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Thursday, 23 August 2012

West Nile virus - is it time you rolled up your sleeve to donate?

CDC is reporting a surge in WNv cases this year, the second highest year on record CDC surveillance sites.  Most of the cases to date have been just west of the Mississippi, which for followers of WNv is a major dividing point for the illness on the continent. In Canada that dividing point is the start of the prairies, however interesting the dominate disease activity has been east of the Manitoba:Ontario border.  Canada’s somewhat dated surveillance system can be found at Canada weekly reports  which operates up to two weeks delayed. 

Springtime tends to be bring thaws, rains, marshy lands and mosquitoes.   If we have been successful in communicating anything in the past few years in North America, it is that mosquitoes are very capable to carrying diseases, and the story of West Nile Fever is a fascinating one in of itself.  Just in its 13th year since likely landing in North America via transatlantic flight, it is spread almost continent wide and caused significant devastation, sparing only Newfoundland, Alaska and the Territories.

Wikipedia offers a classic medical textbook description of the illness although the entry is dated.  Wikipedia.

Missing from many of the public health sites and material written on WNv are some key points.  How many of the following dozen points were you aware of?  (leave a comment about your knowledge level as an informed reader)

·         The neuroinvasive illness results in over 50% of cases with persistent neurological deficits after one year, many of which have yet to resolve, many of which are quite debilitating.  

·         While the classical description of the febrile illness is one of a self-limiting illness, closer inspection of the cases finds a significant minority with persistent symptoms such as fatigue and ‘minor’ neurological deficits which last weeks, a year or longer. 

·         The natural reservoir of West Nile virus are healthy birds.  Only a few species (crows being the most notable) suffer illness and succumb.  The seemingly healthy birds may transport the virus very long distances.

·         Those mosquitoes that swarm us in the wet season are predominately Aedes.  The lone flyer at dawn and dusk is likely Culex and the one more likely to be carrying WNv

·         Mosquitoes only travel a few hundred meters in their lifetime – they are not the natural source of the virus, just the executor of the final blow.

·         It takes prolonged heat to encourage sufficient generations within a year (usually four) of mosquitoes that they bite a less preferred food - human blood.  

·         Bird deaths and mosquito pool identification tend to precede human or equine illness during a summer – it is a rare situation that humans or horses get ill without evidence of the virus in the area (but it does happen!!)

·         Usually human transmission is likely to occur in Canada in mid to late August and cases identified late August through mid-September.   These tend to be times where little about personal protection is communicated.

·         Late season mosquitoes go into a phase called diapause which is sometime in early September and driven by light hours in a day.  Once in diapause they do not feed further, but prepare to try to overwinter in a state of hibernation.  The odd hibernating mosquitoe with WNv may survive to feed on a human in the spring and transmit the illness.

·         Even during outbreaks, only a very small proportion of mosquitoes are positive for WNv (~1%)

·         Where human outbreaks have occurred, the vast majority of the population (85-98%) remain susceptible, suggesting that many recurrent outbreaks may be needed to develop a level of protection commiserate with the natural distribution areas for WNv in Europe. 

The US media are just picking up and carrying messages on WNv risk this year.  Expect the anxiety to spill over the border, just as the peak for human transmission in Canada is likely to occur. 

On the issue of blood feeds, do support the Canadian Blood Services through regular blood donations.  The voluntary nature of the Canadian blood system means we all need to do just that - roll up our sleeves for our neighbours.  

Canada update as of August 22

Quebec: 5 human infections, 1 positive horse, 8 positive mosquito pools and 2 positive birds have been reported.

Ontario: 49 human cases; 4 travel related, 312 positive mosquito pools and 9 positive birds have been reported.

Manitoba: 5 human cases and 98 positive mosquito pools have been reported.

Saskatchewan: 1 positive horse and 4 positive mosquito pools have been reported

US update to August 22
1118 human cases of illness has been reported, 629 (56.3%) have been reported as neuroinvasive and 41 deaths have been reported.  


  1. Thanks Dr. P. I wasn't aware of most of these tidbits. How far they travel (not very!), early spring/late summer being the prime infectious times, and dawn/dusk warriors particularly were interesting to me. I clean my eaves 2-3 times a year, and my bird bath is purely ornamental (I dump out the water after a rain) so I guess that helps explain why my yard remains a pleasant non-biting place :)

    Glad to hear all those ankle/neck bites I got in the Yukon/Alaska this summer were likely safe!

  2. The only one I knew is that the natural reservoir is healthy birds, but I was uniformed about WNv until reading this. What can a person do to limit risk or is it just an "unluck" of the draw kind of deal?

    1. In reply to the question what can one do, being informed is the first step. Avoiding mosquito bites the second. PHAC as one example produces advice that can be found at http://www.phac-aspc.gc.ca/wn-no/protect-proteger-eng.php . The third strategy is in reducing breeding areas around where you live and play as mosquitoes do tend to stay close to "home" where they are hatched. DrP

  3. Isn't being informed just knowing to use bug spray, wearing long clothes and busting up breeding grounds? I don't see how knowing the facts about when and where WNv is most likely to occur does anything to help. It seems like there is only two steps to me.

    1. Ah - if only most people were like you. Generally adherence to protective measures falls off as the highest risk season approaches.