Links to Other ASM Pages:

• Table of Contents
• ASM News Issues

West Nile Outbreak Renews Response Capability Questions

A New York State wildlife pathologist examines a dead crow for traces of the West Nile enchephalitis virus at his lab in November 1999. Dead crows from all over the New York metropolitan area were examined to assess the spread of the virus. Monitoring of wild bird populations is being expanded to a 20-state region along the Eastern seaboard and Gulf Coast. (AP Photo/David Jennings)

Late last summer, microbiologists, public health officials, and clinicians found themselves confronting a surprise outbreak of encephalitis attributed to the West Nile virus in and around New York City that caused seven deaths among 62 cases of severe disease (ASM News, January 2000, p. 4). The virus, which may have persisted locally beyond last winter, is carried by mosquitoes that feed on birds and other animals, including horses. Hence, a wide range of federal, state, and local public health officials as well as other experts who specialize in pest control, migratory bird behavior, weather patterns, and other factors that could affect a potential follow-up epidemic have developed elaborate plans for rapidly detecting the virus and, they hope, preventing a large-scale public health catastrophe.

Outside of a few laboratories, the West Nile virus had not been seen in the Western Hemisphere before last summer. Then, however, it struck with considerable suddenness and force, not only causing the localized several dozen cases of human encephalitis, but also a deadly outbreak affecting mainly crows and other wild birds in the same immediate region and a more diffuse outbreak of encephalitis affecting horses on Long Island. A subsequent survey of residents living near the center of the outbreak in the Queens borough of New York indicated that about 2.5% of that localized population had been infected by the virus without developing symptoms.

The flight of birds and insects and the easy travel of human carriers—thevirus is not transmitted directly between people, but an infected individual can transmit the virus to mosquitoes—loom as potential outbreak-disseminating factors. In part because of the nonlocal nature of this threat, officials at the Centers for Disease Control and Prevention (CDC) in Atlanta, Ga., are now in charge of coordinating the complex monitoring and surveillance effort now in motion to guard against another outbreak of West Nile viral infections, according to Stephen Ostroff, who is Associate Director for Epidemiologic Sciences at the CDC National Center for Infectious Diseases. "It is not feasible to eliminate the virus from the Western Hemisphere, and we may see cases over the years," he says. "There’s no way to predict what may occur. We hope to monitor and control it to reduce the public health threat."

CDC investigators are conducting follow-up studies on the West Nile virus, developing better tests for detecting it, and delving into other lingering questions, such as how the virus landed in the New York area from somewhere in the Middle East. Moreover, CDC is providing $2.7 million to support extensive detection efforts at the state and local levels.

Several other federal agencies are also actively involved in current West Nile virus research, monitoring, and containment efforts, including the U.S. Department of Agriculture (USDA), Environmental Protection Agency (EPA), and the U.S. Geological Survey (USGS). USDA scientists began studying how the virus affects horses after an outbreak last summer in which 25 animals became ill and nine of them died or had to be euthanized, according to Randy Crom of the USDA Animal and Plant Inspection Service. Moreover, faced with concerns that the virus might cause havoc among commercial poultry flocks, USDA scientists have learned from tests on small numbers of chickens and turkeys that the virus infects them but does not appear to cause symptoms or fatalities, according to Crom. Chickens will continue to be monitored this year as "sentinels" to indicate whether the virus is migrating into new areas.

USDA and USGS experts will also be monitoring wild birds for any evidence that the virus is infecting such populations and perhaps migrating with them to other regions of the United States, Canada, or neighboring countries to the south, according to Robert McLean of the USGS National Wildlife Health Center in Madison, Wis. So far, however, the only virus-infected bird found outside New York was a crow found in Baltimore, Md., he says. But, because the virus can infect wild birds that migrate great distances, monitoring plans extend to a 20-state region along the Eastern seaboard and Gulf Coast, he says. Officials elsewhere in the United States, Canada, and at the Pan American Health Organization have also been notified about how best to monitor for the virus, according to Ostroff.

The West Nile virus, which is thought to have originated in Uganda, ordinarily is found throughout the Middle East, Africa, and West Asia. It has caused outbreaks there and also in Eastern Europe, notably Romania. Although not transmitted person to person, mosquitoes pick up the virus from infected birds and transmit it when they feed on humans or other species. Thus, most control measures focus on reducing mosquito populations, and USDA and EPA officials are encouraging those responsible for implementing control measures to follow integrated pest management approaches because they are considered the most productive and safest in terms of minimizing ecological and public health safety risks.

Concerns were raised early during the investigation of the West Nile virus outbreak last year that its appearance in the U.S. might be an instance of biological terrorism. However, Ostroff says that this possibility is now considered to be of "very low likelihood." For instance, most of the time the virus causes only mild, subclinical infections, making it an ineffective and therefore unlikely weapon, he says. Moreover, he adds, genetic analysis indicates that the virus found last year in New York "appeared to be naturally evolved" from that found circulating and causing disease the year before in the Middle East, suggesting that "it is not a strain that someone would have had in a collection from 10 years ago" as might have been true for a bioweapon under development.

Jeffrey L. Fox
Jeffrey L. Fox is the ASM News Current Topics and Features Editor.