Feature
Flying Fever

by Robert H. Boyle photography by Brian Smale

Borne by mosquitoes and hosted by birds, the West Nile virus killed seven New Yorkers last year. Now it's back. Will it spread?

In June of last year, Dr. John Charos laughed when a woman walked into the Bayside Animal Clinic, in Queens, New York, with a large crow perched on her arm. Because the crow was so calm, he thought it was her pet, an imprinted bird. "Basically, it showed no fear," he recalls. "It wasn't vocal, as crows normally are. You could just pick it up and put it down like a stuffed animal or a bookend."  

The crow, which was sick, recovered, but it was just one of perhaps 30 sick crows brought to Dr. Charos, a veterinarian who is known for taking care of wildlife. "We had others that were wobbly, falling over, and couldn't right themselves," he says. "And either the birds were dead within a 48-hour period or they were back to normal--showing some fear, crowing, and cackling."

The crow, which was sick, recovered, but it was just one of perhaps 30 sick crows brought to Dr. Charos, a veterinarian who is known for taking care of wildlife. "We had others that were wobbly, falling over, and couldn't right themselves," he says. "And either the birds were dead within a 48-hour period or they were back to normal--showing some fear, crowing, and cackling." 

By midsummer, so many sick crows had been brought to the clinic that Charos phoned Ward Stone, of the Wildlife Pathology Unit of the New York State Department of Environmental Conservation in Delmar, near Albany. At the same time, Dr. Tracy McNamara, a veterinary pathologist at the Bronx Zoo, the headquarters of the Wildlife Conservation Society, was getting calls about dead crows. Although McNamara was to play a key role in finally identifying the pathogen that was infecting the birds, at this point neither she nor Charos had a glimmer of what was happening or the havoc it would cause. 

As it turned out, the crows were the first victims of an outbreak of West Nile encephalitis, a mosquito-borne viral disease that can cause fatal inflammation of the brain. When an infected mosquito pierces the skin of a victim to get blood, its saliva--a complex mixture that contains not only the virus but also chemicals that numb the pierced area and serve as anticoagulants--immediately flows into the puncture. 

By last September, bites from mosquitoes that carried the virus had killed 7 people in the New York area, hospitalized 55 others, and infected as many as 1,900 residents of northern Queens, the epicenter of the human outbreak. The virus also killed 27 captive birds at the Bronx Zoo, as well as thousands of free-flying birds, mainly crows, according to Dr. Robert McLean of the National Wildlife Health Center, in Madison, Wisconsin. Thirteen of 22 infected horses on Long Island either died or were euthanized, including Terroir, a thoroughbred broodmare that had raced at Belmont and Aqueduct before being retired. 

The threat posed by West Nile encephalitis is extremely serious. Last October Dr. Durland Fish, a medical entomologist and an associate professor at the Yale University School of Medicine, warned a U.S. Senate subcommittee, "The introduction of a foreign insect-borne virus, never before seen in the Western Hemisphere, is a public health threat unprecedented in modern times. It is reminiscent of the introduction of yellow fever and bubonic plague in past centuries." 

The West Nile virus belongs to the family of flaviviruses, which includes the viruses that cause yellow fever, Japanese B encephalitis, St. Louis encephalitis, and Murray Valley encephalitis. It takes its name from Uganda's West Nile Province, where it was first isolated in 1937, and since then it has caused epidemics in Israel, France, South Africa, Algeria, Romania, and, just last year, Russia, where some 50 people in Volgograd died. In addition to some 40 species of mosquitoes, ticks have served as vectors transmitting the disease abroad. Birds are the primary host of the virus, but in a Russian experiment, a frog, Rana ridibunda, was also able to harbor it. The virus kills not only people, birds, and horses but also sheep and rhesus and bonnet monkeys. As for why a virus kills some people but not others, Dr. Deborah Asnis, the chief of infectious diseases at Flushing Hospital Medical Center, in Queens, says there is no "magic answer" to that question, but that "it may be depressed immunity." Speaking of the West Nile virus, she says, "It affects all ages, but the ones who got seriously ill were the elderly."

If it had not been for the dead birds that McNamara was seeing--both crows and captive birds at the Bronx Zoo--the true identity of the disease might not have become known. When the Centers for Disease Control (CDC) originally declared it to be St. Louis encephalitis, that was the first in a series of blunders by federal, state, and local agencies. The extent of the spread of the virus also set off alarms about the failure of the CDC to recognize that animal populations--birds, in this case--can serve as sentinels for human health.

