Agencies join forces against possible West Nile outbreak

Public health plans for the upcoming mosquito season reflect lessons learned from last year's appearance of the virus.

By Stephanie Stapleton, amednews staff. May 15, 2000.

Washington -- Public health experts at the Centers for Disease Control and Prevention, other key federal agencies and at the state and local level have joined forces to step up monitoring and surveillance efforts to prevent -- or at least minimize -- the possibility of a repeat performance of last year's West Nile virus outbreak. A scientific briefing was held in April at the National Institutes of Health campus to detail the national plan.

The 1999 outbreak occurred in Queens, N.Y. Epidemiological studies demonstrated that the virus' transmission link from birds to humans occurred via mosquitoes, mostly during the late summer and fall. Postoutbreak blood samples found traces of the disease in about 2.5% of the population in the affected area. Overall, 61 people were struck with severe cases and seven people died. Officials now say that the experience demonstrates an instance in which the public health system was taken by surprise.

West Nile virus: the facts
Links

The result: throughout the winter, representatives from disciplines ranging from public health to bird migration, mosquito habitats and arbovirus detection have been working to develop an arsenal of tools and a coordinated plan to undermine the virus' ability to cause human illness.

"This virus is here," said Stephen M. Ostroff, MD, associate director for epidemiologic sciences at the CDC's National Center for Infectious Diseases. "It's not feasible to think about eliminating it." But the important thing, he added, is to implement control and monitoring measures to reduce its public health threat.

Although much of the strategy focuses on virus detection among birds and mosquitoes early in the transmission cycle, the ability of physicians to act as sentinels in detecting unusual illness patterns among their patients is central to the process.

"Clearly the experience from last fall in New York [demonstrates] the critical role of inquisitiveness and questioning by physicians in identifying the outbreak," Dr. Ostroff said. It also offers a classic example of why it is "very important that front-line physicians have a working relationship with public health agencies" and for them to know that these departments will be responsive, he added.

Additionally, Dr. Ostroff said the CDC also works with physicians through the Infectious Disease Society of America to study cases of encephalitis beyond those specifically related to West Nile virus. The goal is to uncover other possible triggers for the illness.

Learning from the disease detectives

But much of this specific effort now under way involves preventing the infection from reaching humans.

What is known so far is that the virus' ecology is very complex in that it must be transferred repeatedly between infected mosquitoes and animals -- usually birds -- before it poses a risk to humans. "Birds can be an early warning system," said Robert G. McLean, PhD, director of the U.S. Geological Survey/National Wildlife Health Center in Madison, Wis.

The purpose of active surveillance, then, is to detect this viral activity at this level. The geological survey has been involved in tracking the behavior, susceptibility and vulnerability of birds to the virus. "For now, we're concentrating efforts on the Eastern seaboard," Dr. McLean said. "But we will also provide assistance in other locations."

Meanwhile, the CDC has distributed more than $2.7 million to 19 state and local health departments on the eastern seaboard and Gulf of Mexico to assist in planning and implementing surveillance and laboratory diagnosis programs. Part of this support involves standardizing diagnostic testing to provide greater reliability of results from public health and veterinary labs. It also includes training personnel to enable diagnostic tests to be done at the local level, thereby reducing turnaround time. The CDC has also been involved in long-term epidemiological follow-up of last year's outbreak. In addition, along with federal agencies such as the geological survey, the Dept. of Agriculture and the Environmental Protection Agency, it has been involved in projects to map mosquito breeding sites, implement eradication strategies and even develop an antigen test to allow a more rapid detection of infected mosquitoes.

The fact that a lot more is now known about the virus is an encouraging sign.

Studies emerged from last year's experience linking it to one that has been circulating in Israel since 1997. Although epidemiologists say there may never be a conclusive answer about how the virus reached New York, it was likely imported by either a bird (legally or illegally brought into the United States) or a person. But early fears that it could have been introduced as a bioterrorist assault have been virtually dismissed.

"The chance ... is very low," said Dr. Ostroff. "I don't think we would ever say never, but overwhelming evidence suggests not." First, it is an unlikely choice as a weapon pathogen because the infection rate is low and the vast majority of cases result in no or only mild symptoms. For every severe case of encephalitis, there are about 50 asymptomatic infections. Also, the strain itself appears to be naturally evolved.

West Nile virus is also closely related to St. Louis encephalitis, a virus "we've been studying for 50 years," said Dr. McLean. At least in humans, the two offer presentations that are virtually indistinguishable, added Dr. Ostroff. The clinical manifestation is similar and the disease groups and severity are basically the same.

Thus, as the summer months approach, plans are proceeding with cautious optimism.

"Last fall was clearly a unique situation," said Dr. Ostroff. "And we can't reasonably predict what could happen over the coming months." But in many of the areas involved in last year's outbreak, there has since been aggressive pest management. "Those efforts will reduce the likelihood of outbreaks. If we do see them, they will be smaller."

ADDITIONAL INFORMATION:

West Nile virus: the facts

What is it? A common virus in Africa, West Asia and the Middle East, closely related to the St. Louis encephalitis virus; West Nile was first detected in the United States last year.

How many U.S. cases in 1999? Sixty-one cases of severe disease and seven deaths occurred in areas around Queens, N.Y.

How is it transmitted? People are infected by mosquitoes that acquire the virus when they feed on infected birds. Virus activity is most likely in late summer or early fall.

Who is at risk? Most cases are mild. People older than 50 or those who are immunocompromised are at greatest risk of severe disease. Case fatality rates range from 3% to 15%.

What are the symptoms? Fever, headache and body aches, often with skin rash and swollen lymph glands. More severe infections may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and death.

How to treat? No specific therapy. In more severe cases, intensive supportive therapy is indicated.

Source: Centers for Disease Control and Prevention