In the age of SARS and mad cow disease, there's another public-health danger to worry about, and health officials think it will kill birds, horses,


Killer Mosquitoes

West Nile Virus has arrived in the Northwest.

In the age of SARS and mad cow disease, there's another public-health danger to worry about, and health officials think it will kill birds, horses, and maybe even humans this summer. West Nile virus has arrived in the Northwest.

When veterinarian Ken Leisher took the ferry to Whidbey Island last October, West Nile virus was low on his list of suspects. His patient was a 17-year-old gelding with a 105-degree fever. "The owners noticed he was less interested in his feed, mopey, and lethargic," says Leisher. "It sounded like influenza." But then Leisher saw the horse. "He was unsure, unsteady on his hind legs. It was evident that we were dealing with a neurological problem," he says. The veterinarian started supportive medications and sent off blood tests for a handful of diseases. The horse got worse in the next two days. "He was stumbling a little bit, more weak," says Leisher. "By the fourth day he had muscular twitches and he was apprehensive. Everything was just really saying West Nile."

West Nile virus was known to be in Western Washington. A month before, tests on a dead crow in Snohomish County had come back positive. The horse's blood tests confirmed West Nile, making Leisher's patient the third known infection in the state, the second in Western Washington, and the first in a mammal.

The next month, last November, veterinarian Sharon Hoofnagle encountered a case in Whatcom County. "When I went out it sounded like a case of colic," she says of the 14-year-old mare. "But it didn't sound right. Alarm bells were going off. She had a dazed, glazed look in her eyes. Whether she was in pain, I don't knowbut I believe she was. She was fearful. Animals, like humans, do show fear in their eyes."

Both horses were healthy and well cared for. Both had been given the West Nile virus vaccine but came down with the infection before immunity could be established. Both horses have recovered completely, perhaps due to partial immunity from the vaccine. Once horses become sick enough to see a veterinarian, though, they have a 30 percent mortality rate.

Severe acute respiratory syndrome (SARS) and mad cow disease have been getting all the headlines lately. But West Nile virus is more certain to make a mark on this region. It is expected to attack birds, horses, and humans statewide this summer, starting sometime after mosquitoes start hatching. In fact, state health officials said last Friday, May 30, that they think a man in Franklin County has the virus. If that's confirmed in a few weeks by tests, he would be the first human infected in Washington. West Nile first appeared in North America in 1999, in New York, and started marching west. No one knows how it reached New Yorkby ship or plane, or in a mosquito or a bird. Last year the virus reached the West Coast. In 2002, 4,156 humans and more than 14,000 horses across North America got sick enough for a physician or veterinarian to conduct tests that found the virus. Complications from the virus killed 284 people and a third of the horses. No one knows how many more thousands of humans and horses were not sick enough to be tested.

We do know that, of humans who have antibodies in their blood indicating that they have had the infection, 80 percent do not remember being sick, and 20 percent remember a flulike illness with fever and muscle aches. Only one of 150 infected people develops encephalitis (inflammation of the brain), the most dangerous complication, which is fatal in one out of 10 cases. The only proven risk factor is age. Eighty percent of West Nile encephalitis sufferers last year were over 40. The median age of those who died was 78, with a range from 24 to 99.

Humans and horses sit in the same place in the West Nile virus ecosystem. From the point of view of the virus, mammals are dead ends. Mosquitoes deliver it to humans, horses, and other mammals. But mosquitoes cannot acquire the virus from mammals. Infected mammals just don't build up enough virus in the bloodstream to make a mammal-sucking mosquito a carrier. So where do they get it, and how does the disease spread? Infected birds. They turn into factories for the virus, producing enough viral particles in the bloodstream to infect a mosquito that lands for a blood meal. Wildlife biologists are not sure which birds, or how many, spread the virus.

People can protect themselves and each other with clothing, insect repellent, and mosquito habitat reduction. Animals can't protect themselves, obviously, and are at far greater risk. The virus might turn out to affect people more by killing animals and altering ecosystems than by directly infecting humans. But the risk to humans is real, and while horses can be vaccinated, there is no vaccine for people.


"Thousands upon thousands of birds have died," says Emi Kate Saito, veterinarian at the National Wildlife Health Center of the U.S. Geological Survey (USGS) in Madison, Wis. While more than 140 bird species are susceptible, the hardest hit are the corvid family. Ravens, jays, magpies, and especially crows have nearly 100 percent mortality. Wherever the epidemic has passed, it has left behind stories of crows literally hitting the ground.

