The West Nile Virus and Mosquito Control - Frequently Asked Questions

What is West Nile virus?

West Nile virus (WNV) is a member of the family Flaviviridae (genus Flavivirus) that is closely related to members of the Japanese encephalitis virus complex that includes St. Louis encephalitis (SLE), Japanese encephalitis, Kunjin, and Murray Valley encephalitis viruses, as well as others (1,2).

How many mosquito species are there in the State of Texas? How many of these mosquito species are potential carriers of West Nile Virus?

The Texas Department of Health estimates that there are approximately 82-84 mosquito species in the State of Texas. Only about 12 of these mosquito species, however, have been implicated in the transmission of serious diseases. Although it is difficult to determine how many of these species have the potential to carry West Nile virus, approximately 10 of these species have been identified as positive for West Nile Virus in other areas of the country. The mosquitoes most often found positive for WNV are typically Culex spp..

Where did West Nile virus come from?

West Nile virus has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East. West Nile virus was first isolated in the West Nile province of Uganda in 1937 (3,4). The first recorded large epidemics occurred in Israel during 1951-1954, and the largest recorded epidemic to date occurred in South Africa during 1974. Large human outbreaks of WN Encephalitis occurred in Israel in 2000 and have occurred in Southern France (1962), southeastern Romania (1996) and in south-central Russia (1999) (5,6). Until 1999, West Nile encephalitis had not been documented in the Western Hemisphere. It is not known where the U.S. WNV originated, but it is most closely related genetically to strains found in the Middle East.

How long has West Nile virus been in the U.S.?

It is not known how long it has been in the U.S., but Center for Disease Control and prevention (CDC) scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999, possibly longer.

How many cases of West Nile encephalitis in humans have occurred in the U.S.?

The 2001 Summary of West Nile virus surveillance produced by the Center for Disease Control and Prevention (CDC) estimated a total of 50 confirmed human infections of West Nile virus during 2001. According to the CDC, there were five deaths nationwide from West Nile Virus during 2001. Of the 50 cases, 6 were in CT, 11 in FL, 1 in GA, 1 in LA, 2 in MA, 6 in MD, 8 in NJ, 12 in NY, and 3 in PA. As of July 18, 2002, there have been 3 cases of human disease due to West Nile virus infection reported and confirmed by Center for Disease Control and Prevention in the USA. This number will increase as additional cases are confirmed.

I understand West Nile virus was found in "overwintering" mosquitoes. What does this mean?

One of the species of mosquitoes found to carry West Nile virus is the Culex species that can survive through the winter, or "overwinter," in the adult stage. An infected mosquito can likely harbor the virus during the winter months and can transmit the virus in the following year.

How do people get West Nile encephalitis?

People become infected by the bite of a mosquitoes infected with West Nile virus.

What is the basic transmission cycle?

Mosquitoes become infected when they feed on infected birds, which will have the virus in their blood. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood. The virus is harbored in the mosquito's salivary glands, and during blood feeding the virus may be injected into the animal or human along with mosquito saliva. The virus may then multiply, possibly causing illness.

If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?

No. Even in areas where mosquitoes do carry the virus, very few mosquitoes-much less than 1%-are likely infected. Even if the mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The Maine Environmental Policy Institute (MEPI) estimates that in areas where West Nile Virus is endemic, approximately 1 in 1000 mosquitoes actually carry the virus. The MEPI also estimates that only 1 in 300 people bitten by a West Nile Virus infected mosquito will show any signs of sickness. Even if you contract the WNV and become ill, most people will only exhibit mild flu-like symptoms (7). The chances you will become severely ill from one mosquito bite are extremely small.

How many types of animals have the potential to be infected with West Nile virus?

Although the vast majority of infections have been identified in birds, WN virus has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.

How does West Nile virus actually cause severe illness and death in humans?

Following transmission by an infected mosquito, West Nile virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.

What proportion of people with severe illness due to West Nile virus die?

Among those with severe illness due to West Nile virus, case-fatality rates range from 3% to 15% and are highest among the elderly. Less than 1% of those infected with West Nile virus will develop severe illness.

Is there a vaccine against West Nile encephalitis?

No, but several companies are working towards developing a vaccine.

What can I do to reduce my risk of becoming infected with West Nile virus?

Stay indoors at dawn, dusk, and in the early evening.
Wear long-sleeved shirts and long pants whenever you are outdoors.
Spray clothing with repellents containing permethrin or DEET, since mosquitoes may bite through thin clothing.
Apply insect repellent sparingly to exposed skin. An effective repellent will contain 35% DEET (N,N-diethyl-meta-toluamide). DEET in high concentrations (greater than 35%) provides no additional protection.
Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children.
Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product.
Note: "ultrasonic" devices are NOT effective in preventing mosquito bites.

What is the status of horse (equine) West Nile Virus Infection?

From January 2002 through July 15, 2002, equine illness caused by West Nile virus (WNV) infection has been confirmed by the National Veterinary Services Laboratories (NVSL) in 16 horses from 4 states: Florida (7), Louisiana (7), North Dakota (1), and Texas (1). The equine cases in North Dakota and Texas are the first ever detected in those states.

Of the 16 horses confirmed by the NVSL, 8 (50 percent) were reported as having died or been euthanatized. The numbers of equine deaths associated with WNV infection by State are: Florida, 4; Louisiana, 2; North Dakota, 1; and Texas, 1.

