Usually, an arbovirus is adapted to no more than a handful of mosquito species. By contrast, lab studies show that West Nile can take up housekeeping in at least 36 species of mosquito, making it one of the most versatile arboviruses ever. Michael Turell, an entomologist at the United States Army Medical Research Institute of Infectious Diseases at Fort Detrick, in Maryland, has shown that among the North American vectors are the northern house mosquito (Culex pipiens); the southern house mosquito (C. pipiens quinquefasciatus); a common pest mosquito in the West (C. tarsalis); and the recently arrived Asian tiger mosquito (Aedes albopictus), an aggressive daytime biter that may have played a significant role in last summer’s West Nile outbreak in Louisiana, which was known to strike about 330 people and kill 25. Human victims are essentially innocent bystanders who happen to get between mosquitoes and the virus’s original host, birds. The CDC currently estimates that less than 1 percent of people bitten by a mosquito infected with the West Nile virus will become severely ill.
To cause human disease, a mosquito must first bite an infected bird and pick up the virus. (Only female mosquitoes bite; they need blood protein to lay eggs.) The virus rides the slurp of blood into the insect’s gut, where it must infect the gut cells, replicate, pass through the gut wall, replicate again, and spread throughout the insect’s body until it reaches the salivary glands and the saliva itself. When the mosquito next bites bird, beast or person, it injects saliva, and the virus may then be passed along.
The complex transmission cycle depends on a great many factors. Consider temperature. If the temperature outdoors is 70 degrees Fahrenheit, Turell explained, it takes more than three weeks for West Nile virus to multiply throughout the body of a northern house mosquito, and only 10 percent of the mosquitoes will be able to transmit the virus. At 80 degrees F, the virus multiplies in two weeks, and 20 to 25 percent of the insects are infectious. But when the temperature goes to 90 degrees F, it takes only a week for the virus to multiply—and about 75 percent of the insects can transmit disease. It’s no coincidence that human outbreaks of West Nile virus disease typically begin in the late summer: the higher temperatures favor viral transmission, the number of infected birds is high and conditions are also prime for mosquito breeding.
West Nile virus causes unusually severe infections in crows and blue jays, according to one CDC-led study that measured the number of virus particles in the birds’ blood, or viremia. “I couldn’t believe the incredible viremias these birds cook up,” Monath, of Acambis, said of the study. “There’s no precedent for it. There were from a trillion to ten trillion viral particles per milliliter of blood”—that is, in an amount of blood equal to one-fifth of a teaspoon. “That’s beyond no precedent. That’s almost beyond belief. No selfrespecting bird can gin up a viremia higher than 100,000 particles with St. Louis encephalitis (SLE) virus. So some birds are almost a billion-fold more infectious with West Nile than with SLE.”
At first, scientists hoped that West Nile might fail to survive the North American winter. But the virus can lurk in dormant mosquitoes. “They go into storm sewers during the winter, go dormant, and just sit there resting,” said Stephen Higgs, a biologist at the University of Texas Medical Branch at Galveston. “The walls of some of these sewers are just furry with mosquitoes.” Researchers suspect that the virus might also survive in mosquito eggs that overwinter and hatch in the spring. There are already hints that West Nile has reached Mexico and the Dominican Republic, where the warm climate, researchers say, may lead to year-round disease activity rather than just seasonal outbreaks.