Send Letter to Editor
A virus on the move | Transmitting WNV
Birds and animals get stung as well
Protecting the natural and the home environment
A long-term relationship | Protect yourself from WNV
In December 1937, a 37-year-old African woman in the West Nile region of northern Uganda became ill with a fever of 100.6°F. From her blood, doctors isolated and identified a flavivirus as the cause of her illness.
Now known as the West Nile virus (WNV), it appeared periodically in the Eastern Hemisphere (Africa, West and South Asia, the Middle East, Eastern Europe) into the mid-1970s. Epidemics of the mosquito-borne viral infection affected primarily horses and humans. Birds were identified as the hosts of the virus.
Although many bird populations in the region carried antibodies against WNV, indicating the birds had been exposed to the virus, there was no rise in bird mortality rates due to WNV.
Between 1975 and 1993, WNV virtually disappeared from the world stage, with no reported epidemics. The WNV story changed significantly starting in 1994, as the disease began to spread across North Africa, Europe, the Middle East, and finally into North America, causing more frequent and severe human and equine epidemics.
WNV was first detected on the North American continent in August 1999 in New York City. Researchers there discovered that the same infectious agent was responsible for human encephalitis cases and an increase in crow deaths. Originally thought to be St. Louis Encephalitis (SLE), a mosquito-borne infection that has been present in the Americas for decades, pathologists thought it unusual to see bird deaths associated with SLE. Further investigation by the Bronx Zoo and the Wildlife Pathology Unit of the New York Department of Environmental Conservation identified the infectious agent as West Nile virus.
The New York WNV epidemic ultimately resulted in 62 human cases, including seven deaths. Because WNV was a new disease to the Western Hemisphere, much attention was paid to the discovery. Virtually ignored by the western news media, however, were much larger WNV epidemics in Romania (1996), Russia (1999) and Israel (2000).
A virus on the move
We do not know how WNV was able to jump continents and hemispheres and turn up in the United States. The existence of a nearly identical strain isolated from a dead goose in Israel in 1998 suggests that the U.S. strain (known as "NY99") originated in the Middle East, possibly introduced by an infected migratory bird. Although there is insufficient evidence so far, migratory birds are thought to be responsible for introducing WNV in other regions of the world. Perhaps it was transmitted locally – WNV antibodies have been detected in resident, non-migratory birds in the Dominican Republic and Jamaica. It's also possible New York City's 1999 outbreak was caused by an imported animal, or by a stowaway mosquito on an airplane, ship or boat.
Another theory has to do with prevailing summer surface winds arising from the southeast, driven by a rotation of highs and lows passing through the jet stream. If a large, humid low pressure system from the south rotates counterclockwise, it would not only carry moist air but mosquitoes as well, since mosquitoes can migrate on moist, southerly winds.
Could an infected person have been the original source for WNV transmission in the United States? That's unlikely. Humans generally do not develop high enough virus levels to transmit the virus to a mosquito. Could it have been an act of bioterrorism? Most health experts don't think so, due to the relatively low proportion of the population affected to date and the lack of evidence that any laboratory has developed WNV into a biological weapon.
The Centers for Disease Control and Prevention (CDC) reported of 4,156 human cases, including 284 deaths, in the U.S. during 2002 – the largest WNV epidemic with severe illness ever recorded. Almost half of the human fatalities occurred in the midwestern states of Illinois, Michigan and Ohio. Wisconsin had 52 cases and three deaths reported last year. That compares to a U.S. total of 149 human cases including 18 deaths from July 1999-2001, so the disease is spreading quickly.
The North American outbreak and recent epidemics in other countries indicate more severe forms of the disease have surfaced. Why? Again, the reason is unknown, but the virulence of the virus, the world's aging population (people 50 years and older are at risk, with 70 years and older considered high risk), and changing environmental conditions may provide clues.
Mosquitoes implicated in other countries as transmitters, or vectors, of WNV to birds and humans belong to the genus Culex. In the 1999 New York City outbreak, C. pipiens – the northern house mosquito – was fingered as the primary vector. To date, WNV has been detected in 36 mosquito species, but it is not known which of the 36 are important in WNV transmission to humans and other animals.
