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9.11.2000 00:36
The what, where and why about West Nile virus
What is West Nile virus?
It's a mosquito-borne virus that is common in Africa, West Asia and the Middle East. The first documented cases of West Nile infection in the United States occurred in New York City in August 1999; 62 people became ill, and 7 died.
This year, West Nile has been found in horses, crows and mosquitoes in New England during August and September. In Rhode Island, several crows, a blue jay and one horse have died from West Nile, but none of the mosquitoes caught and tested by the state have been infected.
How did West Nile virus get here?
It's not known how the virus arrived on this continent. But there was a West Nile outbreak in Israel in 1998, and the West Nile virus found in New York is genetically similar to Middle Eastern strains, so scientists speculate that air travel between New York and the Middle East brought it here.
West Nile virus lives in birds, circulating in their blood for a few days. Although the virus infects many species of birds, only a few -- notably crows -- die from it. A mosquito becomes infected when it feeds on an infected bird. The virus lives in the mosquito's salivary glands, and after an incubation period of 10 days to 2 weeks, an infected mosquito can transmit it to the next bird or another animal that it bites.
The breed of mosquito responsible for the New York outbreak can survive through the winter, and they continued to harbor the virus. Mosquitoes don't travel far, but birds do, and migratory birds bit by the New York mosquitoes brought the infection to New England, where it continued to cycle between birds and mosquitoes until the virus had built up enough to start affecting other animals.
Besides birds and mosquitoes, which animals can get West Nile virus?
Many types of mammals can become infected with West Nile virus, but humans and horses appear to be the only ones that become ill. It's possible that cats and dogs can be sickened by it, too, but there have been few documented cases.
When a horse in Wakefield was put to death on Aug. 28 because it was ill with West Nile, awareness of the infection crossed a new threshold in Rhode Island. The horse's illness was proof that the virus has spread to the kinds of mosquitoes that bite mammals, and a person could be next.
What happens to a person who is bitten by a West Nile-infected mosquito?
The mosquito injects the virus into the bloodstream, where it starts multiplying. If the infected person's immune system does not first kill the virus, in 3 to 15 days after the mosquito bite, the virus can cross the blood-brain barrier to infect the brain, interfering with nervous-system function and causing inflammation. The person may experience fever, severe headache, eye pain, drowsiness, extreme muscle weakness and enlarged lymph nodes.
For most people, the symptoms are mild. A small percentage of people -- usually the elderly or those with weakened immune systems -- become seriously ill, sometimes progressing to tremors, convulsions or death. The death rate is around 3 to 15 percent.
Can I catch West Nile virus from an infected person or horse?
As far as anyone can tell, the only way to get West Nile virus is to be bitten by an infected mosquito. There is no evidence of person-to-person, animal-to-animal or animal-to-person transmission of the disease. People and horses are considered "dead-end hosts" because the virus doesn't build up in the bloodstream enough to be picked up by another creature.
How does West Nile virus compare with Eastern equine encephalitis (EEE)?
Both are transmitted by mosquitoes and both cause encephalitis -- an inflammation of the brain.
EEE is more rare and more deadly than West Nile. EEE is carried by many species of swamp-breeding mosquitoes, but usually not by those that bite humans. Even when it does invade the people-biting species, the virus seems to have trouble getting a foothold in the human body. So it is extremely rare for a person to get sick from EEE; there are only about 5 to 10 cases in the entire nation each year. The last Rhode Island case was in 1998.
But when EEE makes someone sick, it makes the person very sick. One-third to one-half of its victims die, and survivors often suffer brain damage.
Compared with EEE, West Nile is relatively benign, but much easier to get, because West Nile virus is at home among people-biting species of mosquitoes, particularly those that breed in urban areas. But people usually don't get very ill from West Nile; because the symptoms are similar to so many other ailments, many won't even know they have it. The death rate (3 to 15 percent) is much lower than that of EEE.
What can I do to avoid getting mosquito-borne illnesses?
First, protect yourself when you go out, especially in the evening and early morning when mosquitoes are most active, by applying insecticide containing 30 percent DEET to your skin and clothing, and wearing long-sleeved shirts and long pants.
Second, keep mosquitoes out of your home by keeping your screens in good repair and making sure they sit snugly in their frames.
And third, don't grow mosquitoes in your yard. Mosquitoes breed in stagnant pools and puddles. (Fish ponds are okay because the fish will eat the larvae.) Change the water in your birdbath twice a week, clean your rain gutters so water doesn't pool there, don't let puddles form in your swimming-pool cover, and remove any tires, buckets, garbage-can covers and other receptacles.
What is the government doing to prevent the spread of West Nile?
Shortly after the virus was identified in New York, the U.S. Centers for Disease Control and Prevention began planning for its spread. It distributed more than $2.7 million to 19 state and local health departments on the Eastern seaboard and Gulf of Mexico and held a planning meeting in November 1999.
Rhode Island received $250,000, and in the spring the state Department of Environmental Management and Department of Health launched a three-pronged effort: to kill mosquitoes, to educate the public and to track the disease.
Municipal officials were asked to distribute larvicide -- a pesticide that prevents mosquito larvae from developing into flying bugs -- in some 77,000 catch basins around the state. The state provided the larvicide and training in how to use it. Municipalities were also trained in spraying adult mosquitoes, should that become necessary.
Health and DEM officials met with media representatives, distributed brochures and ran advertisements instructing the public in mosquito protection and abatement.
And the DEM beefed up its mosquito-testing program, adding traps in urban and suburban areas. Procedures were established for collecting and testing dead birds, and the state laboratory prepared to identify West Nile virus.
Given that the virus has now spread to mammal-biting mosquitoes and a horse has died, does that mean the state's effort failed?
Not at all. Although no one could be sure what would happen with West Nile, the government never expected to be able to stop it in its tracks. The goals were keep a close eye on the virus's spread, make the public aware of it, and reduce the risk of infection.
It's too early to assess how well that worked, but Alan D. Gettman, who oversees mosquito abatement for the state, says that in Rhode Island every aspect of the program has been carried out as planned. The larvicide distribution went smoothly -- and may have helped keep the mosquito population down despite wet weather that was extremely conducive to mosquito breeding, Gettman said.
Why is the state now spraying pesticides
in limited areas?
The spraying started as soon as dead crows were found to have West Nile virus. When a crow becomes infected, it is usually ill -- and spreading the virus to mosquitoes that bite it -- for several days. So when a crow is found with West Nile virus, that's evidence that many mosquitoes in a given area have been infected. The goal is to kill as many of those mosquitoes as possible, so communities where infected crows have been found are spraying within a two-mile radius, using equipment loaned by the DEM.
What exactly are they spraying?
It's a pesticide called Sumithrin, a man-made chemical similar to natural pesticides produced by flowers such as the chrysanthemum. Similar pesticides are used in pet shampoos, flea sprays and household insect sprays.
Spraying will be at concentrations great enough to kill adult mosquitoes (and any other invertebrates that get exposed to enough) but too low to harm humans.
What should I do if there is spraying in my neighborhood?
The Health Department recommends:
Avoid direct exposure to pesticides.
Keep all family members and pets inside during spraying.
Stay inside at least 10 minutes after spraying -- longer if you can.
Close windows, shut off air conditioners.
Bring toys, clothes and small equipment inside.
Cover outdoor tables and big equipment.
Close car windows and turn air to re-circulate mode.
If exposed, wash yourself off.
If fruits and vegetables are exposed, wash them off before storing, cooking or eating.
Where can I call for more information?
For human health questions, call the Health Department at 1-800-942-7434.
To report a sick or dead bird, call 788-3698.
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