Monday, 19 August 2013
West Nile Virus - a reminder of the annual threat
West Nile Virus season is hitting full speed now. Mid-August is the time the mosquitoes may be into their fourth generation and have had sufficient previous feeds to have potentially contracted WNv from its natural host in birds and now competing for a blood feed, some species get less picky on their preferred blood source and may accept the less desirable blood of a human. Moreover, early sunsets mean that dusk activity for humans increases, and that is the preferred feeding time for vector species of mosquitoes.
If you look at most messaging however, you would think that the greatest risk for West Nile comes with the hoards of spring mosquitoes (mostly Aedes sp. and Coquilletidia perturbans) and by now the messaging has petered out, when the risk of transmission is at its greatest.
Moreover, while not exclusive to Culex sp. , Culex is the mosquito most likely to feed on birds and mammals – and our western provinces are at greater risk because of a particular species C. Tarsalis which is even less finicky than the more common eastern variety C. Pipiens
Those that are interested in more on West Nile Virus and appropriate messages are referred to DrPHealth posting from August 2012.
Surveillance for West Nile ramps up considerably after the initial summer anxiety in the media. Kudos to PHAC for actually trying to message appropriately at this time of the year. However, data reporting in the busiest time period is still biweekly, and often weeks out of date. August 3rd reporting only identified two Canadian human cases, with positive mosquitos from Quebec through Saskatchewan. Alberta surveillance is far from up to date and must be suspect. BC reported a positive mosquito pool in an August 15th update. The BC report alludes to five Canadian human cases, 97 mosquito pool positives and positive birds from Ontario through to Saskatchewan.
The US is reporting nearly 200 human cases predominately in states other than the Eastern Seaboard.
While it may overall be a cooler and wetter year for most of Canada, conditions that do not favour Culex species, it is a healthful reminder that now is the time to be most cautious on avoiding mosquito bites.