24 June 2011

Unhealthful News 175 - You cannot avoid mosquitoes

Pretty insightful title, huh?

Those of us who write about public health with a libertarian bent (or what I prefer to call a humanitarian bent – we treat people as people, rather than mere biological processes, and care about what they care about) often point out that the behavioral preventive measures that the "health promotion" types favor are often not worth the costs.  But sometimes they are not even preventive.

I was just noticing the usual annual flurry of news stories about mosquitos and west nile virus (no link – I am not basing this on any particular story or claim).  Many of them offer suggestions about avoiding being exposed (lots of repellant, staying indoors at certain times of day), usually in conjunction with a warning about WNV being detected in mosquitoes in the local media market.  Some stories are about government efforts to eradicate the exposure vector with over-the-top chemical attacks on the local mosquitoes.  The problem is that it is probably impossible to avoid exposure to WNV.  Maybe I am thinking about this because I was just in northern Alberta feeding approximately 500 mosquitoes per day.  No WNV there, but if there was, I would definitely be exposed.  And during my time being outdoorsy in Texas, Minnesota, and other places, I have no doubt I have been exposed.  And so have most of you who have lived in places where it is endemic. 

At least that is my guess.  The last time I followed the science on this topic was about five years ago, and at that time no one had a good idea of what portion of the population in areas with WNV had antibodies (which proves exposure, though people without detectable antibodies may have been exposed too).  Maybe knowledge has improved since then, but I have not heard anything.  Anyway, the point is that it is a better assumption that everyone in areas with lots of WNV mosquitoes is eventually exposed than to make the usual assumption implicit in the news reports (and the public policy).

What is that assumption?  That only those people with diagnosed cases of west nile disease have been exposed.  This is absurd, but it is remarkably common among those people who are supposedly expert on infectious disease.  Think about numbers you hear, like 75% mortality rates for some exotic new disease.  What that means is that if you look at only the people who show up at a hospital because they are in critical distress from SARS, bird flu, etc., lots of them die.  Not exactly the same as what is being claimed, since thousands or millions may have gotten the infection but showed little or no symptoms.

So the precautions against WNV assume that most people who have never suffered a disease case from the virus must have not been exposed, so they should protect themselves (or the government should protect them) from exposure.  But, again, there is absolutely no basis for this assumption.  Frankly, it is kind of crazy.  It is pretty clear that, among people in endemic areas, what keeps someone from becoming a disease case is much more likely to be genetics or other personal characteristics, not the absence of exposure to the virus.

This is a case where you might choose to let the press off the hook (though it would be good if they were better at what they do) because they are being fed a steady stream of nonsense from the supposed experts.  You can avoid getting HIV by practicing safe behavior.  But if a substantial fraction of mosquitoes where you live carry WNV, it will find you (unless you can get them all to use condoms).  And as long as the experts keep up the fiction that avoiding exposure is the best way to prevent disease, we are unlikely to find a solution for the majority of people who are not willing to seal themselves up from bugs for months every year.  This naive fixation is annoying and absurd, and yet strangely true to form for official "public health" people.

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