PUBLIC HEALTH SUPPLEMENT:West Nile Virus, A Status Report

It started on August 23, 1999. Two elderly patients from the same community in Queens, N.Y., both had unusual cases of encephalitis. Oddly, both lived in the same 2-mile square area. Then, a short time later, officials at the Bronx Zoo sent two dead birds to the United States Department of Agriculture (USDA) to determine the cause of death. USDA was able to isolate the virus that killed the birds but wasn’t able to identify it. USDA then sent the virus isolates to the Centers for Disease Control and Prevention (CDC), where the virus was identified as West Nile virus (WNV).

Just months later, there were 62 cases, including seven deaths, of West Nile virus confirmed in New York City. The reports created headlines and concern across much of the country.

WHAT IS WNV? West Nile encephalitis is an infection of the brain caused by West Nile virus, a flavivirus commonly found in Africa, West Asia and the Middle East. According to Dr. Harry Savage of CDC, no one really knows how WNV entered the United States. He suggests several ways that this could have happened:

• Via an infected human host;

• Via a human-transported vertebrate host, either legal or illegal;

• Via human-transported vector(s); or,

• Via a storm-transported vertebrate host (bird).

This is how the virus is spread: mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals when they take a blood meal.

Although most infections are mild, symptoms of WNV include fever, headache, and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death. The incubation period for WNV is usually three to 15 days.

Fortunately, West Nile encephalitis is not transmitted from person-to-person. For example, you cannot get West Nile virus from touching or kissing a person who has the disease, or from a health-care worker who has treated someone with the disease, CDC reports.

All residents of areas where virus activity has been identified are at risk of getting West Nile encephalitis; people older than 50 years have the highest risk of severe disease, according to CDC.

1999 PROGRAM. After the virus was identified last year, mosquito control was initiated in parts of New York City on Sept. 8, 1999. But at that time, the virus had already hit its peak. "It’s hard to tell if control is working when it’s applied after the number of human cases have peaked," Savage said. "But it did reduce Culex pipiens numbers. Cases decreased in treated areas, but cases continued to occur at low rates in Nassau and Suffolk Counties, where mosquito control wasn’t conducted."

After the WNV outbreak, health officials decided it was important to see just how many people had been infected. After going door to door, it was estimated that there were 12,000 WNV cases total even though not nearly that many were reported. The reason? Most people could fight the virus off without seeking medical attention. Most affected were the elderly.

In the winter after the outbreak, officials made two trips to sewers in New York City to test the overwintering mosquitoes for the virus. It was found that when the Culex species survived through the winter, in the adult stage, the virus survived within mosquitoes at low rates. This allowed the virus to overwinter and transmission to occur during the summer of 2000.

Health officials decided they needed to put a plan in place to address a potential WNV outbreak the following year.

2000 WNV Surveillance. In 2000, a special West Nile virus surveillance program was funded by an emergency congressional appropriation of $4 million total in 17 states and two cities.

"The money was used primarily to improve the infrastructure of the state health departments," Savage said. "Most chose to improve their diagnostic facilities and hire technicians to do the testing."

In addition, officials have been collecting virus data on a weekly basis for the following:

• sentinel chicken flocks;

• testing mosquitoes for virus;

• dead bird observations; and,

• enhanced human surveillance.

Each of these four produced massive amounts of data. This research is important so when the first virus activity is detected in a community, prior to the occurrence of human disease, rapid mosquito control measures (such as targeted application of adulticides and larvacides) can be implemented.

Preventing WNV. Although infected mosquitoes are the primary source for West Nile virus, according to CDC, ticks infected with West Nile virus have been found in Asia and Africa, although their role in the transmission and maintenance of the virus is uncertain. However, there is no information to suggest that ticks played any role in the cases identified in the United States.

According to CDC, WNV has been identified primarily in birds through September 2000. But CDC also has received reports of WNV infection in horses, cats, bats, chipmunks, skunks, squirrels, domestic rabbits and raccoons.

Here are some tips from CDC to help protect PCOs and consumers from WNV:

• Stay indoors at dawn, dusk and in the early evening.

• Wear long-sleeved shirts and long pants whenever you are outdoors.

• Spray clothing with repellents containing permethrin or DEET, since mosquitoes may bite through thin clothing.

• Apply insect repellent to exposed skin. An effective repellent will contain 35% DEET (N,N-diethyl-metatoluamide). DEET in high concentrations (greater than 35%) provides no additional protection.

• Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer’s directions for use as printed on the product.

A. "Encephalitis" means an inflammation of the brain and can be caused by viruses and bacteria, including viruses transmitted by mosquitoes. West Nile encephalitis is an infection of the brain caused by West Nile virus, a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States.

A. It is not known how long it has been in the U.S., but CDC scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999, possibly longer.

A. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood. The virus is located in the mosquito’s salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.

A. Most infections are mild, and symptoms include fever, headache, and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death.

Number of cases of West Nile Virus from May 1 to October 31, 2000:

Connecticut 1 1,077

District of Columbia 0 3

Maryland 0 50

Massachusetts 0 442

New Hampshire 0 7

New Jersey 4 1,133

New York 13 1,228

North Carolina 0 1

Pennsylvania 0 29

Rhode Island 0 71

Vermont 0 1

Virginia 0 2