When Robins Head South, W. Nile Risk Buzzes Up — Mosquitoes Bearing Virus Target Human Meals in Late Summer, Fall

Human risk of contracting West Nile virus surges in late summer and fall in the United States because that’s when mosquitoes carrying the virus increasingly bite people rather than birds, researchers reported Monday.

Since it was introduced into the United States in 1999, the virus has became the most widespread mosquito-borne disease in the country, spreading to the 48 contiguous states and infecting as many as 1 million people as of last summer.

Fortunately, most people don’t get severe symptoms with the virus, but some 20,000 come down with flu-like illness, and a smaller number have sustained nerve damage and other severe effects. The federal Centers for Disease Control and Prevention reports that nearly 700 people have died from the virus in this country since 1999.

The extent of the outbreak has puzzled researchers when they compare how the virus has spread and behaved in Europe, with only sporadic outbreaks affecting much smaller numbers.

The virus is transmitted by mosquitoes in the Culex family and usually cycles between birds that the mosquitoes feed on. But the new study, published online by the open-access journal Public Library of Science Biology, suggests that the major carrier of the virus in the United States, Culex pipiens, changes its feeding habits from their preferred hosts, American robins, over to American humans in the late summer and fall as the robins head south.

Such shifts in feeding patterns appear to be a “continent-wide phenomenon” that may explain why West Nile outbreaks have been more severe here than in Europe or even Africa, where the disease was first identified in humans, the researchers said.

The researchers were led by Marm Kilpatrick, a senior research scientist with the Consortium for Conservation Medicine in New York. From May through September of 2005, Kilpatrick and his team collected mosquitoes and caught birds at six sites in Washington and Maryland.

Their findings showed that from May to June, the American robin, while accounting for just 4.5 percent of the bird population at the test sites, provided 51 percent of the Culex pipiens’ meals. But as the summer wore on, the robins dispersed from their breeding grounds.

And even though the overall number of birds in the test areas continued to rise as new broods hatched, the mosquitoes that carry West Nile passed on starlings and sparrows and turned to people. By August, the probability that a mosquito’s last meal came from a human had increased sevenfold.

The researchers noted that studies done back in the 1960s of feeding habits among Culex tarsalis in Colorado and California found a similar pattern of feasting on robins early in the season, then a shift to mammals. The mosquitoes are now the main carriers of West Nile Virus in the western United States.

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Mid-South cases in ’05

Tennessee: There were 17 West Nile cases; one death was reported in September by Shelby County health officials.

Arkansas: 26 human cases. In October, state health officials reported the first death of an Arkansan from the virus since 2002, when five human deaths and 43 cases were reported.

Mississippi: 70 human cases.

– Centers for Disease Control and Prevention and staff reports

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