Mosquitoes that bite both infected birds and humans are known as "bridge vectors." During late summer, when newly fledged young swell U.S. bird populations, infected mosquitoes and humans may find themselves together on long, warm summer nights. WNV is most actively transmitted and detected during the summer and early fall in temperate zones, and likely is transmitted year-round in the warmer Gulf States.
In addition to mosquitoes, "hippoboscid" flies – commonly known as flat flies or louse flies that parasitize birds and mammals – have tested positive for WNV. Studies are underway in laboratories to determine whether these flies, as well as other ectoparasites such as ticks, may be important WNV transmitters.
Preliminary studies at the CDC found some birds can be infected if they consume infected prey or mosquitoes. In 2002, health officials identified new modes of WNV transmission in humans: blood transfusion, organ transplantation, through breast milk, intrauterine infection to the fetus during pregnancy, and accidental needle pricks in the laboratory setting. These findings have spurred efforts to develop testing methods for blood banks and to evaluate safety issues for laboratory workers, wildlife biologists, bird handlers and hunters. There is currently no evidence to suggest a person can acquire WNV by handling live or dead infected birds or other infected animals.
If a WNV-infected mosquito bites you, it may take a few days to two weeks to develop symptoms. An estimated 20 percent of people infected with the virus will develop relatively mild flu-like symptoms: fever, headache, muscle ache, skin rash, swollen lymph glands, nausea and vomiting. Symptoms can last three to six days. Less than one percent, or approximately one in every 150 people infected, will develop a more severe illness often referred to as "West Nile meningoencephalitis," a potentially lethal swelling of the brain and the membranes that enclose the brain and spinal cord.
How long does WNV stay in an individual's blood? The virus has been detected for only a few days, and chronic infection is not thought to occur. What remains in the blood are the antibodies the body produces to fight the virus. These antibodies provide future protection from the virus and may confer prolonged, if not lifelong, immunity to WNV.
Birds and animals get stung as well
Birds are generally considered the natural host and reservoir for WNV. Infected birds may become "amplifying" hosts, meaning they build up sufficient levels of the virus in their blood to infect the mosquitoes that bite them. Mosquitoes with WNV carry virus particles in their salivary glands and infect birds, humans, and other animals when they bite them for blood meals. Horses and other mammals, including humans, are believed to be "dead end" or "incidental" hosts – once infected, their bodies do not carry sufficient levels of WNV to infect mosquitoes that may bite them.
During 1999-2002 in North America, corvids – crows, ravens, jays, magpies – had the highest mortality rates. Raptors, especially red-tailed hawks and great horned owls, were also hit hard in 2002 in the Midwest.
The disease's spread in Wisconsin mirrors its rapid march across the nation. WNV was first detected in Wisconsin in a crow picked up in Milwaukee at the end of July 2001. By the end of the 2001 mosquito season, WNV had been detected in 58 wild birds primarily from the Milwaukee area, but also from Kenosha, Waukesha, Racine and Dane counties. In 2002, WNV was detected in birds from 65 of the 72 Wisconsin counties. The vast majority (96 percent) of Wisconsin birds testing positive for WNV last year were crows, some blue jays and a raven.
Nationally, threatened and endangered species such as the California condor, which number fewer than 100 in the wild, could be jeopardized if the WNV juggernaut hits the West Coast in 2003. Robust raptor populations generally can endure periodic high mortality from a severe disease outbreak or from a random catastrophic event. The condor, teetering on the edge of extinction, cannot.
Horses are particularly susceptible to WNV. Veterinarians diagnosed over 14,500 cases in 2002 alone; about 30-40 percent died or were euthanized because of chronic neurological problems. An equine vaccine introduced in 2001 has proven to be over 90 percent effective in field trials.