Thanks to ever-increasing international travel and transport--and to government neglect--Americans live in a country that is wide open to foreign pathogens, a situation made worse by the fact that next to nothing is spent on monitoring the health of wildlife. At best, the United States has only a dozen experts in mosquito-borne diseases. In 1983 and again in 1992, the National Academy of Sciences warned that there is insufficient field research about animals that can host diseases, and that the threat posed by the introduction of new agents of disease is a serious one. As Fish said in his testimony, both Congress and federal agencies ignored those warnings.
 

"It wasn't vocal, as crows normally are," says Dr. John Charos, a veterinarian who is known for taking care of wildlife, holding a pigeon. "You could just pick it  up and put it down like a stuffed animal or a bookend."


Possibly as a result of official ineptitude, the virus survived in overwintering mosquitoes in Queens. On March 29, 2000, led by Representative Joseph Crowley, a Democrat whose district includes northern Queens, 69 members of the House of Representatives, from Maine to Maryland, asked President Bill Clinton to appoint a West Nile virus coordinator "to oversee all of the activities surrounding this issue on a federal, state, and local level." In April Donna E. Shalala, the secretary of the Department of Health and Human Services, appointed Dr. Stephen Ostroff of the Centers for Disease Control to coordinate efforts to control the West Nile virus. 

It it still not known when or how the virus entered the country last year, but it probably arrived with an infected mosquito in June. At one point, the CDC said that "imported infected birds may have played a role," but the odds are against this. All imported birds that arrive in the New York area are held in 30-day quarantine at Stewart International Airport, in Newburgh, New York, 55 miles northwest of the city, and there was no outbreak of the virus there. Moreover, an infected bird has the virus in its bloodstream for only 7 to 10 days. Either it dies then or it develops antibodies that kill the virus, and survives. 

As summer went on, McNamara kept hearing reports of more sick and dying crows. Then, on August 8, crows that frequented the zoo grounds began dying. McNamara sent a number of them to Ward Stone,  but when Stone could not come up with a definitive finding--not surprising, given the circumstances--she began doing necropsies on dead crows herself. She found hemorrhaging of the brain and inflammation of the heart--classic symptoms of encephalitis caused by a virus. 

"In August we started receiving crows from a very wide area--Westchester County, Nassau County, Queens, the Bronx--and many of those crows appeared to have a virus," Stone says. "But I had no idea what the hell it was." 

On August 12 a 60-year-old man from the Whitestone section of Queens was rushed by ambulance to Flushing Hospital Medical Center. Deeply tanned and previously in good health, he had felt weak for several days and had begun to run a temperature. He did not want to go to the hospital, but his wife insisted, and when he was admitted he had a 104-degree fever.  Three days later an 80-year-old man, also from Whitestone, was admitted with cardiac arrest, but he also displayed symptoms of having been infected by a virus. On August 23, after tests of the blood and the spinal fluid from both patients suggested a viral infection, Dr. Asnis called Dr. Marcelle Layton, the head of the Bureau of Communicable Diseases in New York City's health department. While Layton's staff was reviewing the cases, Asnis called to report a third patient; while she was on the phone, another of the hospital's doctors interrupted to tell her about yet another case. The two new patients were also from northern Queens. At that point, Layton notified the CDC. On September 1 the 80-year-old man died, and another patient died the next day. 
 

"We're in the midst of a human encephalitis outbreak, and I've got lots of dead birds with encephalitis," said Dr. Tracy McNamara, a veterinary pathologist at the Bronx Zoo. "


On August 28, after a fifth case was reported at another hospital, Layton and an associate went to Flushing Hospital, also in Queens. While they were reviewing charts, a sixth patient arrived, prompting Layton to query hospitals throughout the city for reports of similar cases. By the end of the week, eight patients had turned up. Interviews with families of the patients revealed one common thread: The patients had all spent time outdoors.

On Friday, September 3, the Centers for Disease Control announced that the outbreak was caused by St. Louis encephalitis, a mosquito-borne virus previously confined to the southern parts of the United States. (The first outbreak occurred in and around St. Louis in 1933.) The mistaken identification stemmed from the fact that the CDC based its finding on human serum--blood and spinal fluid--instead of on brain tissue. It was not looking for the West Nile virus, which was then unknown in this country. 

With that, New York City, which had no mosquito surveillance and prevention programs in effect, began the city-wide spraying of Malathion, an organophosphate pesticide, by helicopter and truck. Counties on Long Island and north of the city followed suit. Malathion, which attacks the nervous system, is toxic to flying insects and fish, and city authorities told residents to avoid direct contact with the spray. Although the city and the CDC both declared that the spraying was safe, many residents later complained that they had not been notified of when spraying would take place, and that even when they were, the sprayers were sometimes off schedule.

