April showers bring May flowers – and a new season of mosquitoes that may carry West Nile Virus (WNV).
WNV, which first appeared in the U.S. in 1999, poses a threat mainly to certain wild birds and some livestock, especially horses.
But the virus can infect people, too. It happens when a mosquito bites a virus-laden bird and then bites a human. An estimated 20 to 25 percent of people who become infected develop the flu-like symptoms of West Nile fever, and a much smaller number of those people go on to develop West Nile encephalitis, an inflammation of the brain that can be fatal.
The people at greatest risk from WNV are those 50 and older and those with weakened immune systems. Younger people also can acquire the infection, but their risk is significantly lower. In 2002, 4,156 Americans developed a WNV-related illness, and 284 died. In Missouri last year, WNV caused illness in 168 people, and seven of them died.
The question is, how great a threat will WNV pose this year, and how soon will the WNV season begin?
“That’s difficult to say,” says Michael Diamond, M.D., Ph.D., assistant professor of medicine, of molecular microbiology and of pathology and immunology at Washington University School of Medicine in St. Louis, “but the first clue will come from the number of last year’s infected mosquitoes that survived the winter or managed to pass the virus to their progeny before dying.”
Public health officials have set mosquito traps to answer that question and learn how many of those insects carry the virus.
“The season will start earlier if many mosquitoes infected with the virus survived the winter, and last winter was colder than the previous winter in much of the country,” says Diamond. “If few survived, the season starts later. If none survived, the cycle won’t start until infected birds or mosquitoes come into a particular area, so that could delay the spread by several weeks.”
Another clue will come from the numbers of susceptible birds that begin dying. Crows, blue jays and some sparrows are the most important carriers of WNV.
Last year, says Diamond, birds began dying in late June. Cases of human illness then followed a few weeks later, in mid- to late-July.
The time lag between bird deaths and the onset of human illness occurs because WNV infection first must reach a high level in the bird population before it spreads efficiently to humans. Then the virus must spread from mosquitoes that primarily bite birds to mosquitoes that primarily bite humans.
That takes time. The bird deaths also provide a useful warning. Like the canary in the coal mine, they indicate that human infections are soon to follow and that people should take precautions to reduce the risk of infection.
Diamond recommends using skin treatments that deter mosquitoes, especially those containing the chemical DEET, minimizing time spent outdoors at dawn and dusk when mosquitoes are most apt to bite, and wearing long pants and long-sleeved shirts if one must go outside during those hours. He also recommends eliminating standing water where mosquitoes tend to breed, especially in gutters, backyards and birdbaths.
Vaccines to prevent West Nile fever and encephalitis are being tested, says Diamond, but they won’t be available for at least two or three years. An experimental vaccine for horses is available, but it’s not yet clear how long the immunity lasts.
Symptoms of West Nile fever, usually a mild illness that lasts a few days, include fever, headache, and body aches, and sometimes a skin rash and swollen lymph glands.
One in 150 individuals infected with the West Nile virus develops West Nile encephalitis or meningitis. Signs and symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.