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Date: Dec 10, 2002
Posted by: Dominick Ninivaggi (Dominick.Ninivaggi@co.suffolk.ny.us)

This sounds like something for CDC to round up, perhaps by querying the labs that do the work (there are relatively few of them). However, knowing for sure human biters are infected is a lagging indicator of risk to people. If we find positive Culex (presumptively bird-biters) in the presence of substantial numbers of human-biters that are capable of transmission ("bridge vectors"), our concern level goes up substantially, even if none of the "bridge" species turn up positive in our samples. Infection rates in bridge vectors tend to be low, so the fact you've not detected it doesn't mean spillover to them hasn't happened, unless your sample size is quite large. In addition, even if spillover to other vectors has not yet occurred, all the ingredients are there for it to happen when you find them with infected Culex. If you wait for proof positive that bridge vectors are infected before intervening, you could be too late. Plus, there is a time lag between sampling and getting results. In our case, that's 10 days or more. I would not want infected, aggressive mosquitoes flying around for 10 days while I was waiting to confirm they are infected. I think it's better to bring down the numbers of human biters in an infected area down immediately, rather than wait til they turn positive. By the time you know that, you may have lost the opportunity to prevent transmission to humans. This is a tough call to make, because we have to accept the limitations of what our sampling can tell us. In particular, while positive results indicate virus is present, negative results do not mean it is absent. The possibility that the bridge vectors could be contaminated by Culex is a serious issue. That possibility, should not cause one to conclude that control is not necessary when contamination can't be ruled out. Just finding positive Culex in the presence of other vectors is cause for concern. Being sure bridge vectors are positive just raises that concern to the highest level.

-----Original Message-----
From: C. Back [mailto:christian.back@gdg.ca]
Sent: Monday, December 09, 2002 11:07 AM
To: WESTNILEVIRUS-L@cornell.edu
Subject: Vectorial role of mosquito species for West Nile virus -
cross-contamination of pools

Vectorial role of mosquito species for West Nile virus - cross-contamination of pools

West Nile virus has hit Quebec in 2002, with the consequence of dead birds, human cases and infected mosquito pools. Efforts are under way to prepare for 2003, but many questions regarding the optimal strategies must be answered.

One of the key issues is which mosquito species to target, whether the aim of control is to prevent amplification of West Nile virus in the bird reservoir or to prevent bridge vectors from infecting people and domestic animals.

There are currently two main sources of information to assess the vectorial role of mosquito species: competence studies in the lab, and analysis of field-collected mosquitoes. Competence studies are certainly useful but have limited predictive value for real-life situations. Recent breakthroughs in identifying blood meal sources of field-collected mosquitoes are providing additional insights (http://esa.edoc.com/medical/v39n5/v39n5p777.pdf), but field-collected mosquitoes from surveillance programs have provided the core of information about the vectorial role of North American mosquito species for West Nile virus.

In 2002, the Quebec mosquito surveillance program resulted in 46 mosquito pools testing positive by PCR, out of a total of 3,210 pools representing 33,000 mosquitoes. The detection of West Nile virus was performed by the Zoonotic Diseases and Special Pathogens section, National Microbiology Laboratory (Health Canada, Winnipeg) using TaqMan RT_PCR. The vast majority of positive pools (40) were Culex pipiens/restuans. The remaining 6 positive pools consisted of Coquillettidia perturbans (1), Ochlerotatus canadensis (1), Aedes vexans (2) and Anopheles spp (2). The positive pools of Aedes vexans and Anopheles sp. (mostly punctipennis) all came from CDC light trap samples in which pools of Culex pipiens-restuans were also positive. In at least one case, the pools of Aedes vexans and Anopheles spp were weakly positive, and the PCR had to be pushed to confirm the presence of West Nile virus. It was concluded that, at least in this case, the pools of Aedes vexans and Anopheles sp. were probably contaminated by body parts of infected Culex pipiens/restuans from the same sample. The risk of contamination is inherent to the capture and sample processing techniques and cannot be eliminated. One leg of a heavily infected Culex could contaminate a sample to a level detectable by PCR. The probability of detecting it with the VecTest would be lower.

Therefore my question is: In the published lists of positive mosquito pools of species other than Culex pipiens/restuans, how many came from samples where there were also infected Culex pipiens/restuans?

My guess is that if authors of these published lists could check the original data, we would end up with a stronger prominence of Culex pipiens/restuans in positive pools. This information would be significant for the elaboration of West Nile prevention & control strategies.

Christian Back

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