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
Received on Tue Dec 10 13:37:44 2002