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The answer to the very good question below is that you are part right and
part wrong. An organisms ability to 'spread' a virus is dependant
upon numrerous factors. When looking at this you need to differentiate
the difference between the ability to be infeted +/- becoming clinically
ill and the ability to amplify the virus during the viremic (infectious)
period and then spread it. Dead end hosts usually mean that one can
be infected and even get sick and die but do not produce enough of the
virus (for long enough) to infect something else. Thus, while humans
can be infected and get sick, they are not very good amplifiers (reproducers)
of the virus and so the likelihood of spreading it over long global distances
is low. However Im not sure the level of viremia and the length of
the viremic period in people have been reliably established yet so there
is some doubt in my mind.
<p>Corvids are very good reservoirs (transmitters) because they shed (release)
alot of virus for a long enough period of time to come into contact with
and infect mosquito vectors, which in turn infect others etc.
<br>
<p>Dominic Travis DVM, MS
<br>Veterinary Epidemiologist
<br>Lincoln Park Zoo
<br>Chicago, IL 60614
<br>(P) 312-742-7225
<br>(F) 312-742-7823
<br>
<br>
<p>Ellen Paul wrote:
<blockquote TYPE=CITE>In response to the comment that humans are not likely
to be WNV vectors:
<p>I'm equally baffled by the response.
<p>I'm not a medical doctor (and I don't even play one on TV), so perhaps
I just don't get it. But couldn't ANY host serve to move the virus? If
I'm infected and carrying active virus, and I get on a plane and go to
Argentina, or any place, for that matter, and I get off the plane and a
mosquito bites me, wouldn't that be as effective a source of transmission
as an infected bird flying to Argentina?
<p>What am I missing?
<p>I'm not clear how the fact that humans and other mammals are incidental
hosts makes any difference. There may be fewer infected humans than infected
birds, but since most humans have, at most, a mild illness, it would
seem that we'd be more effective vectors. We'd be more likely to be able
to travel long distances while infected than would birds.
<p>What is the known range of incubation times in birds? It is my understanding
that WNV has been studied in the lab in only a handful of species of wild
birds - crows, mallards, Sandhill Cranes (the latter a vaccine study).
We would need to know more about the range of incubation times and the
range of the duration of migration (it varies not only with distance, but
with species, with weather patterns) to be able to estimate the probability
that infected, migrating birds could be vectors to a given place.
<p>Cleveland Parks Metrozoo has a statement (no research cited) that "<font face="Arial, Helvetica, sans-serif">WNV
is sustained in a bird's blood stream for 1 to 4 days after exposure. After
this incubation period, birds develop immunity against WNV.</font>"
<p>Scanning SIS and CRIS very quickly , I'm not coming up with anything
on incubation times or other species.
<p>So for instance, a bird that has arrived in Florida, replenished its
body fat for a couple of days, then gets bitten by an infected mosquito
in Florida, and, with favorable wind conditions and body conditions, departs
that night for a Caribbean island, would be more likely to make it than
a bird bitten in Maine that will fly from point to point along the coast
or ridges of the U.S., stopping for one or more days at each stopover location
to eat and wait for favorable conditions, and then continue in this manner
across the gulf or through Mexico, to points in South America ranging as
far as Argentina.
<br>
<p>Ellen
<pre class="moz-signature" cols="$mailwrapcol">--
Ellen Paul
Executive Director
The Ornithological Council
<a href="Mailto:epaul@concentric.net" class="moz-txt-link-freetext">Mailto:epaul@concentric.net
</a>Ornithological Council Website: <a href="http://www.nmnh.si.edu/BIRDNET" class="moz-txt-link-freetext">http://www.nmnh.si.edu/BIRDNET
</a>"Providing Scientific Information about Birds"</pre>
<p>
<br>
<p>Environmental Risk Analysis Program wrote:
<blockquote type="cite"
cite="midp0520000dba4a1c2fab6c@%5B132.236.139.173%5D">A
second batch of compiled responses to the question posted yesterday (re:
possibility of connection between bird die off in Argentina & West
Nile Virus):
<p>(4)
<br>From: Ashley Lovell <a href="mailto:Ashley.R.Lovell@aphis.usda.gov" class="moz-txt-link-rfc2396E"><Ashley.R.Lovell@aphis.usda.gov></a>
<br>Date: Tue, 14 Jan 2003
<p>I find it surprising that a previous ornithologist* made the comment
that WNV might arrive in Argentina (or anywhere for that matter) via an
infected human. It is quite unlikely that this would occur since
humans, like horses (for the most part), are incidental hosts. Yet
I see this statement made in information publications, by scientists, and
mentioned in the popular media quite often, and it baffles me. In addition,
it is plausible (and in fact believed by many) that a bird infected with
WNV would and could migrate through its normal migration range. Several
species of birds that might become infected with WNV are capable of migrating
because incubation times vary from species to species, and viremia levels
vary from bird to bird.
<p>Ashley Lovell
<br>USDA/APHIS - Wildlife Services
<br>1445 Federal Drive, Room 222
<br>Montgomery, AL 36107
<p>[*WestNileVirus-L moderator's note: this is a reference to comments
made by Ellen Paul <a href="mailto:epaul@concentric.net" class="moz-txt-link-rfc2396E"><epaul@concentric.net></a>
#(3) in the previous posting]
<br>
<p>(5)
<br>From: Timothy Male <a href="mailto:tmale@environmentaldefense.org" class="moz-txt-link-rfc2396E"><tmale@environmentaldefense.org></a>
<br>Date: Tue, 14 Jan 2003
<p>Concerning the Argentine bird deaths, as the authors below note, it
seems unlikely that any of the species mentioned are migrants directly
involved in spreading the virus from North America to South America.
However, this overlooks the more likely scenario in which migrants spread
the virus to South America and then the virus appears, potentially, in
both resident and migratory bird faunas.
<p>That said, it seems likely that this is not a set of bird species likely
to be indicative of a west nile outbreak (pigeons, for instance, are not
known for their wnv susceptibility nor are cardinals (Red-crested Cardinal
and Yellow-billed Cardinal are, I believe, non-migratory species present
in Argentina)) and seems more likely to be due to pesticide exposure.
If bird mortalities continue, then perhaps there there would be greater
concern over wnv as the causitive agent?
<p>Timothy Male, PhD.
<p>Wildlife Scientist
<br>Environmental Defense
<br>1875 Connecticut Avenue, NW
<br>Washington, DC 20009
<p>(202) 387-3500
<br>(202) 234-6049 fax
<br><a href="mailto:tmale@environmentaldefense.org" class="moz-txt-link-abbreviated">tmale@environmentaldefense.org</a></blockquote>
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</blockquote>
<p>--
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Received on Wed Jan 15 16:20:21 2003
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