ProMED: Laura D. Kramer PhD
IRAQ WMD "HYBRID MOSQUITOES".
Wed Mar 17 21:49:48 2004
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HYBRID MOSQUITOES AND WEST NILE VIRUS (02)
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Date: 16 Mar 2004
From: Laura D. Kramer kramer@wadsworth.org  [edited]


Epidemiological variables can affect the role of WN vectors
------------------------------------------------
The newly reported hybridization between _Cx. pipiens_ s.s and _Cx.
pipiens_ form _molestus_ may have played a role in the highly
successful establishment of WNV in New York City in 1999 and its
subsequent spread in the northeastern and north-central USA, where
this species is the predominant enzootic vector. But at least 43
other mosquito species have been found infected with the virus and
are competent vectors in the laboratory; and the largest outbreaks in
the USA have been observed in the western states where _Culex
tarsalis_is the predominant vector. [Calculation of] risk of
arbovirus infections in humans has to take into account vector
competence, vector density in relation to its hosts, biting
frequency, and longevity.

Certainly, the rapid spread of WNV in the USA, Canada, and Mexico and
the contrast between virus activity in the USA and Europe cannot be
attributed to the genetics of _Cx. pipiens_ complex mosquitoes alone.
One also must take ito account differences in circulating virus
strains, differences in historical exposure to the virus, and
differences in vertebrate host competency.

Further research is needed to determine whether there is a
correlation between the genetic, behavioral, and biological traits of
_Cx. pipiens_. Recent studies conducted by Kramer et al. in NYS
(unpublished data) have demonstrated spatial and temporal variation
in vector competence of _Cx. pipiens_. Additional studies are in
progress to determine whether these differences correlate with _Cx.
pipiens_ population genetics and other biological traits, since the
exact type of hybrid (as Fonseca et al. noted) present in an area
might be a cause of the observed differences. A dynamic distribution
of hybrids with accompanying variation in their vector competence and
feeding behavior would provide an excellent opportunity to examine
the genetic basis of these 2 traits and the epidemiological
consequences for WNV of the interaction between genetic and
environmental variables.

--
Laura D. Kramer, PhD
Associate Professor, School of Public Health SUNY Albany
Director, Arbovirus Laboratories
Wadsworth Center
New York State Dept Health
kramer@wadsworth.org

[We are most grateful to Dr Kramer for emphasizing the complexity of
the epidemiology of West Nile in the USA and drawing attention to the
many variables that can affect vector competence. This helps to
clarify the news report on hybrid mosquitoes and West Nile virus
found at:
http://www.knoxstudio.com/shns/?action=detail&pk=WESTNILE-03-04-04 >
and which can also be accessed from the archival reference cited
below - Mod.MS]

[see also:
Hybrid mosquitoes and West Nile virus 20040306.0647]
............ms/pg/lm

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APFN Messageboard 2004-03-06 Archive
... HYBRID MOSQUITOES AND WEST NILE VIRUS — PROMED, Sat Mar 6 15:06: ... Inquiry into Intelligence
on Iraq's WMD — RESEARCHER, Thu Mar 4 18:14: Find The Culprits For ...
http://www.apfn.net/messageboard/03-06-04/discussion.cgi.8.html

Date: 5 March 2004
From: ProMED-mail
Source: Scripps Howard News Service 03/04/2004 [edited]
http://www.knoxstudio.com/shns/?action=detail&pk=WESTNILE-03-04-04


_Culex pipiens_ and _Culex molestus_ hybrids in the USA
---------------------------------------------------
West Nile virus has set up housekeeping in the United States so readily
because of a hybrid mosquito species (_Culex pipiens_ s.s. X _Culex pipiens_
form _molestus_) that transmits the disease by biting both people and birds.

But the disease has not taken hold in northern Europe, where the infection
is limited mainly to bird-biters (_Cx. pipiens_), a new study suggests.

The first case of West Nile virus detected in the Western Hemisphere was in
the New York City area, in 1999. Since then, the virus has spread to infect
humans in 45 states, although 4 of 5 people who become infected have no
symptoms. But for the rest, the virus dishes out a flu-like illness that can
spread to the brain, with fatal effects.

The virus is not typically transmitted between humans, although the federal
Centers for Disease Control and Prevention has reported a single case of
mother-to-infant infection and a second transmission to a breastfed child.

Most of the time, "you must have a mosquito that bites a bird today and a
human tomorrow for transmission to occur," said Dina Fonseca, a genetic
researcher with the Smithsonian Institution's Museum of Natural History.
She is the lead author of the new study, published Friday in the journal
Science.

Humans and other mammals don't seem to carry enough of the virus in their
blood to pass it along to birds or other animals through mosquitoes, "so if
you have a mosquito that only bites mammals, there is no transmission to
humans," Fonseca said.