"I was playing catch with a friend when the helicopter came over," says Josh Kaufman, 24, a Whitestone resident. "It was so low I could see the guy's face, and then the spray hit me." Kaufman says that he felt sick for weeks, with aching muscles and a headache. Others reported that they and their children had been sprayed while in a park, and fans watching a Mets game at Shea Stadium also got doused. (In May the federal Environmental Protection Agency announced that "any potential cancer risk posed by Malathion is not of concern" because human exposure to the pesticide was so low.)

On September 7, 8, and 9, five Bronx Zoo birds native to this hemisphere--a bald eagle, a snowy owl, a guanay cormorant, and two Chilean flamingos--died. McNamara's necropsies revealed that they had suffered the same symptoms as the crows, and she wondered if the same virus was killing both birds and people. 

On September 9 Ward Stone shipped crows to the National Wildlife Health Center, but the center, he says, "wasn't able to run down the virus, or even pin it to a virus." Next he sent some tissue to the Agriculture Department's National Veterinary Services Laboratory, in Ames, Iowa. McNamara, too, sent tissue samples from zoo birds and a crow to the Veterinary Lab, which she called to say, "It's some kind of encephalitis virus. I don't know which one, but we're in the midst of a human encephalitis outbreak, and I've got lots of dead birds with encephalitis."

That same day, McNamara sent blood samples from the flamingos to the CDC "and told them I wanted them to pursue or rule out that there might be a connection between the bird deaths and human deaths." The CDC's reaction? "Skeptical," she says now. "Never been reported before, therefore did not exist. I kept trying to convince them to work on the material, and after a few days the Veterinary Lab did electron microscopy and showed virus particles of a diameter that ruled out eastern, western, or Venezuelan encephalitis. The virus particle was 40 nanometers. I said, ‘That's it. It's a flavi-virus.' They couldn't take the diagnosis any further because no veterinary laboratory in the United States has the reagents to take the test any further, and this was the first time it had been reported that a flavivirus killed birds.

"I called the CDC to reiterate the need to pursue the possible link between the bird deaths and human deaths," McNamara continues. "They were swamped with the human specimens, and doing bird testing is not part of the mission of the CDC. So I got in touch with a friend at the Armed Forces Institute of Pathology, and he put me in touch with Fort Detrick"--in Frederick, Maryland-- "because their mission is rapid diagnosis of infectious agents. We started a scientific collaboration, and they pulled out all the stops and were rapidly able to pin this down." 
 

"When you talk about bald eagles and peregrine falcons, species that . . . have very low populations in the whole Northeast . . . and a new disease causes serious mortality . . . that would be serious," says Ward Stone, a wildlife pathologist, here with a red-tailed hawk."


Last fall the spread of the virus could literally be tracked as the crow flies--or, in this case, as the crow dies. Dead crows with the virus were found 60 miles into eastern Connecticut, 150 miles up the Hudson River Valley in Saratoga, and 200 miles south in Maryland. 

According to Ward Stone, the family Corvidae--crows, ravens, and rooks--may be unusually susceptible to the West Nile virus; no one is sure why this should be the case. "In lab studies done in Egypt back in the 1950s, researchers inoculated some hooded crows, and they all died," Stone says. "However, when they studied hooded crows in the wild in Egypt--and it was the United States Navy that did this--they found lots of hooded crows with high antibodies. So obviously, in the wild, they all didn't die from West Nile. The stress of captivity in the lab may have been an aid in the pathogenicity of the virus, but the indications certainly were that it was highly pathogenic to the corvids.

"Even if thousands of crows die in New York City and the suburbs," Stone adds, "they have such a wide distribution elsewhere in the state--in the Catskills and the Adirondacks--that they're going to be nesting in places where mosquitoes are not very abundant. Thus we would expect them to rebound. But when you talk about bald eagles and peregrine falcons, species that number only in the dozens or the low hundreds in New York and have very low populations in the whole Northeast, and a new disease causes serious mortality--say 10, 15, 20 bald eagles every year in the Hudson Valley, where they're now spending more time--that would be serious. We don't know what the hell this virus is going to do to wildlife, and it's going to be a long time before we find out." 

At the Senate hearing last October, Dr. McLean of the National Wildlife Health Center said that the virus had infected 19 species of birds. In addition to those already named, there was a red-tailed hawk, broad-winged hawk, Cooper's hawk, sandhill crane, blue jay, robin, mallard, kestrel, and black-crowned night heron. Overseas, McLean said, infected birds are not normally killed by the virus, but "the disease in birds in the New York area that is producing high mortality in crows and other birds is unusual. . . suggesting that this introduced West Nile virus is more virulent to the native bird species or represents a new virulent strain of the virus."

