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April 27, 2004

Predicting West Nile

According to a new study, the number of dead crows in a particular area serves as a fairly accurate predictor of the number of human West Nile cases that will occur two weeks later.

On the face if it that seems like an fairly banal observation--the more dead birds, the more sick humans. But since crow mortality peaks around twoo weeks before the huiman population is affected, municipalities have not only a heads up on when they need to start seasonal mosquito control programs, but where to concentrate them.

Even more interesting is this seemingly throwaway sentence.

Whether a similar spatial association between early-season crow deaths and residences of WNV-infected case-patients will be evident in future seasons is unknown, as an estimated 81% of the Chicago-area crow population is thought to have died in 2002

If one assumes that the crow mortality rate observed in Chicago is the same outside of Chicago, there should be many fewer West Nile cases in Illinois in 2003 than there were in 2002--as indeed there were. In 2002, when the crow corpses in the study above were counted, there were 884 cases of West Nile in Illinois. In 2003, after the crow population crashed, presumably, there were 54.

Might there be even fewer this year? Though other avian species are known to carry West Nile, it historically has affected the crow population more severely, possibly because smaller birds are more likely to regard mosquitos as a potential meal and devour them before they have a chance to pass on the virus. As well, no other population has, as of yet, shown the high correlation between individual deaths and human infection rates that the corvid population has.

Crow numbers will eventually recover. No disease ever kills 100% of a population, and the survivors of the yearly West Nile outbreaks will be increasingly immune to the virus--shutting down, if not the most prevalent, at least one of the the most obvious vectors of virus transmission to humans.

Alternatively, it's possible that crows could become hosts for the disease, but the high mortality rate in the species would argue against it. It's far more likely that another species would fill that role.

Given time, possibly very little time indeed, West Nile activity in the U.S. may come to resemble that of its close cousin, St. Louis Encephalitis, which normally pops up less than 150 times a year.

Which should then raise an obvious question in many minds-- Was there a similar population crash in the avian species that host St. Louis Encephalitis in 1933*, when that disease first established itself? It apparently does not cause disease in avians now, but American populations have had 70 years to adapt to the virus since it first appeared.

*Oddly enough, 1933 was also the same year that another cousin of West Nile, Eastern Equine Encephalitis, was first isolated in the United States. Just five years earlier, the era of transatlantic flight had been inaugurated by the Graf Zeppelin.


Posted by Bigwig at April 27, 2004 04:22 PM | TrackBack
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You’re probably right; West Nile cases are usually high the first year, and not so bad the next (I say this will all the wieght of two years of history, so it’s not exactly rock solid). Hopefully the virulent strain that appeared in 2002 will, in fact, go the way of itty-bitty, minor diseases nobody worries about.

Posted by: Daniel Morris at April 28, 2004 02:29 PM
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