Thursday, June 29, 2006

West Nile Virus Depends on Robin Distribution

A recent study of West Nile Virus transmission in the DC area found that the number of infections in humans depends on the density of the local population of American Robins. Researchers set up mosquito traps baited with dry ice in several local parks: the 26th Street Dog Run, National Mall, and Fort Dupont Park in DC; and Camden Yards, Takoma Park, and Bethesda in Maryland. Birds at the same sites were caught in mist nets for blood tests.

Robins, it turns out, appear to be taking the hit forhumans, getting sick and dying as did thousands of crows that wereinfected in the first wave of West Nile virus after it arrived in NorthAmerica. Thanks to the robins, humans who frequent the 26th Street dogpark and similar areas have a lower chance of contracting the virus, atleast in spring and early summer months. The reason? To mosquitoes,robins are far more tempting meals.

Then the scene changes.

"Robinsbegin to migrate south in late July and August," Kilpatrick said,"leaving mosquitoes on the hunt for blood from another source."

That source turns out to be Homo sapiens. The number of human infections with the virus shoots up come the dogdays of August. Then it's mosquito vs. man or woman, instead ofmosquito vs. robin.

The authors of the original article in PLoS Biology argue that the pattern of infection - birds in early summer and humans in late summer - has increased the prevalence of the disease in both mosquitos and humans:

Feeding shifts have two synergistic effects on the intensity of WNV transmission to humans. First, we have shown that the increase over time in the probability of Cx. pipiens feeding on humans results in a greater number of human WNV infections than if the mosquitoes fed on humans with the same probability as in early summer (Figure 1C). Second, feeding primarily on WNV-competent avian hosts during the amplification period of the epidemiological cycle maximizes the intensity of the epidemic in mosquitoes.... This is because mosquito WNV prevalences are already beginning to decline (possibly as a result of increased acquired immunity in juvenile birds) when mammals begin to make up an important fraction of the blood meals. Thus, the shift in feeding from competent hosts early in the season to humans later leads first to greater amplification of the virus as transmission intensifies between birds and mosquitoes and subsequently to an even greater number of human WNV infections.

The continuing presence of the West Nile Virus in the DC area is a reminder to take reasonable precautions against mosquito bites, especially in late summer: do not keep standing water around your house; use insect repellent on bare skin or keep skin covered. (See this CDC advice on avoiding bites.)

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