Before the Senate, Fish testified that "every conceivable effort must be made to prevent West Nile virus from reemerging next spring." Otherwise, the virus "is quite likely to survive the winter months, either in infected wildlife or within millions of mosquitoes hibernating in buildings and tunnels in the New York City metropolitan areas. . . . Every conceivable effort must be made to control virus transmission early enough to prevent human infection and avoid the use of widespread aerial nsecticide application to control an epidemic in humans. A repeat of last season's response must be avoided at all costs."

Fish was right on the mark in stating that the virus was quite likely to survive the winter if health authorities did nothing. In April 2000, tests of mosquitoes hibernating in Fort Totten in northern Queens, right next door to last summer's epicenter, confirmed that the virus was alive. This spring another redtail was found to be infected. The city health department announced plans to apply larvicides, where possible, in areas with stagnant water that could not be drained. Meanwhile, city residents were advised to remove empty buckets, old tires, and any outside objects where water could collect and mosquitoes could breed; to clean and empty clogged roof gutters; and to drain and cover swimming pools not being used and chlorinate those that were.  Residents were also asked to report property owners who did not comply. Those measures might have been too late, however.  Next to nothing was done during the winter to eradicate hibernating mosquitoes, and it would afterward take a trained army to try to control them. 

Health officials continue to monitor the situation by checking mosquitoes for the virus. Caged chickens have been put out in some areas, and if they are infected by mosquitoes, this will help determine where the virus is most prominent and how widespread it is. On May 15 the New York City Health Department announced the start of a public education campaign to help "mosquito-proof" the city. The campaign was designed to increase public awareness of the West Nile virus, to urge personal protection measures, and to eliminate mosquito breeding sites.

Only adult female mosquitoes bite--they need protein for egg development--while the males get by on nectar and plant juices.  Four species (one with two subspecies) are threats in the spread of the West Nile virus. The first is Culex pipiens pipiens, the northern house mosquito, which overwinters by the million in any place that offers protection from the cold, such as basements, outbuildings, caves, and tunnels. It can produce multiple generations in a year; depending on the temperature, the egg-larval-pupal-adult life cycle takes only about 10 days. Besides the West Nile virus, this species serves as the vector for Western equine encephalitis, St. Louis encephalitis, fowl pox virus, bird malaria, and Dirofilaria immitis, the organism that causes heartworm in dogs. 

Next is C. pipiens pipiens' mysterious alter ego, Culex pipiens molestus, a possible vector that is so truly domesticated that it rarely, if ever, ventures outside the house and that is active in winter. The females can produce their first batch of eggs without requiring blood. After that they bite. 

In its immature stage, another species, Adedes vexans, thrives in temporary pools formed by rain, seepage, and flooding. The adults are known carriers of the West Nile virus, and they can fly long distances from their breeding places. 

Then there are a couple of nasty foreign biters: Adedes japonicus and Adedes albopictus, whose common name, the Asian tiger mosquito, says it all. Both are very efficient vectors of the West Nile virus, and vicious biters, and unlike their American relatives, they bite in the daytime. A. japonicus, which arrived in the late '90s, is found in Connecticut, and A. albopictus, which arrived in the early '80s, is found in northern New Jersey. Since New York City hasn't done a survey, it's not known if they are present there. Still, it's an odds-on bet that they are. 

As for the future, Fish says, "What happens depends more on the weather than on whatever health authorities can do about the mosquitoes. If the virus comes out early, the more time it has to amplify in the bird population and the more dangerous it's going to be." 

Robert H. Boyle writes extensively about environmental issues. 

 


Fighting Back 

You can do a number of things to help control mosquitoes, which breed in standing water. You should remove any outside objects, such as discarded tires and empty buckets, that hold water in which mosquitoes can breed. If objects cannot be removed, they should be emptied, turned over, or covered to prevent water from collecting. Drain water from roof gutters.  Keep swimming pools clean and chlorinated, and cover them when they are not in use. Change the water in birdbaths every other day. Report property owners who are doing nothing to correct problems.

Mosquitoes are most active at dawn, dusk, and in the evening. Wear long-sleeved shirts and pants outdoors. Apply a safe insect repellent to exposed skin, but do not apply repellents to very young children. Spray clothing with safe repellents; mosquitoes can bite through thin clothing. --R.H.B.
 
 

© 2000  NASI

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