As the initial furor over the West Nile virus' emergence in the United States subsided along with the warm temperatures in which the infectious mosquito vectors thrived, health officials began taking advantage of the break afforded by Mother Nature to develop an extensive surveillance network in parts of the country where it is feared the deadly encephalitis may surface this spring.
Since October, when it first became evident that an exotic flavivirus had been introduced in New York state, New Jersey, and Connecticut, federal and state health agencies banded together to close the safety net as it were, in anticipation of another possible outbreak.
Of the 60 known human infections, seven resulted in death. Scores of birds, mostly crows, have also died, and several horses in New York tested positive for the virus. An indeterminable number of infections in humans, birds, horses, and other vertebrates are believed to have occurred. The virus was also diagnosed in a cat in New Jersey.
In the wake of the West Nile virus outbreak, countries including those in the European Union have temporarily banned or restricted US imports of horses and poultry. Chicken inoculation studies conducted by the USDA National Veterinary Services Laboratories have been completed, and as of press time, the data are being compiled. Initial reports show the chickens developed high, short-lived viremias and shed the virus in feces. Contact birds in the same cages as the inoculated birds did not appear to be affected.
Equine studies are under way at the Agriculture Department's Plum Island Animal Disease Center; no clinical signs were observed in a group of inoculated horses through day 17. Results from turkey inoculation studies by the USDA Agricultural Research Service are not yet available.
Although mosquito activity in the Northeast has been curtailed by cold weather, there are concerns that infected migratory birds will introduce the West Nile virus to new regions within the United States. When the human outbreak was initially thought to be St Louis encephalitis, a flavivirus similar to the West Nile virus, routine testing of sentinel birds in New York state by the Centers for Disease Control and Prevention revealed subclinical infections in such species as mallards, pigeons, and Canada geese.
The virus was later found outside the three initially involved states when it was diagnosed in a crow in Baltimore.
As of press time, no new pools of infected mosquitoes have been discovered since Oct 14, 1999, and the last diagnosis was Oct 29 in a wild bird in New York.
In November, some of the world's leading experts on the West Nile virus met in Colorado to strategize for the coming year. The CDC, in cooperation with USDA-APHIS Wildlife Services and Veterinary Services, the US Geological Survey's National Wildlife Health Center, and state and local health departments, is conducting a 20-state surveillance network. The system includes monitoring for dead birds — especially crows, which appear to be the species most sensitive to the virus — and some live bird testing.
Participating states are located in the East and Southeast, and on the Gulf Coast. Other states can join in.
The surveillance is expected to end in May 2000.