The epidemic of encephalitis in the New York City area that broke out at the end of August seems to be abating. As of Oct 4 there had been 50 confirmed cases with five deaths (including one in Westchester County, north of the city). In August, crows around the Bronx Zoo and in other areas were dying. Then other bird species in the zoo died, with evidence of brain haemorrhage. On Aug 23, two patients with encephalitis were seen at a hospital in the northern Queens borough of New York City. An astute infectious-disease doctor notified the city's health department, and an epidemiological investigation and a public education and control programme began.
The responsible virus was initially identified by the US Centers for Disease Control and Prevention as St Louis encephalitis virus. The CDC now say that the causal agent is West Nile virus. In a research letter in today's issue, Kunjin/West Nile virus is proposed as the cause (page 1261). West Nile virus has not been reported in the Americas before. The last major epidemic of West Nile encephalitis outside Africa occurred in Romania in 1996, with nearly 400 cases and 17 deaths (Lancet 1998; 352: 767—71).
In practical terms the identity of the virus is largely immaterial. Because all three viruses are transmitted via mosquitoes that bite infected birds and then pass the virus to human beings, the measures brought in to control the outbreak (aerial and ground spraying with insecticide, advice to avoid mosquito bites) will be effective for all three. So will the drop in temperature as New York's summer, and the mosquito season with it, ends.
The veterinary detective work is starting to fall into place, although the original source may prove impossible to track down. According to a spokesperson for the Bronx Zoo, birds have not been imported from Africa into the zoo for over 6 months. Birds outside the zoo succumbed before any in it, so birds in the zoo are an unlikely source. Pet shops may have imported African birds, although any infected birds should have died during quarantine. Other importations may have been illicit, however. Africa as the source of West Nile virus does not explain the suggestion of Kunjin virus as the cause, since this virus is found mainly in Australia. Some may even see a humorous side to the story that, thanks to the omission of an “r”, cow deaths due to West Nile virus were reported in Connecticut.
The way the outbreak was dealt with, and the story behind the outbreak, are destined to become classics in infectious-disease annals. The investigation and the handling of the epidemic form a success story, even though susceptible individuals have been badly or fatally affected by their infection.
Amazingly, given such a litigious and sometimes anti-authority society as in the USA, New Yorkers were willing to be exposed en masse to malathion spraying organised by the city's officials. The successful implementation of the control programme was due to openness by the city powers, including the mayor and public health and emergency-response officials, rapid publicity at the local level, and sensible reporting of the outbreak by the media. There were few signs of headlines of the “Killer virus loose” variety. The public-health necessity of mass (and personal) mosquito control was explained fully, and the sprayed insecticide is relatively harmless. Congratulations are justified—officialdom and the media worked well together for the public good.
There are other lessons to be learned from the New York outbreak. Given that international travel is so common, as is the trade in exotic animal species, it should come as no surprise that infectious diseases stray from their traditional grounds. Unusual diseases in animals may act as sentinels for a human outbreak, so clinicians must be alert to warnings from veterinary colleagues. Doctors also need to remember (as happened in New York City) that a very small cluster of unusual infections is enough to call in the authorities. Finally, the encephalitis outbreak in New York City shows how public-health and emergency-response medicine remains as vital a discipline now as when mass infections struck in previous eras.