Of Mosquitoes, Dead Birds and Epidemics

By LEON JAROFF

Tracey McNamara had never seen anything like it. In July, dead crows began turning up by the dozens at New York City's Bronx Zoo. By August, McNamara, the zoo's staff pathologist, had collected carcasses of 40 birds. Meanwhile Dr. Deborah Asnis, an infectious-disease specialist at Flushing Hospital in Queens, reported the admission of two elderly patients with muscle weakness, fever and confusion.

Although unsuspected at the time, these two seemingly unconnected events presaged an alarming encephalitis epidemic. By last week lab tests revealed that the disease had stricken 36 people in metropolitan New York and caused at least five deaths in the region. The tests also provided the first clues in what became a fascinating medical-detective story about a virus never before seen in the U.S.

Within days, Flushing Hospital admitted four more patients with similar symptoms. All were elderly and all spent time in their backyards. That suggested bites by mosquitoes carrying the St. Louis encephalitis virus, a particular menace to the elderly, though usually in the South. Tissue analysis by the Centers for Disease Control and Prevention seemed to confirm the suspicion. As the tally rose, the city began an anti-mosquito spraying program in a 4-sq.-mi. area of Queens. That quick action seemed to contain the epidemic.

At the Bronx Zoo, however, pathologist McNamara was becoming increasingly concerned that the coincidence was too unlikely to ignore. Over the Labor Day weekend, several rare birds in the zoo's collection had suddenly died, and her autopsies showed heart and brain damage. She promptly sent tissue samples to a U.S. Department of Agriculture veterinary lab in Ames, Iowa. Finding no evidence of equine encephalitis or other suspected pathogens, the lab forwarded her samples to a CDC lab in Fort Collins, Colo., for further study.

All the while, the epidemic was spreading. Hundreds of dead crows were turning up in the metropolitan region, and cases of the disease in humans were reported elsewhere in the city and suburbs. Using trucks and choppers, the city loosed clouds of pesticide in the middle of the night, to the distress of many residents. Nearby towns quickly followed suit.

Then came a break in the case. Scientists at the Fort Collins lab and at the University of California at Irvine, who had scrutinized human tissue, identified the real culprit. It wasn't the St. Louis virus but its West Nile cousin, or something very like it. That would account for the many patients with encephalitis symptoms who had nonetheless tested negative for the St. Louis virus. But it presented a broader mystery. Usually found in Africa, but also responsible for epidemics in the Middle East, Europe and Asia, the West Nile virus had never before been identified in the Americas. How did it make its way here?

Thomas Monath, an expert on mosquito-borne diseases, says it could have been carried by someone recently arrived from southern Russia, currently the site of a large West Nile outbreak. If mosquitoes had gorged on his blood, they could have transmitted the virus to birds by biting them in turn--thus starting an infectious cycle deadly for some humans and birds, though never for the carrier Culex pipiens. It's a scenario, says Monath, that's become increasingly common in a jet-setting age.

The National Center for Infectious Diseases believes the epidemic has now peaked. "With the cold weather and the control efforts," says director James Hughes, "I expect transmission to cease." But questions remain. Will migrating birds carry the virus south, and will it re-emerge next spring? Most intriguing, why does its overseas counterpart not kill birds?

Answers may come this week as CDC investigators fan across Queens, seeking blood samples in an effort to learn more about a most unwelcome visitor.

--Reported by Edward Barnes/New York and Dick Thompson/Washington

With reporting by Edward Barnes/New York and Dick Thompson/Washington