Important Health Information about the West Nile Virus

October 12, 2000
The following message is from Carol Garvey, M.D., M.P.H., Health Officer, and

Lynn Frank, FACHE, Chief, Public Health Services, Department of Health and Human Services


As you have no doubt already learned from the media, two crows infected with West Nile Virus were found in Montgomery County last week. If you live in one of the areas where an infected crow was found, you have already been notified that an infected crow was found in your neighborhood and been told that mosquito spraying will be done as soon as weather permits.

This message is to bring others up to date about what we are doing, why we are doing it and what we will do if other infected crows are found in the county.

First of all, let us emphasize that NO PEOPLE HAVE BECOME INFECTED with West Nile Virus in Montgomery County or anywhere else in the State of Maryland.

Although these infected crows were found in Montgomery County, we believe they became infected elsewhere and didn't begin to feel sick until they were in the county. Here is why we say this.

There are two elements in the spread of West Nile Virus: crows, who have a very large flying range, and mosquitoes (specifically culex pipiens), who have a very small flying range. As we have explained in previous messages, mosquitoes become infected by biting infected crows. They then pass the virus to more crows and to people when they bite them. Until last week, we had no evidence that West Nile Virus was present in Montgomery County. And we've been conducting active surveillance, looking for it in many ways.

Since this spring we have regularly tested mosquitoes, dead birds, "sentinel" chickens and people who had encephalitis and menningitis. All tests were negative. (Because chickens get infected by West Nile Virus but do not get sick from it, they have been placed strategically around the state where they were likely to be bitten. Their blood has regularly been tested. Therefore, they are "sentinels" to give us an early alert that West Nile Virus is present in mosquitoes. West Nile Virus is just one of many viruses that cause encephalitis and menningitis in people.)


Because these two infected crows are the first evidence of West Nile Virus, it is likely that they flew here from wherever they became infected. When they were in Montgomery County, they probably began to feel ill and to move slowly. At that point they were easy targets for mosquitoes, but only mosquitoes that were already in the area of the crow.

Therefore, the Maryland Department of Agriculture will try to interrupt the cycle of bird to mosquito to more birds to more mosquitoes transmission by spraying those mosquitoes that were already in the area of the sick crows. The spray they will use will be in a very weak solutionstrong enough to kill mosquitoes but not much else. The chemical being used is the same chemical used in over-the-counter shampoo for head lice and for dipping dogs with fleas. The amount used in one shampoo will be used to spray a half acre. No spraying will be done within 100 ft. of bodies of water to protect aquatic life that might be sensitive to such small doses.

And our current cold weather is slowing the mosquitoes movement too, shrinking their already normally small areas. After the first hard frost (around 25 degrees F. for at least a half hour) the mosquito season will be overuntil next spring.


The Maryland Department of Agriculture will spray as soon as weather conditions are favorable. The temperature must be above 55 degrees F. Wind must be less than 10 mph. It must not be rainy.

When spraying is scheduled, affected residents will see posters in their neighborhoods. If weather conditions change and spraying must be rescheduled, residents can get the most up-to-date information from the Department of Agriculture's Spray Hotline at 1-800-492-2105. We will also put such information on our web site ( But the Hotline will have the information first.

Spray will be applied between dusk and dawn when most people are in their houses. We advise people to stay indoors with windows and doors closed during the spraying and for about a half hour afterwards. The spray quickly dissipates and does not leave residua.

Many people have raised concerns about effects of the spray on the environment or on people's nervous systems. The key to evaluating risk versus benefit is the concentration of the spray. In this case, the benefit is great and the risk, at the dilution used, is negligible. Effects on the nervous system are much less than neuro-toxins that people voluntary take. For instance, alcohol is a neuro-toxin. This spray is weaker than one alcoholic drink.


If an infected bird is found, residents of that neighborhood will be notified IMMEDIATELY. A spraying schedule will be instituted, weather permitting. As the weather gets colder, mosquitoes begin to die or go into hibernation for the winter. This year we would welcome an early frost. A balmy, Indian Summer would be most unwelcome!


There are two basic prevention methods: continue your efforts to eliminate places where mosquitoes can breed and protect yourself from mosquito bites.

Eliminating places where mosquitoes may breed means getting rid of any standing or stagnant water. Any place that can hold even a few spoonfuls of water can breed mosquitoes. At this time of year clogged gutters are a major problem. Mosquitoes can breed in as little as a half-inch of water and water in gutters will stay warmer than water on the ground.

Look for standing water around your house and in your gardens. Turn over buckets, watering cans, wheelbarrows and other containers where water can stand. Saucers under potted plants should be emptied regularly or have a hole in them for water to drain out. Water in bird baths and pet dishes should be changed at least two or three times a week. Be sure gutters and downspouts are clean. In the fall, leaves can quickly block gutters. Check flat roofs for standing water. Get rid of puddles from window air conditioners and repair leaking outdoor faucets. Turn over canoes or boats, remove tarps over log piles or equipment stored outside. Keep garbage cans covered. Dispose of old tires.

Protection from mosquito bites means staying indoors at dawn, dusk and through the early evening when mosquitoes that carry West Nile Virus are most active. Use fine-mesh screens on open windows and doors. Repair tears or holes on screens.

If you must go out during these times, be sure to wear long-sleeved shirts and long pants and use insect repellants. If you will be out for a long time where there are mosquitoes such as in the woods, spray clothing BEFORE YOU PUT IT ON so that the pesticide will be on the outside of clothes and not on your skin. Be sure to follow directions on containers.

More information is available on the county web site at Click on the mosquito icon. The web site will be updated regularly with the number of birds tested and the number infected. You also can get more information about mosquitoes or West Nile Virus from our Communicable Disease Program at 240-777-1755.

Finally, let us thank you for your help is spreading information about West Nile Virus. We have heard from many people who learned from you personally, or through the emails about West Nile Virus that you forwarded to them, and took efforts to rid their areas of standing water. Public health means the health of all of us. We can't do our job alone, and we thank you for all your help.

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