Contact Information Henry Hagedorn UA entomology dept. 520-621-5358 hagedorn@ag.arizona.edu

by Henry Hagedorn, Kathleen Walker and Frank Ramberg

The ecology and epidemiology of West Nile is not simple. As a result it is not going to be easy to combat. Early preventative action is essential and the primary target is the aquatic larvae rather than the adult. To understand why this is so requires an understanding of the nature of the disease and how it is transmitted.

West Nile is actually a disease of birds that only occasionally affects humans, horses and other mammals. Mosquitoes are the vectors that transmit the virus that causes the disease. Bird migration patterns have quickly spread the disease from the East Coast to the rest of the country. Birds fly south in winter, where the virus is transferred between them by mosquitoes before they return. Because Arizona is an important route for migrating birds the arrival of the virus will occur soon. Natural and artificial wetlands, attractive to both birds and mosquitoes, are likely to be sites where virus is transmitted to local birds.

An unusual feature of West Nile virus is that it can be transmitted by many different species of mosquitoes. Those species of mosquitoes that transmit the virus among the birds may not be the same as those that transmit it to us and to other mammals.

The mosquitoes involved in transmission have changed as the virus moved across the country. Culex quinquefasciatus and Culex tarsalis, small brown mosquitoes that prefer to feed at dusk, are present in Arizona and have been shown to be able to transmit the West Nile virus. Both species prefer to feed on birds, but will also occasionally feed on mammals.

Because of the complexity of the disease, the development of an epidemic in an urban area develops gradually. It begins when virus is introduced into the bird population and spreads among them. The virus infects more than 100 species of birds. Most recover rapidly and are then immune. A few species die very quickly, and the death of large numbers of birds is an early warning signal that the virus has arrived. Infection of horses and humans occurs later in the epidemic because the likelihood of transmission to mammals increases as the percentage of the infected bird population rises. By the time humans and horses are infected the epidemic is essentially over. The death rate of infected horses can be as high as 40 percent, but it is far lower in humans.

It is difficult to determine exactly what percentage of infected humans have severe symptoms. After a major epidemic in Romania in 1996, the percent of the population exposed to the virus was estimated by checking antibodies in human blood samples. Health officials found that about four percent of Bucharest-area residents (125,000) had been exposed. Of those about 400 were hospitalized with serious symptoms and 17 died. While the death rate of those with severe symptoms is about four to five percent, the total number of infected people who died was actually less than 0.014 percent.

This tells us two things. First, the number of people exposed to the virus can be quite large. Second, and more importantly, the danger to most humans is lower than is often stated.

Arizonans are faced with the immanent arrival of the West Nile virus. What can we do? Given the facts discussed above, it is clear that reducing the mosquito population should be our most important consideration. The best way is to eliminate mosquito breeding sites. Mosquito larvae live in standing water and there is plenty of that in our urban areas, particularly in back yards and nearby washes. Identifying breeding sites is not difficult, although numerous sites may exist in a single backyard.

Remove containers that catch rain water. Remove those plant saucers. Get rid of those old tires, or put them in the garage. If you have ponds or fountains you can kill mosquito larvae by using a natural microbial insecticide called Bti, available commercially as "Mosquito Dunk." Adding fish to your pond that eat mosquito larvae is another natural way to eliminate them. Ensure that water does not collect in washes after it rains. Water that remains for 5 days can breed mosquitoes.

Personal protection is also important. Use screens on your doors and windows. Don't go outside in the evening, or if you must, wear clothes that cover as much skin as possible. There are mosquito repellants available. Compounds that contain DEET are widely available and work well, but they are not the only type available. Recently, several products that contain lemon eucalyptus oil have become available and have been found to be quite effective. All of these products contain chemicals that might cause allergic reactions, so exercise care when using them.

In contrast, devices that produce sound are useless. See the recent article by Fradin and colleagues in the New England Journal of Medicine, volume 347, pages 13-18, for an excellent study of the efficacy of insect repellents.

When West Nile virus appears there will be calls to spray pesticides to control adult mosquitoes. Spraying may give people a comforting sense that the government is taking action, but it may not add much protection against the disease. Mosquitoes tend to hide outdoors in thick vegetation, making them difficult to reach with pesticides. Heavy spraying can temporarily knock down the number of flying mosquitoes, but there is no clear evidence that this reduces the disease-carrying mosquito populations overall or that it reduces transmission of West Nile virus.

Furthermore, pesticides sprayed over wide areas kill a lot more organisms than just mosquitoes. The Centers for Disease Control recommend spraying for adults only as a last resort when larval breeding site reduction fails.

In contrast to spraying for adults, controlling juvenile mosquitoes in aquatic breeding sites is more effective and less environmentally disruptive. Reducing and treating larval breeding sites is far more cost effective and much less damaging to the environment than pesticide use. If breeding sites are not removed, the adult mosquitoes that come from them will quickly make more larvae and more adults, and make the killing of adults a Sisyphean task.

In New Orleans, for example, malathion, the pesticide most often used in mosquito control, was sprayed every 12 hours for five-and-a-half days. Flying mosquitoes declined by 75 percent, but the mosquito population recovered to normal levels one week after spraying ended, according to a 1987 article in the American Journal of Tropical Medicine and Hygiene.

We can defeat West Nile virus transmission by taking proactive action to reduce the number of mosquitoes present. That can be done if we are diligent about reducing mosquito breeding sites. It is collectively our backyard problem.

You can learn more about West Nile at the Centers for Disease Control and Protection at http://www.cdc.gov.

The authors are members of the department of entomology and Center for Insect Science at the University of Arizona