The virus is transmitted when a mosquito bites an infected person, bird or other animal, picks up the microbe and then bites someone or something else. Study of West Nile has been complicated because only certain mosquitoes are efficient carriers. And it has high mobility, because most of its victims have been birds. Scientists are not sure how West Nile got here from its native territory in Europe, Africa and the Middle East, the CDC says. There have been no human cases in the Washington area.
Experts believe hundreds of others went undetected.
Only one infected bird had been found locally the year before, in Baltimore. This year, experts said, one of the main concerns will be how far the virus spreads after its dramatic geographic expansion last year, well beyond the New York City area, where it emerged the year before. The CDC, mindful of the recent jump of the more deadly Japanese encephalitis from Asia to Australia, suggested this month that West Nile surveillance start in every state in the continental United States and as far away as South America. One federal wildlife expert is capturing birds in Florida to see whether West Nile has spread to species there, and another will travel to Puerto Rico later this month to assist surveillance there. State and federal experts said they anticipate less pesticide spraying this year for adult mosquitoes, a practice that created intense controversy in some areas last year.