Sunday, September 11, 2005

Katrina and mosquito-borne disease

The specter of outbreaks of mosquito-borne disease, particularly West Nile fever, has been raised repeatedly in the wake of the flooding in the Gulf states. This seems like a reasonable fear, given the fact that mosquitoes are associated with standing water and West Nile virus (WNV) is endemic in area birds and there have been a number of human cases. But on closer examination it is not obvious there will be a problem and jumping to conclusions can make matters worse. Already we are hearing that Louisiana state officials are planning an aerial spraying program to curb the hatching of mosquitoes and flies in the flood waters around New Orleans. Since aerial spraying is used against adult mosquitoes, not eggs or larvae, it is hard to understand why this would be done. Instead it will introduce another biologically active toxin into an environment which already has plenty. Time to take a clearer view of the the threat.

The immediate effect of a flood is not to increase the kind of mosquitoes that transmit WNV but to reduce them by washing away their larvae. The "bridge vectors," i.e., the species of mosquitoes that transmit the virus to humans by biting both infected birds and humans are the kind that ordinarily breed in small containers, not floodwaters. Moreover it is not at all clear what has happened to the reservoir for WNV, the wild birds. The Hurricane has almost certainly disrupted the usual bird-mosquito-bird which is the normal transmission cycle, humans and horses being accidental dead-end hosts not particularly important in the virus's natural history.

The problem will come when most of the city is drained and small containers and puddles of standing water are left. Even then, breeding mosquitoes have to be infected with the virus, and we don't know if that will be the case, given the drastic ecosystem disruption that has occurred. At that time the proper intervention will be to empty containers and larvicide small pools and catch-basins. Killing adult mosquitoes has not been shown to interrupt human disease transmission. Mosquitoes reproduce exponentially quickly and adulticiding reduces their numbers only temporarily and arguably may reduce their natural predators to allow an even greater population to return.

WNV is not the only serious mosquito-borne virus that can be a problem. Eastern Equine Encephalitis (EEE) and St. Louis Encephalitis (SLE) are also theoretical threats, but Western Equine Encephalitis (WEE) is found in the north central plains states and the west, not in Louisiana and the Gulf, and past experience suggests none are likely to be a post-hurricane problem. Only the Red River floods of 1975 in North Dakota and Minnesota had any significant mosquito-borne disease in their wake, mostly WEE and some SLE. None of Hurricanes Hugo (1989), Andrew (1993), the Mississippi floods of 1993 or Tropical Storm Alberto (1994) produced any outbreaks of mosquito-borne disease in humans in the southeast (source). Since there is no significant malaria or yellow fever in the US, no outbreak of either would be expected.

We can expect an increase in "nuisance mosquitoes" (i.e., non-disease-carrying biters) and it will be important to reconstitute the mosquito surveillance apparatus in southern Louisiana, Mississippi and Alabama as soon as possible. Prudence suggests the situation bears watching. In the meantime we doubt there will be any significant mosquito-borne disease as a result of the flood and broadcast aerial spraying with the idea that it is to be expected is not indicated, counter-productive and ineffective. The Mayor's widely-publicized warning that mosquitoes were feeding on dead bodies is utter nonsense. Only female mosquitoes bite and they do so to obtain a blood meal to allow egg-laying. Human remains are not sources of circulating blood, nor do they have the viruses we worry about.

None of these facts will deter tabloids and uninformed officials from hysterical predictions. Here is my current favorite (from The Sun, a British scandal sheet):
BILLIONS of disease-carrying mosquitoes have erupted from the toxic waters of New Orleans.

Health officials have warned ANYONE in the flood zone will be bitten 200 times a MINUTE unless they take drastic precautions.

Most of the mozzies are carrying untreatable West Nile Fever — which can leave victims paralysed and brain damaged.

It can also put people in a coma, make them blind, give them convulsions or cause fever.
These guys almost make Fox News look like a legitimate news outlet. Almost.