Mosquito-borne disease in droughts and hurricanes

Bird flu isn't the only virus dangerous to humans that finds its primary home in birds. West Nile Virus (WNV) and other arbovirus infections do, too. WNV is now on the rise in California and seems worse than last year:

There were 18 new human cases of West Nile virus reported this week by the California Department of Public Health, double the number counted since the first case of the year was confirmed on June 20. That brings the state total to 27, and puts the California count slightly ahead of 2005, when the virus sickened 935 and killed 19. Last year, there were only 292 cases statewide and seven deaths.

"It is difficult to predict West Nile virus activity, but right now we are on a par with 2005,'' said Vicki Kramer, chief of the vector borne disease section of the state health department. (San Franciso Chronicle)

This is a surprise to some because there has been little rainfall and the snowpack levels in May were less than a third of normal (the lowest since 1988). The mosquito vector for WNV breeds in standing water. So what's going on?

One plausible explanation is that the constricted water pools is crowding birds into a smaller area where they can be more easily bitten by mosquitos. Humans are incidental and dead end hosts for WNV, which usually just cycles back and forth between birds and mosquito species that like to bite birds. There are a couple of places on a bird's body, such as around its eyes, where a female mosquito can draw blood and get the protein meal she needs to ovulate. If the mosquito can also occasionally bite humans, a so-called bridge vector, a person can contract the disease sort of by accident. Humans aren't a reservoir for the virus because we don't develop high enough viral levels in our blood to allow a mosquito who bites us to pass it on to another person.

The paradoxical behavior of a mosquito-borne disease increasing in a period of drought is mirrored by the surprising lack of mosquito-borne disease after major floods. A recent letter to CDC's journal Emerging Infectious Diseases reported on the lack of vector-borne disease on the Gulf Coast following Hurricane Katrina.

Rarely has the aftermath of a natural disaster in the continental United States resulted in increased transmission of mosquitoborne viruses. However, on August 29, 2005, Hurricane Katrina struck Louisiana and Mississippi, where mosquito-borne West Nile (WNV) and St. Louis encephalitis viruses are endemic. Using data from the ArboNET system of the Centers for Disease Control and Prevention, we evaluated the short-term effects of Hurricane Katrina on the reported incidence of human West Nile neuroinvasive disease (WNND) and Saint Louis encephalitis (SLE) in Louisiana and Mississippi using the reported week of onset and the year (2003 - 2005).


Although Hurricane Katrina disrupted WNV surveillance in Louisiana, it did not appear to increase the incidence of WNND and SLE in either Louisiana or Mississippi. In coastal areas, the hurricane destroyed housing and impeded vector control, thus possibly increasing the risk of mosquito-borne infections. However, hurricane-force winds and heavy flooding might have actually decreased the risk of WNV and SLE transmission by dispersing or killing birds and mosquitoes, and destroying their habitat. (Lehman et al. in Emerging Infectious Diseases; cites omitted)

Once again, our intuitions are wrong. The authors caution that if flooding occurs during another time in the transmission cycle, the results could be different, but the reality is that natural disasters rarely cause outbreaks of disease except when connected to poor sanitation amongst displaced residents living in temporary shelters.

We pointed this out at the time of the Hurricane when dire predictions of a mosquito-borne epidemic of disease was being written about in the media and used as justification for doubtful policies by state officials.

It's nice to be right, sometimes.

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Another explanation was that the eggs in the ground for the mosquitos were so throughly innundated with water that they drowned. Another was that there was so much contamination from petroleum products that it got them that way.

By the same token, if those same media people had just shown the distance from the dome to the area above water and rescue, there wouldnt have been such a stink. It was 500 yards worth of wading for most, those others who were too infirmed would have still needed a medevac dustoff but Katrina goes into history as a really big disaster followed by an even bigger one.

For once we got a break with the mossies, very small break indeed.

By M. Randolph Kruger (not verified) on 23 Jul 2007 #permalink

Other factors potentially contributing to a bad WNV year include the extreme heat of the summer of 2007, which decreases the maturation time of the mosquito vector, and increases the replication of the virus.

Another possible factor, at least west of the Mississippi River where Culex tarsalis breeds effectively in irrigated settings, is the increased irrigation that occurs during a drought.

C. tarsalis numbers are highly elevated in the high plains states this summer with infection rates in mosquitoes on the increase.

Relatively few people live in the most affected states, so they don't get the media attention of the two coasts. N. Dakota, S. Dakota, Wyoming, Nebraska, and Colorado have been the hardest hit states since WNV first appeared in the U.S. (See this recent Institute of Medicine presentation,, slide 23)

I live in South Dakota and we have had very little rain. I think the main reason for the high cases of WNV here is because they do not spray for mosquitos here like they do in the east. When I was in Florida, the trucks and planes would spray almost daily for mosquitos. I have not seen any spraying (whatever the word for it is) since moving here.

By Rapid Heartbeat (not verified) on 24 Jul 2007 #permalink

Rapid: There is very little evidence that adulticiding (spraying to kill adult mosquitos) interrupts the transmission of WNV, so it is unlikely that is the reason.