UA research entomologist Frank Ramberg studies mosquitoes that infect birds with West Nile virus. (Photo: Lori Stiles)
By Lori Stiles
Aug 08, 2003
Arizona, and Alaska, Hawaii, Oregon, Utah and Nevada, are the only states that have yet to report an animal or human case of West Nile virus.
However, the mosquito- and bird-borne virus is likely to reach Arizona soon, University of Arizona entomologists say. "We are faced with the imminent arrival of the West Nile virus in Arizona," they emphasize.
Colorado had its first human case of West Nile virus last month, and infected horses are showing up in Colorado near the Four Corners area of Arizona, said UA research scientist Frank Ramberg of the entomology department. West Nile virus-positive horses are also being recorded in New Mexico, and Valencia County, N.M. has had a human case this summer, he added.
"Now that the weather has changed, West Nile virus could show up here any time," Ramberg said.
Ramberg, UA entomology Professor Henry Hagedorn, Kathleen Walker of the UA Center for Insect Science, and UA assistant professor of entomology Elizabeth Willott work with state and county health officials in monitoring for West Nile and advising the public on measures that will help check an outbreak.
Common brown mosquitoes transmit West Nile virus between birds, and also infect mammals. (Photo: Lori Stiles) |
"Early preventive action is essential because this virus is not going to be easy to combat," Hagedorn said. "And that action should target mosquito larvae rather than adult mosquitoes."
West Nile is a disease of birds that only occasionally affects humans, horses and other animals. Mosquitoes transmit the virus that causes the disease when they feed on birds. Infected birds spread the disease during their seasonal, cross-country migrations. Arizona is an important route for migrating birds. And when birds migrate in Arizona, they tend to follow natural and artificial wetlands, which is exactly where local bird populations and mosquitoes congregate.
West Nile infects about 140 bird species, and birds remain infectious for 4 to 5 days. Most recover rapidly and are then immune. But birds of a few species develop extremely high levels of virus in their tissues and quickly die.
"Songbirds, crows, grackles and jays are especially vulnerable," Ramberg said.
Large numbers of dead birds is an early warning signal that the virus has arrived, the UA scientists say. It presages an urban epidemic. As the fraction of the infected bird population rises, so does the likelihood that mammals will become infected.
Keep watch on birds in your neighborhood. If you notice they are dying, collect them for state and county health officials. Pima County officials have a dead bird collection number, 520-740-2760.
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The death rate of infected horses can be as high as 40 percent. Ramberg noted that the Arizona Department of Agriculture recently issued a letter urging horse owners to have their animals vaccinated. This is a two-step process, and effective only after the second vaccination.
However, West Nile virus is far less fatal to humans. The Centers for Disease Control currently estimates that less than one percent of people bitten by a mosquito infected with the West Nile virus will become severely ill.
While it is difficult to determine exactly what percent of the infected humans will develop severe symptoms, Hagedorn said, chances of developing immediate, severe symptoms are slim.
He cited an epidemiological study conducted after a 1996 West Nile virus epidemic in Romania. Researchers found that 125,000, or about four percent of the Bucharest population, had been exposed. Of those, 400 people were hospitalized with serious symptoms and 17 died. That is, although the death rate of those with severe symptoms was about 4 to 5 percent, the death rate for all those exposed to the virus was only about one-hundredth of one percent.
But a disturbing, growing body of evidence says that "at least with birds and wildlife, infected animals may show no symptoms at all but still sustain neurological damage," Ramberg said.
"Arizona has 45 mosquito species, by my latest count," Ramberg said.
Two of these species have been shown capable of transmitting the West Nile virus. Culex quinquefasciatus, the common brown house mosquito, and Culex tarsalis , which are common around natural and artificial wetlands, feed on birds and on mammals, including humans. Both prefer to feed at dusk and throughout the evening.
"These mosquitoes are night fliers," Ramberg said. "It is important to distinguish Aedes aegypti , which is the mosquito that transmits dengue fever, from those that transmit West Nile virus because Aedes aegypti feeds during the day while the West Nile mosquitoes feed from dusk to dawn. You can be walking your dog at 11 at night and be bitten. "
Mosquitoes breed in standing water available in backyards (including drained swimming pools and neglected hot tubs), golf courses, irrigated orchards and fields, natural washes and wetlands.
One good defense against an urban epidemic is to rid backyards and natural washes of standing water. Toys, cups, old tires and plant saucers need to be dumped after a rain. Water that stands for 5 days can breed mosquitoes.
Be wary of indoor standing water, too, Ramberg advised. He has found that Arizona's common brown mosquitoes breed in large indoor plant pots, and can be abundant inside humid buildings.
Water that can't be dumped or drained should be treated with a natural microbial insectide called Bti, commercially available as mosquito "dunk," Hagedorn said. Bti is non toxic to other organisms; it specifically kills mosquitoes. Adding fish that eat mosquito larvae to your pond is another natural way to eliminate them.
Screen windows and doors, wear long-sleeved shirts and long pants, wear mosquito repellent or simply stay indoors at night. Mosquito repellent that contains the compound DEET or lemon eucalyptus oil is effective, but may cause allergic reactions. Noise-generating devices and electric insect "zappers" are useless.
Heavy, widespread pesticide spraying is not recommended.
"When West Nile virus appears, neighborhoods will call for spraying pesticides to control adult mosquitoes," Hagedorn said. "It may give people a comforting sense that the government is taking action, but it may not give much added protection against the disease. Mosquitoes tend to hide 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 overall that it reduces transmission of West Nile virus.
"Furthermore, pesticides sprayed over wide areas kill a lot more organisms than mosquitoes. (Widespread spraying) carries too great a cost to the environment, human health, and budget," Hagedorn said.
Meanwhile, keep watch on the birds in your neighborhood. If you notice they are dying, collect them for local county or state health officials. Pima County officials have a dead bird collection number, 520-740-2760.
- Updated: August 8, 2003
