February 15, 2003


 West Nile epidemic of 2002 worst in Western Hemisphere

Posted Feb. 1, 2003

Public health officials say the West Nile virus epidemic of 2002 marks the largest arboviral meningoencephalitis epidemic in the Western Hemisphere. Forty-four states and the District of Columbia reported West Nile virus activity last year, compared with 27 states and D.C. in 2001, according to the national Centers for Disease Control and Prevention.

A total of 3,389 human cases of West Nile illnesses were reported in 2002, compared with 149 between 1999 and 2001. Of the reported cases in 2002, 69 percent had West Nile meningoencephalitis and 21 percent had West Nile fever, while the remainder had an unspecified illness.

Since the virus was first detected in the Northeast in 1999, it has spread across the continental United States within three years. In this latest epidemic, five states—Illinois, Michigan, Ohio, Louisiana, and Indiana—accounted for 64 percent of human illnesses. The CDC attributes 241 deaths to West Nile virus.

Large numbers of infected birds, horses, and mosquitoes were also reported as part of a West Nile epizootic. State officials and the Department of Agriculture's National Veterinary Services Laboratories confirmed 14,515 equine cases in 40 states. An estimated 14,000 bird cases were also documented, along with three dogs and eight squirrels—species previously thought to be safe from the virus.

The CDC reports that the virus had been passed to a nursing infant through breast milk and from an infected mother to her unborn child. These newly recognized mechanisms for person-to-person transmission prompted the CDC to encourage pregnant women to take precautions against exposure to West Nile virus and to undergo diagnostic testing when appropriate.

Although the infant infected in utero was born with congenital abnormalities, there's no evidence that West Nile virus was the cause, according to the CDC.

In addition, two laboratory-acquired West Nile infections have been documented in the United States. In both cases, laboratory microbiologists infected themselves while handling animal material containing the virus, resulting in mild illness.

During the 2002 West Nile epidemic and epizootic, numerous laboratories and laboratory staff were involved in arboviral testing. The potential for laboratory-acquired infections has increased. The CDC believes that accidental infections are underreported.

The CDC is, therefore, encouraging that testing for live West Nile virus be conducted under Biosafety level 3 containment. BSL-2 facilities with minor modifications can, however, achieve an acceptable level of safety necessary for routine diagnostic testing involving the virus including bird necropsies.