Additional reports of equine WNV cases have been made by state officials. The total number of equine WNV cases reported from 5 states is 24: Florida (7), Kentucky (2), Louisiana (6), North Dakota (1), and Texas (8). Some of the state reports include cases that have not been confirmed by the NVSL and some reports do not include all the cases confirmed by the NVSL.

What is the most effective means of mosquito control?

According the Center for Disease Control and Prevention, source reduction is most effective and economical method of providing long-term mosquito control in many habitats. The term source reduction refers to the alteration or elimination of mosquito larval habitat. Source reduction can include activities such as the proper disposal of used tires, cleaning rain gutters, emptying and refilling birdbaths on a weekly basis, and any activity that reduces small pools of water. An item as small as a bottle cap can be a potential mosquito breeding ground.

Is larval control an effective means of controlling mosquitos?

Yes. The Center for Disease Control and Prevention considers larviciding, or the application of chemical or biological agents to kill mosquito larva or pupae, as more effective and target-specific than killing adult mosquitoes, but less permanent than source reduction. The objective is to control the immature stages of the insect before adult populations can develop and disperse. If populations of adult mosquitoes are kept at low levels, the risk of arbovirus transmission is small. The Bacillus thuringiensis israelensis (Bti) donuts used by the City of Highland Village are an example of a biological larvicide.

How can I get the Bacillus thuringiensis israelensis (Bti) donuts for my yard?

Bacillus thuringiensis israelensis (Bti) donuts are available at most home improvement stores, farm supply stores and commercial nurserys, such as Calloways, Lowes, Home Depot and Ace Hardware.

Is adulticide and effective means of controlling mosquitos?

Adulticiding, or the killing of adult mosquitoes by ground or aerial applications of chemicals, is considered by the Center for Disease Control and Prevention to be the least efficient mosquito control method. There are several reasons adulticiding is not highly effective. For example, there are several different types of mosquitoes that have the potential to carry disease, each with its own particular preference for flight times and habitats, which makes timing spray events difficult. In areas where there are many houses, trees, and other obstructions, the chance of actually hitting a mosquito with the spray cloud is greatly diminished. Although it is very difficult to measure the efficacy of a large spray event, the results of trap experiments conducted in Houston and Florida suggest that a large spray event will typically reduce the adult population of mosquitoes by only about 30 percent (8).

How far can a mosquito travel?

Depending on the species, adult mosquitoes may fly several miles with help from the wind. Culex pipiens, the species most commonly associated with the West Nile virus, has a maximum travel range of two to three miles and is not considered to be a strong flier.

How long do adult mosquitoes live?

Generally, adult female mosquitoes have a life span of 2 weeks to a month while adult male mosquitoes only live a week.

What is the life cycle of a mosquito?

A mosquito goes through four distinct stages: egg, larva, pupa and adult.

What is the egg phase of a mosquito?

Eggs are laid in clusters and tend to float on the surface of water. They can be stuck together in rafts of hundreds, or laid separately on water or flooded soil. Most eggs hatch into larvae within 48 hours. Adult female mosquitoes can lay eggs every 10-14 days.

What is the larval stage?

In general, mosquito larvae live in water from 4 to 14, days depending on the water temperature. They come to the surface frequently to obtain oxygen and feed on algae and small organisms living in the water. The larva sheds its skin four times while it grows. After the fourth time, the larva becomes a pupa, the stage before the mosquito becomes an adult.

What is the pupa stage?

The pupa stage is a resting, non-feeding stage. Mosquito pupae must live in water from 1 to 4 days, depending on the species and water temperature. When development is complete, the pupa skin splits and the mosquito emerges as an adult.

What is the adult stage?

The newly emerged adult mosquito rests on the surface of the water for a short time to dry and allow all its parts to harden. If nothing eats or kills it, the female adult can live up to a month, the male typically only a week.

References

De Madrid, AT and JS Porterfield 1974. The flaviviruses (group B arboviruses): a cross neutralization study. J Gen Virol 23:91-96.
Claisher, CH, Karabatsos, N, Dalrymple, JM, Shope, RE, Porterfield, JS, Westaway, EG, and WE Brandt, 1989. Antigenic relationships between flaviviruses as determined by cross neutralization tests with polyclonal antisera. J Gen Virol 70:37-43.
TP Monath ed. 1989. The Arboviruses: epidemiology and ecology. CRC Press, Boca Raton, FL: 59-88.
Hubalek, Z, and J Halouzka. 1999. West Nile Fever - a reemerging mosquito-borne viral disease in Europe. Emerging Infectious Diseases 5: - 643-650.
Tsai, TF, Popovici, F, Cernescui, C, Campbell, GL, and NI Nedelcu. 1998. West Nile encephalitis epidemic in southeastern Romania. Lancet 352:767-771.
Platanov, AE, Shipulin, GA, Shipulina OY, Tyutyunnik, EN, Folochkina, TI, Lanciottie, RS, Yazyshina, S, Platanova, OV, Obukhov, IL, Zhukov, AN, Vengerov, YY and VI Pokrovski. 2001. Outbreak of West Nile Virus Infection, Volgograd Region, Russia, 1999. Emerg Infect Dis 7:128-132.
The Maine Environmental Policy Institute, May 16, 2001 press release, Hallowell, Maine. Available at http://www.meepi.org/wnv/release.htm
Fagin, Dan. 2000. Doubts about spraying - some experts call it ineffective against West Nile Virus. Newsday 11/8/2000. Available at http://www.cfe.cornell.edu/erap/wnv/WNV-Larchive/11-8-00.html

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