the main host for this virus, and Culex
mosquitoes (which are generally "container" breeders and not open water or
marshland breeders) transmit it from bird to bird and from bird to mammal,
including humans.  There is very little, if any, transmission by mosquitoes
from mammal to mammal or mammal to bird.  Robins are, as the epidemiologists
say, a highly competent host and the authors present evidence that a shift
in mosquito feeding behavior during the late summer, due in part to changing
robin behavior, may account for much of the late summer increase in human
WNV infections.
 

Chuck Nicholson

Norris, TN


 


A New Model for Predicting Outbreaks of West Nile Virus


Liza Gross

DOI: 10.1371/journal.pbio.0040101

Published: February 28, 2006

Copyright: C 2006 Public Library of Science. This is an open-access article
distributed under the terms of the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.

Citation: Gross L (2006) A New Model for Predicting Outbreaks of West Nile
Virus. PLoS Biol 4(4): e101

  _____  

Infectious diseases were wreaking widespread havoc long before scientists
had any idea what caused them. But knowing the pathogenic agents behind
today's scourges is just the first step in protecting against deadly
outbreaks. Roughly 75% of emerging infectious diseases are zoonotic-humans
contract them either directly from infected animals or through vectors that
feed on infected animals. West Nile virus is the biggest threat in North
America, where Culex mosquitoes are the primary vector. Birds are their main
target, but mosquitoes also transmit the virus to humans, horses, and other
mammals.

Since the virus was first discovered in New York City in 1999, it has
infected 20,000 people and killed 770-in stark contrast to the sporadic
infections in Europe. The factors behind the North American epidemics are
poorly understood, though proposed explanations involve a more virulent
strain, North American birds' ineffectual immune response, and a hybrid
species of mosquito that prefers humans over birds. In a new study, A. Marm
Kilpatrick, Peter Daszak, and their colleagues now present evidence that a
shift in Culex pipiens mosquito feeding behavior from birds to mammals is
also driving the epidemics.

A critical factor in predicting the intensity of a zoonotic epidemic
involves determining how the vector's feeding behavior and preferences
change over space and time. Birds appear to be West Nile's most competent
vertebrate host-they transmit the virus to other mosquitoes, which supports
viral reproduction-while humans (and most other mammals) can't transmit the
virus. The researchers hypothesized that if mosquitoes bit mostly birds in
the summer, then switched to humans in the fall, this behavior could
intensify both the summer epidemic in mosquitoes and the subsequent
transmission to humans.

To investigate this possibility, Kilpatrick et al. collected data from six
sites in Maryland and Washington, D. C., from May through September 2004, to
determine the population dynamics of birds and mosquitoes, which taxa Culex
was targeting, and the epidemiology of the virus. They estimated population
densities for mosquitoes and birds at each site, and identified the
morphologically cryptic mosquitoes by sequencing their DNA. Over 90% of
their catches were Cx. pipiens, which were tested for the virus. The
researchers determined species of avian and mammalian targets by sequencing
the DNA from blood in engorged mosquitoes.

From May to June, the American robin, which represents just 4.5% of the
local avian species, accounted for over half of Cx. pipiens' meals. As the
summer wore on, and robins left their breeding grounds, the probability that
humans would provide the blood meal increased 7-fold, while the probability
that Cx. pipiens would feed on robins declined. Since the birth of new
offspring raised the overall numbers of birds during this same period, 

With the data collected from the Washington, D. C., area, the researchers
modeled the risk of Cx. pipiens-mediated viral transmission to humans based
on Culex mosquito abundance, the prevalence of Culex infection, and the
probability that mosquitoes would feed on humans. The model predicted that
the risk of human infection peaked in late July to mid-August, declined
toward the end of August, then rose slightly at the end of September. The
pattern of actual human cases in the area, the authors point out, "showed a
strikingly similar pattern." The model also suggests that the human
incidence of West Nile virus would have been much lower if mosquitoes had
maintained their June feeding rate throughout the season.

The same pattern was seen in California and Colorado, with a peak abundance
of infected Cx. tarsalis mosquitoes in June and July, followed by a
late-summer spike in human infections. Since mosquitoes feed primarily on
birds during early summer, viral load can increase substantially. When
mosquitoes switch to humans, the prevalence of infection among mosquitoes
increases the chances of a human epidemic. If mosquitoes had fed mostly on
humans-wasted meals from the perspective of viral amplification-instead of
birds during early summer, prevalence of infection in mosquitoes and then
humans would have been greatly reduced.

These feeding shifts appear to be a "continent-wide phenomenon," the
researchers conclude, and may also explain outbreaks of other avian zoonotic
viruses. This study highlights the importance of understanding how vector
behavior affects transmission of zoonotic pathogens to humans-a crucial step
in developing strategies to prevent and control a potential epidemic.



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