Ebb and Flow of West Nile Virus Linked to the Rituals of Robins
The departure of American robins southward or to rural areas in the late summer and early fall helps to explain why West Nile virus infections among humans begin to surge soon afterward, according to a study published this week in the journal Public Library of Science Biology.
The robin is the favored food source of the Culex pipiens mosquito, one of the dominant carriers of the virus. When the birds begin to disperse after mating, the mosquitoes then turn to humans as a new food source.
The study found the mosquitoes feed on humans at a rate seven times higher in the late summer than earlier in the summer.
The study is the first to examine the effect of feeding shifts on West Nile transmission, said A. Marm Kilpatrick, an ecologist at the Consortium for Conservation Medicine in New York and the lead author of the study.
Kilpatrick and his team trapped 30,000 mosquitoes at six sites around Maryland and Washington, D.C., from May to September 2004. They determined the mosquitoes’ last meals by sequencing the DNA of the blood in their guts.
In the U.S., the number of human West Nile infections typically peaks in August and September, though some Southern states also see cases from June to December, according to researchers at the Centers for Disease Control and Prevention.
By the end of October, many mosquitoes have died off and others have begun to hibernate because of lower temperatures.
West Nile virus was named after the district in Uganda where the disease was first isolated. The virus first appeared in the United States in 1999 in New York and has since spread across the country. California reported its first human case in fall 2002.
About 80% of the people infected develop little or no illness and never know they are infected. A small percentage, however, develop a severe form of encephalitis or meningitis -- a swelling of parts of the brain -- that can be fatal, especially in the elderly, young children and those with compromised immune systems.
