Saturday, July 30, 2005

West Nile: another disease from birds

With West Nile season is on us again, we are reminded that bird flu is not the only disease we blame birds for. West Nile virus also likes to hang out in birds, but its mode of transmission to humans is via mosquitoes that feed on infected birds and then bite humans. There is a rather puzzling story today on the CNN website bringing the "surprising" news that it might be the American robin, not the crow, that is responsible for harboring the disease. It also reports some unnamed "experts" as being skeptical of the new report, just published in the journal Emerging Infectious Diseases.

What is really surprising about this story is that I know hardly any real experts in West Nile who think crows are important in propagating the disease. The only thing unusual about crows is that they get sick and die from West Nile. The virus has been found in over a hundred other species of birds and last time I checked the reservoir hadn't been pinpointed with any specificity but the leading candidate was the common house sparrow. If the robin is also implicated as another reservoir that would not be at all surprising. Crows are important in West Nile surveillance precisely because they are large, obvious birds whose carcasses are easy to spot. They are "dead end hosts" (like humans), meaning that they are not the actual reservoir of the disease in nature but incidental victims, like people. The birds that are the real reservoir don't get sick. There is a lot yet to learn about West Nile epidemiology, such as exactly which species of bird is the most important reservoir and in which regions (it probably differs in different parts of the country); whether there are non-bird reservoirs (this virus is able to infect many species, including mammals); which species of mosquito is responsible for transmission from bird to bird and which ones from bird to humans (the so-called "bridge vectors"); etc.

The way to avoid contracting the disease, which is usually mild or inapparent but in a small percentage of cases can produce a disabling or fatal meningoencephalitis, is to avoid being bitten by mosquitoes. The best way to do that is to have as little exposed skin as possible when in an area cohabited by mosquitoes (a windy beach, for example, usually isn't mosquito infested but just inland from the beach there may be many breeding grounds so when you leave the beach you can cover up), and to use insect repellent. Until recently the only proven effective repellent was DEET (found in many preparations, such as Off!), but recently CDC has approved two more, Picaridin (KBR 3023), known as “Bayrepel” outside the US; and the active ingredient in oil of lemon eucalyptus, p-menthane 3,8-diol (PMD). CDC has a good webpage on insect repellent use and safety.

The most contentious issue in the management of West Nile disease in any locality relates to killing adult mosquitoes with broadcast applications of insecticides (usually pyrethroid agents with additional ingredients to enhance their effectiveness). We addressed this issue herea while back. It is not at all obvious there is any benefit to this and some evidence it might cause additional harm. It is difficult to get public officials, including public health authorities, to acknowledge this, however. They want to be able to show they are "doing something" and many believe what they are doing works. The most effective means of mosquito control is a systematic program to kill them in the spring in the larval stage. By the time West Nile season rolls around, that period is over.

So while you are fretting about bird flu, this is another thing to keep in mind. We inhabit this planet with many other species and when we screw up the balance of things we have to expect things to happen we don't expect and don't like. But the real issue, as President Bush reminds us so cogently and so often, isn't pandemic influenza or West Nile disease. It is Freedom for the Iraqi People by bombing the living shit out of them. Have to keep things in perspective, after all.