Although some species, like crows, die so quickly that they would be poor disease transmitters, others, like chickens, develop immunity without showing disease symptoms. The best pattern for transmitting the disease would be a bird species that develops high blood levels of virus but survives for a long time with moderate disease symptoms. "Don't take it out on the crows," says John Marzluff, a corvid and raptor researcher at the University of Washington College of Forestry. "House sparrows and house finches seem to be more effective reservoirs."

In many counties, bird infections preceded human infections, warning health officials to increase preventive measures. "Crows are the canary in the coal mine," says Marzluff. He hopes to decode what the crow-canary is going to tell us this summer. "I would like to see Seattle document the effects carefully," he says. "Document the dead birds."

The first corvid to hit the ground in Washington was a raven in Pend Oreille County, near the Idaho border, last September. According to state veterinarian Robert Mead of the Washington State Department of Agriculture, a homeowner saw two ravens on a tree branch in his backyard. One bird flew off. The other fell out of the tree. "He was stone dead when he hit the ground," says Mead. "They seem to hang on to their perch branch as long as they can. They let go, and they die."

Before the horses were infected, West Nile showed itself in Western Washington last October in Snohomish County, where a bird-watcher saw a dead crow by the side of the road. The man had seen something in the newspaper about West Nile, so he called the Snohomish Health District. He was told to freeze the bird and bring it in.

Public Health-Seattle & King County will pick up dead birds for analysis if they meet certain criteria, according to spokesperson Hilary Karasz-Dominguez. The bird must be freshly dead (within 24 hours), or the virus will not be detectible if it is present. Birds will not be accepted on Friday, Saturday, or Sunday. Precautions are recommended in handling dead birds. "Use gloves and a shovel," says Karasz-Dominguez. It is not possible to catch West Nile virus by touching an infected animal or human, but any wild animal might have other diseases. Even if the bird does not meet criteria for pickup, reporting is helpful to understand the movement of the virus. "We are mapping reports of all dead birds, whether we pick them up or not," says Kim Moore, health and environmental investigator with Public Health-Seattle & King County. (To report a dead crow in King County, call 206-205-4394, 8 a.m. to 5 p.m. Monday through Friday, or consult the resources listed in the box on p. 33.)

Across the Midwest last year, so many birds dropped dead that health departments, inundated by calls, stopped testing carcasses. No one counted the bodies. "It's rare to find a dead bird, they're scavenged so quickly," says Seattle Audubon Society science adviser Herbert Curl. "Finding dead birds in the hundreds is really phenomenal." Says Marzluff: "Something like this might be a correction. Birds have been out of balance. Suburban growth is good for crows, but not good for other birds."

Bird counts can drop drastically after a severe West Nile epidemic. The Chicago region was hard hit in 2002. Illinois led the nation in human cases and deaths. Last fall, the Chicago Audubon Society found that in the areas with the highest numbers of human infections, crow counts were down 81 percent, and black-capped chickadees had almost disappeared. "Rare species can't recover," says Curl, who worries about the spotted owl and peregrine falcon. Raptorsowls, hawks, eagles, and falconsare dying of West Nile virus, but little data are available. "This might be the third nail in the coffin for the spotted owl," says Marzluff. "Habitat loss, [competition from] the barred owl, and now West Nile."

Saito is cautiously optimistic. "No major species will disappear," she says, and she expects birds to adapt to the new disease by developing resistance and shifting to different habitats. "But some localized species may dwindle," she says. "What happens when West Nile hits Hawaii? It's an already fragile ecosystem. We consider all the birds at risk."


Helping Western Washington meet the threat of West Nile virus, Marzluff of the UW has met with government agencies and environmental organizations he's never worked with before. "It's an interesting intersection of human and wildlife health," he says. For the most part, local governments plan to educate the public; monitor storm-water ponds, retention ponds, and catch basins; and, if necessary, actively kill mosquitoes. There are two ways to kill mosquitoes. Adulticides are chemical sprays that kill flying adult mosquitoes. They can harm beneficial insects, animals, and humans and are relatively inefficient, because their targets are spread over such a wide area. Larvicides keep aquatic mosquito larvae from hatching into flying adults. The safest larvicides are natural soil bacteria, which are natural enemies of mosquitoes. Bti, Bacillus thuringiensis israelensis, affects only mosquitoes and a few other equally annoying insects, such as the blackfly. Larvae devour the larvicide. It clogs the digestive system of the larval mosquito like a hairball in a cat. The larvae either burst or starve. Larvicides can be sprayed, but most Western Washington agencies are only planning to use slow- release larvicide tablets that last for a month.