Fonseca and Richard Wilkinson, of the Walter Reed Army Research Institute in
Silver Spring, MD, collaborated with other scientists around the world to
study the genetic makeup of the _Cx. pipiens_ complex of mosquitoes, which
is strongly linked to the West Nile outbreak in the United States.

The scientists analyzed mosquito samples from 33 locations in various
countries, including Australia, Great Britain, Germany, Italy, Japan, Jordan
and the United States. The researchers found that there are 2 different
forms of the same species residing in northern Europe that tend to bite
either birds or humans but not both. The people-biting form lives
underground, in places such as subways and basements, while the form that
bites birds lives in the open.

Although the 2 groups live in the same areas and appear morphologically
identical, the analysis showed that the northern European _Culex pipiens_
s.s. mosquitoes are not swapping with the closely related _Culex pipiens_
form _molestus_ DNA in some key regions.

But among the US samples, 40 per cent of the _Culex_ mosquitoes were
hybrids. In southern Europe, where outbreaks of West Nile are somewhat more
common, about 10 per cent of the mosquitoes were hybrids.

Among _Culex_ mosquitoes in Japan, Australia, and the Middle East, the
distinction between above- and below-ground populations doesn't appear to
exist, the researchers said. The findings make sense to other scientists who
have tried to understand how _Culex pipiens_ that had long been considered
a bird-biter seemed to be a key factor in spreading the disease to humans in
many places.

"It appears that what we've been calling pipiens are hybrids," Larry Kramer,
Director of the insect virus lab at the New York state Department of Health,
told _Science_. "It fits with what we've been seeing."

But the CDC reports that at least 43 species of mosquitoes around the United
States are infected with West Nile, including several others known to bite
humans.

Many other public health experts think West Nile spread so rapidly across
the United States, and also much of Canada, because the virus is new to the
Americas and neither birds nor other animals, including humans, had any
immunity to it, while Europeans have been exposed to it for centuries.

But Fonseca said it's clear that the existence of so many infected hybrid
mosquitoes in the United States points to a different pattern of natural
hosts and a difference in infection rates. "It's not the only reason, but it
could be a component of this difference," Fonseca said.

On the Net: http://www.sciencemag.org  >
http://www.cdc.gov/ncidod/dvbid/westnile/site!26index.htm

[byline: Lee Bowman]

--
ProMED-mail
promed@promedmail.org

[West Nile virus is unusual in seeming to be transmitted by so many
different mosquito species; at least 43 species in 10 genera have been found
positive for WN virus in the USA since 1999. The virus can also infect
ornithophagic ticks (_Argas hermanni_ and _Hyalomma asiaticum_ ), but their
role in the epidemiology of WN remains unclear. Nevertheless it appears that
_Culex_ species, in particular _Cx. pipiens_, are the principal vectors in
the USA. Elsewhere, such as in Europe and Africa, _Culex_ mosquitoes are the
main vectors, including some species that provide a bridge by biting both
birds and humans.

The publication by Dina M Fonseca,_et al, Emerging vectors in the _Culex
pipiens_ complex (Science 2004; 303: 15535-8, on which the above news report
is based, will undoubtedly generate much interest amongst virologists, and
I would guess also considerable controversy as to whether hybridization in
the _Cx. pipiens_ complex explains, or helps explain, how WN has spread so
rapidly in the USA.

Another recent and relevant publication is by S Higgs, K Snow, and EA Gould,
"The potential for West Nile virus to establish outside of its natural
range: a consideration of potential mosquito vectors in the United Kingdom"
(Transactions of the Royal Society of Tropical Medicine and Hygiene 2004;
98; 82-7. - Mod.MS]

[see also:
West Nile virus, human death - USA (NC) 20040224.0583
West Nile virus, bird - USA (Texas) 20040212.0464
2003
---
West Nile virus update 2003 - Canada (05) 20030504.1115
West Nile virus update 2003 - Canada (03): correction 20030429.1060
West Nile virus update 2003 - Canada (02) 20030428.1046
2002
----
West Nile virus, humans - USA (VT, MT ex OH): prob. 20020911.5281
West Nile virus, birds - Mexico (Yucatan): suspected 20020828.5172
West Nile virus & weather
20020807.4984
West Nile virus update 2002 - USA (14) 20020801.4911
West Nile virus - USA 2001: final report 20020613.4491
West Nile virus, mosquitoes - Israel (02) 20020520.4276
West Nile virus review papers online 20020304.3683
2001
---
West Nile virus surveillance 2001 - USA (NYC) 20011213.3017
West Nile virus, mosquitoes - USA: update 20010925.2339
West Nile virus, mosquitoes - USA (Massachusetts) 20010831.2068
West Nile virus surveillance 2001 - USA (18) 20010825.2017
West Nile virus surveillance 2001 - USA (17) 20010817.1946
West Nile virus surveillance 2001 - USA (16) 20010810.1890
West Nile virus surveillance 2001 - USA (15) 20010804.1532
West Nile virus - Romania (Bucharest) 20010731.1499]

............ms/pg/sh
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