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Date: Dec 12, 2002
Posted by: Dominick.Ninivaggi@co.suffolk.ny.us

Excellent points. Until we know more about how likely various species are to actually transmit to humans, it's hard to make the best possible decisions regarding control. That will require knowing for sure whether the various species really do become infected in the field, especially species like Ae. vexans that generally prefer mammals. Blood meal studies combined with competance studies will help that a lot. You are right to be suspicious of positive vexans from the same trap as positive Culex. As for Cq. perturbans, I thought that it was capable of transmitting WNV, although not very efficiently. That's a species of concern for us, since we have an area where we have had WNV repeatedly (including a human case) and where perturbans is common and has been found WNV positive. Even worse, it's a half mile from my home! It would be a very good thing if I could be assured that the perturbans that get into my house are not a problem, but for now, I have to assume the worst.

-----Original Message-----
From: C. Back [mailto:christian.back@gdg.ca]
Sent: Thursday, December 12, 2002 11:09 AM
To: WESTNILEVIRUS-L@cornell.edu
Subject: RE: Vectorial role of mosquito species for West Nile virus -
cross-contamination of pools

Your points are well taken. I fully agree that, in the course of surveillance and operations, the presence of a WN-positive pool of Culex and/or other species is a clear indication of active transmission and a justification for mosquito control initiatives.

We use mosquito surveillance for two different purposes: 1) to detect or confirm the circulation of WN virus in a given area and to define the extent and intensity of the transmission focus. In this case we use mosquitoes as bird blood samplers and virus amplifiers. 2) to understand the key factors and key players in WN virus transmission, in order to identify the most efficient targets of preventive or curative control.

The purpose of my question was to better assess the respective vectorial roles of the various species of mosquitoes for West Nile virus, in the context of strategic planning and outside of the operational season. One reason for doing this is to establish priority criteria for the collection and analysis of pools of the various species. Another reason is to allocate properly resources for preventive mosquito control between Culex pipiens/restuans and the other species.

The published compilations of the number of positive pools per species are used to assess these roles. For that matter, you will agree that we could find contaminated positive pools of moths in CDC light trap samples including heavily infected Culex, and that no one would conclude that moths are vectors of WNV. Perhaps the high frequency of Aedes vexans positive pools is partly due to the fact that it is a late season mosquito frequently associated in CDC light trap samples with Culex pipiens.

Checking the published lists of positive pools per species could help reconcile disparities between the conclusions of competence experiments and of field observations. One major information from the competence studies is whether a given species has a transmission barrier at the salivary gland level, as has been suggested for Coquillettidia perturbans.

Christian Back 
GDG Environnement

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