Last year, both adulticides and larvicides were used in various parts of the country. "I hope we don't end up using adulticides here," says Dr. Ward Hinds, health officer of the Snohomish Health District. "We've never done adulticiding." Says Curl of Audubon, who is worried about mosquitoes breeding in storm-water ponds: "Larviciding is the one and only thing that we can and should do."

Many Northwest health officials are concerned that citizens will panic this summer and spray toxic insecticides around their homes, doing more damage to themselves and their environment than the disease ever could. At the other extreme, inadequate mosquito control may have contributed to last summer's epidemic in Chicago.

Twenty percent of the West Nile cases in the country last summer came from two areas of suburban Chicago. As reported in The Chicago Tribune, Cook County officials are saying that the mosquito-control district responsible for the southern suburbs was part of the problem. Crews there did not treat mosquito-breeding catch basins with preventive larvicides until August, when increasing numbers of Chicagoans were already hospitalized with West Nile encephalitis. This was months after other mosquito districts had treated their catch basins, and months after dangerously high levels of infected mosquitoes were seen.

In the northern suburbs, even as patient counts rose and crows fell, many homeowners objected to spraying adulticides to kill infected mosquitoes. If several residents of a block objected, their street was not sprayed.

The decimation of Chicago's black-capped chickadees and crows last year may have been due to the spraying or to the virus or both. The environmentally sensitive approach to pest control is taken for granted in the Northwest, but not in Chicago. Much of the larvicide used there was methoprene, which is not species-specific, meaning it harms other insects and fish as well as mosquitoes. Adulticide spraying is a last resort in Northwest mosquito-control programs, but it was used regularly around Chicago.

However, if mosquito-control chemicals did kill birds, it is paradoxical that the worst bird declines were found in the same areas that had the most human infectionswhich were the areas with the least use of mosquito-control chemicals.

The two Chicago-area mosquito- control districts with the most thorough mosquito surveillance and the best- documented mosquito control had the fewest number of human infections. Bird counts in those areas were normal or only moderately low.

The City of Seattle Integrated Pest Management Plan for Mosquito Control, issued by the Office of Sustainability and Environment, de-emphasizes even larvicide. The plan ranks actions in order of priority. The first four steps are public education and information gathering. Larvicide is the seventh and last step. Next to last, before larvicide, is placing larval predators, such as native frogs and fish, and nest boxes for birds and bats at mosquito-breeding habitats. The effectiveness of larval predators is debatable. According to Saito of the USGS, "I would say the jury's still out."


Storm-water ponds, retention ponds, and catch basins are different names for similar things. They might be open ponds or covered by grillwork under city streets. "Storm-water ponds reduce flooding downstream and reduce pollutants before the water gets to a stream," explains Dan Mathias, principal engineer for Everett. When it rains, they overflow into streams, but between rains they are standing water, ideal mosquito-breeding habitat. "Every city in King County has storm- water ponds," says Dan Willott, project manager for storm-water services in the King County Department of Natural Resources and Parks. "Seattle only has a few, because it was developed before ponds were required." In unincorporated parts of King County, there are 700 ponds on public land, and another 700 on private land.

King County will perform mosquito surveillancechecking the ponds for mosquito larvaethis summer, according to Willott. The decision about whether to use larvicide has not been made. If larvicide is necessary, the county will use Bti tablets.

Seattle has 10 storm-water ponds, according to Tracy Dieckhoner, West Nile virus coordinator for Seattle's Office of Sustainability and Environment. Routine mosquito surveillance of the ponds is not planned. In natural wetlands, according to both Willott and Dieckhoner, Seattle and King County are not planning to check for larvae or use larvicide because it is thought that natural predators are sufficient.

Everett is not waiting for animal or human infections before placing larvicide in its 75 storm-water ponds, according to Mathias. Mosquito larval monitoring began in late April. When a pond reaches a minimum level of larvae concentration, it will be treated with a Bti tablet.

In Portland, mosquitoes are monitored year-round, and larvicide, mostly Bti, is sprayed 11 months of the year, according to Dave Turner, field supervisor of Multnomah County's Vector Control District. (A vector is anything that carries disease.) The Portland mosquito-control program began in the 1930s, when Portland had mosquito-borne malaria. Multnomah County partners with Clark County, Wash., for aerial spraying of larvicide over the mosquito-breeding Columbia River wetlands. Oregon is the only state on the West Coast that has not yet seen any West Nile infections in animals or humans.