Currently, no human or bird-specific vaccine is available, but vaccines are under development and may be available in a year or two. During fall 2002, the CDC in Colorado tested a new genetic-based WNV vaccine, and preliminary studies showed a 60 percent increase in survival rates in crows upon exposure to the virus. Although still experimental, the vaccine has been given to the California condors in hopes that it will help protect the species and prevent major setbacks to ongoing recovery efforts. To protect resident birds at zoos, veterinarians are vaccinating birds with the approved equine vaccine even though its effectiveness in non-equine animals is unknown.
Protecting the natural and the home environment
Although wetlands can be mosquito-breeding sites, the mere presence of wetlands does not necessarily increase the prevalence of WNV and other mosquito-borne diseases. In fact, the Culex mosquito subspecies generally breed in shallow manmade sources of water, not wetlands. The important role wetlands have in the natural ecosystem health needs to be considered whenever officials begin to discuss wetland elimination as a means of reducing the risk of WNV infection. Even where WNV is prevalent, relatively few mosquitoes actually carry the virus. Realistically, the chances an infected mosquito will bite you are low.
The EPA has approved ground and aerial spraying of small quantities of organophosphates or pyrethroid formulations to control adult mosquitoes if surveillance data indicate a moderate to high level of risk for human infection. Minor eye and skin irritations, as well as breathing problems (asthmatic or allergic) have been associated with such applications, mainly in persons sensitive to the active ingredients or in those with pre-existing respiratory conditions. Contact your local health department to find out if spraying is planned in your neighborhood and take the recommended precautionary measures.
What are the environmental consequences of spraying to control mosquitoes? When applied correctly, these pesticides are reported to have low toxicity to wildlife. Even so, non-target species such as other insects, birds and aquatic organisms may be affected. Data on more than 4,000 birds gathered by the New York Wildlife Pathology Unit as part of that state's WNV surveillance program found approximately 48 percent of the bird deaths could be attributed to pesticides or other chemicals commonly used around the yard or in agriculture. Spray programs are now the focus of several laboratory and field studies.
Homeowners can make their immediate surroundings less congenial to mosquitoes by eliminating any stagnant pools of water in containers. Once or twice a week, empty water from flowerpot saucers, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels and cans. Turn over wheelbarrows, wading pools and boats when not in use. Check and clean clogged rain gutters, and remove discarded tires and other items that collect water. If you have a flat rooftop, consider sweeping it regularly to deprive mosquitoes of breeding spots. Contact your local environmental health or county mosquito control program to report suspected mosquito breeding sites in your neighborhood.
To control mosquitoes in still or stagnant waters, municipal authorities are turning to larvicides with a biological organism (Bacillus thuringiensis var. israelensis and B. sphaericus) as the active ingredient. Methoprene, a biochemical regulator that interferes with mosquito larvae maturation, has also proven effective. These larvicides have not been associated with any serious health effects to date, nor are they known to harm other aquatic life according to public health officials. Some regions have introduced populations of larvae-eating mosquito fish (Gambusia spp.) into bodies of water as an alternative.
A long-term relationship
What are the long-term consequences of WNV for birds? No one knows, but the general consensus among ornithologists is that species such as the American crow and blue jay will likely co-evolve with the virus and eventually develop sufficient immunity, as have the birds where WNV is endemic in Africa. We hope to learn more about WNV's long-term effect on birds from established traditional monitoring programs, such as the Audubon Society's Christmas Bird Count, the federal Breeding Bird Survey, and other long-term initiatives to monitor birds.
For humans, the future of WNV can be glimpsed from a look at Africa, where the virus has existed for a long time. Most people in the African endemic areas are likely exposed to WNV during childhood and subsequently develop antibodies, which provide prolonged immunity. In North America, it has been predicted that WNV will persist in the environment at low levels, circulating among birds and mosquitoes, and causing occasional human and equine epidemics. The hope is that we, too, will adapt along with the virus, develop immunity and witness fewer severe outbreaks.
Sumner W. Matteson is a DNR avian ecologist. Emi Saito is the West Nile Virus surveillance coordinator for the USGS National Wildlife Health Center. Both are based in Madison.