These analyses showed a slight indication of St. Louis encephalitis but a vastly stronger reaction for West Nile virus. Named for the province of Uganda where it was first described in the 1930s, the West Nile virus spreads via mosquitoes and causes encephalitis in people and horses in Africa, Australia, the Middle East, and parts of Europe. Tests in government and academic labs soon confirmed that in New York, people, horses, crows, and zoo birds had succumbed to the West Nile virus. Just as epidemiologists predicted, the outbreak died down after mosquito spraying. Then, cold weather presumably curbed the adult mosquitoes that had been carrying the disease.
Crows seem again to be doing more flying than dying.
During this merciful lull, as the public health community plots its strategy for next year, many of the big questions involve animals. Adults of Culex pipiens, the mosquito suspected of most frequently spreading the disease, can survive the winter, perhaps lurking in New York's sewers, says Nicholas Komar of CDC in Fort Collins. Once infected, a mosquito bears the virus for life. Yet the presence of virus-carrying mosquitoes doesn't guarantee another outbreak. One important factor will be how the species that the mosquitoes bite react.