Whatever local governments do to eradicate mosquitoes, individuals should protect themselves this summer. They can also do things that protect other people and animals. A virus-bearing mosquito might come from your own backyard or your neighbor's backyard, or it may come from miles away.

People can prevent mosquito bites by staying inside at the prime mosquito-biting times of dawn and dusk, by wearing long pants and long sleeves, and by using mosquito repellent. DEET has been in use worldwide as an insect repellent since 1946. "This substance has a remarkable safety profile," according to Mark Fradin of the University of North Carolina, in a 1998 paper on insect repellent safety. "Twenty years of empirical testing of more than 20,000 other compounds has not resulted in another marketed chemical product with the duration of protection and broad- spectrum effectiveness of DEET," he wrote.

For a 2002 research paper in the New England Journal of Medicine, 15 volunteers applied various mosquito repellents to their arms, stuck their arms in cages with hungry mosquitoes, and timed how long it took for the first bite. A product containing 24 percent DEET protected volunteers' arms for an average of five hours. A 5 percent DEET product gave one to two hours of protection. In other studies, DEET effectiveness leveled off above 50 percent concentration.

Two plant-based products were as effective as 5 percent DEET. A soybean oil product called Bite Blocker, marketed by HOMS, protected from bites for one and a half to two hours. Repel and Fite Bite, two brand names for a eucalyptus oil formula, caused a rash in one subject but gave bite protection of one to three hours. The nationally marketed citronella products tested in this study only gave about 20 minutes protection. In other studies, some citronella products that are only marketed locally or regionally scored about two hours of bite protection. Wristbands impregnated with various mosquito repellents gave no protection beyond two inches from the wristband.

No mosquito repellent is completely free of side effects. None should be used around the eyes, and any of these products will cause rashes in a few individuals. The American Academy of Pediatrics recommends using no stronger than 10 percent DEET on children.

Even a mild West Nile virus infection results in immunity, and younger people are more likely to have milder infections. Seattleites who consider themselves young might be tempted to let the mosquitoes bite them while they're young enough to fight the virus. Warns Jeff Duchin, chief of Communicable Disease Control, Epidemiology and Immunization Programs at Public Health-Seattle & King County: "We really can't advise people to intentionally get infected. A small proportion, even of young, healthy people, get very sick. Some never fully recover neurologically. Are you going to be one of the lucky ones? DEET is really very safe."

There is no vaccine approved for humans, as there is for horses, but drug companies are working on it.


Mosquitoes lay eggs in standing water as little as a Dixie cup left on the deck or as large as a pond. "People don't realize how small a volume of standing water can serve as a breeding site," says Saito, the wildlife veterinarian at the USGS National Wildlife Health Center. "Even a bottle cap."

Mosquitoes begin as sesame-seed-sized eggs laid in water. They hatch in the water into tiny wiggly worms called larvae, as long as a dime is thick. They molt into winged adult mosquitoes. Only adult female mosquitoes bite.

An essential step in controlling mosquitoes is making water unavailable to them. People can protect themselves and others by eliminating standing water around their homes and businesses. Water should be emptied twice a week from birdbaths, flower-pot saucers, kiddie pools, old tires, wheelbarrows, gutters, or anywhere that it collects.

As the disease swept across the country last year, public-health personnel got panicked calls from citizens. King County has produced a 25-minute educational video available for loan to groups. It is also in streaming video on the county's Web site, at "Our basic message is habitat reduction and personal protection," says Moore of the county health department.

Preventing infection with West Nile virus means depending on the home and yard maintenance of other people. That confounds traditional frontier notions of privacy and private property.

"People call up and complain about their neighbors" leaving water in mosquito-breeding containers, says Everett's Mathias. "I tell them, don't get too confrontational about this." At a public meeting in Everett in March, concerned citizens thought owners of ponds on private property should be required to larvicide them. "We don't think we have the legal authority to do that, and we probably wouldn't use it even if we thought we did," says Mathias. King County has "no authority to require anyone to do anything" about neglected standing water, says Karasz-Dominguez.

"One neglected pool could really contribute to a focal outbreak [of disease] in a neighborhood," says Turner from Oregon. "You're putting your neighbors at risk."

Merilee D. Karr is a University of Washington- trained physician and freelance writer who lives in Portland.

comments powered by Disqus

Friends to Follow