West Nile Virus and Wildlife

O 23 August 1999, the director of infectious diseases at the Flushing Medical Center in Queens notified the New York City Health Department that three admitted patients had an apparently neurological illness. The symptoms included fever, weakness, and confusion. As the number of similarly ill patients grew to five, doctors noted that many were elderly and had spent time outdoors on previous summer evenings. On the basis of this information, and the fact that one of the patients appeared to have encephalitis, the causative agent was suspected to be a mosquito-borne virus. Meanwhile, American crows (Corvus brachyrhynchos) had been dying in large numbers in Queens, New York, since June. Unfortunately, this information was not reported to the Centers for Disease Control and Prevention (CDC) until 4 September, one day after St. Louis encephalitis (SLE) virus had been diagnosed as the causative agent in the human outbreak. Since birds infected with SLE are asymptomatic, public health officials viewed the crow die-off as unrelated to the cases of human illness. At the Bronx Zoo, Tracey McNamara, a wildlife pathologist, had been conducting necropsies of crows since August. By September, a bald eagle (Haliaeetus leucocephalus), a snowy owl (Nyctea scandiaca), flamingos (Phoenicopterus spp.), cormorants (Phalacrocorax spp.), and other birds had unexpectedly died at the zoo. McNamara sent samples to the US Department of Agriculture’s National Veterinary Services Laboratories (NVSL) in Ames, Iowa, suspecting that the birds might be ill with the same disease. NVSL workers discovered that the causative agent was a flavivirus, a family of viruses that includes both SLE and West Nile virus (WNV). This finding explained the CDC’s positive test results for SLE in the human epidemic. At this point, NVSL contacted the CDC, because such viruses require Biosafety Level 3 containment facilities. Though skeptical of the NVSL findings, the CDC requested a tissue sample from McNamara on 19 September. McNamara also contacted the US Army Medical Research Institute of Infectious Diseases (USAMRIID) at Fort Detrick, Maryland. Both USAMRIID and the CDC confirmed that a flavivirus had killed the birds, and they started testing the samples against other flaviviruses. On 24 September 1999, USAMRIID and the CDC concluded that the birds had been infected with WNV (Steele et al. 2000). By the month’s end, it seemed clear that humans and birds had died not from SLE but from WNV, a virus not previously detected in North America.