West Nile Virus and the Asian Tiger Mosquito

A Gardener's Alert

Gardeners spend many hours outside during the day tending their growing plants. Because of the time required to plant, prune, weed, fertilize, and inspect growing annuals, perennials, and crops, gardeners are likely to encounter the Asian Tiger mosquito (Aedes albopictus). Why? This mosquito (unlike other species which feed primarily in early morning and late evening hours) is active and feeding throughout daylight hours. As the Tiger mosquito population grows, people who enjoy gardening will become more exposed to bites resulting from this distinctly Asian Tiger mosquito behavior. Gardeners need to be informed about this insect pest, which is a newcomer to the southern Maryland area. The Asian Tiger Mosquito, as well as other local mosquito species, may be vectors for the West Nile Virus.

The Asian Tiger Mosquito

A tropical insect, the Asian Tiger Mosquito is found primarily in Asia and West Africa. Because mosquitoes are cold-blooded insects, their populations are not great in areas with prolonged cold weather such as that found in most of North America. But with the right combination of mild winters, dry springs, and hot summers, the Asian Tiger Mosquito can proliferate in non-tropical geographical areas. ("The Weather Turns Wilde", Nancy Shute, U.S. News and World Report, February 5, 2001, p. 48.) The picture shown is the Asian Tiger Mosquito. The photos shown may be helpful for identification of this relative newcomer to North America.

The Asian Tiger Mosquito has distinct white body striping, helpful in identifying the species. Its breeding habit is similar to other mosquito species in that it lays eggs which hatch into larvae that grow into mature adult insects. However, the Asian Tiger Mosquito prefers to lay eggs on the edge of stagnant water sources. It may breed in water sources as small as a water-filled twist-off bottle cap or soda can. Rain or other water sources (such as hose watering of container plants) can wash the eggs into standing water. When the eggs hatch, the larvae grow in the stagnant water source. In about two weeks, the adult mosquitos are ready to launch into the air, feed on unsuspecting hosts, and repeat the egglaying cycle. ("Virus-bearing mosquitoes invade region", Nancy Bromley McConaty, The Maryland Independent, June, 2000.)

West Nile Virus

The Asian Tiger Mosquito is a possible carrier of the West Nile Virus. This Virus has been normally found in West Africa and Asia. The Virus can cause encephalitus (inflammation of the brain). In 1999, the West Nile Virus affected approximately 60 people. In 2000, there were 17 people who were confirmed to have contracted West Nile. (Death is more likely to occur in older people and others with weakened immune systems.)

The Virus is distributed by birds infected by Asian Tiger mosquito bites. Although mainly a bird disease, West Nile Virus afflicts birds, horses, and humans as well. Crows are particularly susceptible to West Nile Virus. However, the Virus has been detected in other wild birds, other mosquito species, humans, and horses. USGS West Nile Summary 2006

In 2002, three Louisiana men had been infected with West Nile Virus. These were the first confirmed cases of the mosquito-transmitted illness. No other cases had been confirmed in 2001, but the Virus has reportedly killed 18 people on the East Coast of the US since 1999. (Washington Post,"Nation in Brief", July 14, 2002, p. A8.)

The Infection Cycle

Birds may travel a great distance after having been bitten by an Asian Tiger Mosquito or other mosquito species infected with the West Nile Virus. Scientists and health officials believe this may be the way that the West Nile Virus has spread in northeastern America.

In the year 2000, the 48 crows and 2 blue jays were found infected with West Nile Virus in Maryland. How do the birds get infected? The illustration shows the typical cycle of disease transmittal. The Asian Tiger Mosquito bites and infects a bird with West Nile Virus. Other mosquitoes bite the now infected bird. These mosquitoes then infect other birds, horses, and humans. ("Onslaught of mosquitoes expected due to heavy rains", David Abrams and Nancy Bromley McConaty, The Maryland Independent, June 2001.)

a graphic from Scientific American describing the infection cycle of west nile virus

They're Here

The Asian Tiger Mosquito is in Maryland. It was first detected in the City of Baltimore in 1999. Investigation revealed identification of the mosquito at a tire recycling business. In September 2001, it was reported that the first human victims of West Nile Virus were believed to be identified in Baltimore County. A 72-year old man became comatose and health officials think the virus may have been the cause of swelling in his brain sac. A 63-year-old woman also was identified as a potential victim of West Nile Virus. (The Washington Post, "West Nile Virus Likely", September 9, 2001.)

The Asian Tiger Mosquito (a West Nile Virus vector) has spread since first identified in Baltimore. It has been reported that the Tiger Mosquito population is particularly high in southern Maryland. Charles and St. Mary's counties have been identified as harboring increasing populations of the Asian Tiger Mosquito. It is likely because of the large areas of wetlands that these Mosquito populations are growing quickly in southern Maryland.

The Maryland State Health Department prepared a final report for the year 2001. In the 2001 Arbovirus Surveillance Final Report, 454 West Nile Virus-positive birds were found in Maryland with 215 of the birds found in Baltimore City. (Only one infected bird was found in Charles County in 2001 and two in 2002.) Maryland reported six humans with the Virus in 2001. (The Maryland Independent, "State testing dead birds for signs of West Nile",Dave Ferris, p.1.) The number of individuals contracting West Nile Virus in Maryland overall increased from 36 in 2002 to 73 in 2003. Officials are concerned that Hurricane Isabel when it uprooted many trees last fall provided ideal places for mosquitoes to breed, leading to an increase in possible West Nile Virus infection of birds, people, and horses. (Michelle Boorstein, "Delayed Punch from Isabel Faces Region this Spring, The Washington Post, Sunday, April 18, 2004, Metro section.)

West Nile Spreads Into Charles County

In the year 2000, the first two dead birds that had been infected with the West Nile Virus were found in Charles County . Health officials indicated that this was the first sign the Virus had reached the Southern Maryland area. Although public officials were reluctant to speculate about future presence of West Nile Virus in Charles County, they believe it is likely that the Virus will become more widespread. As mentioned above, one West Nile infected bird was found in Charles County in 2001, and in 2002 several birds had been confirmed as infected with West Nile Virus. (The Maryland Independent, "Charles County bird tests positive for West Nile", Jay Friess, front page.)

J.B. Hanson, Deputy Director of Public Relations for the Maryland Department of Health and Mental Hygiene, has been quoted: "We definitely expect it to spread..." but "Quite frankly, there's a lot worse things you could get [than the West Nile Virus]".

Even so, the Maryland Department of Agriculture will be spraying mosquitoes in Southern Maryland to reduce the insects' populations and hopefully reduce the risk of West Nile Virus. Spraying in Maryland usually begins in March and continues through October in selected southern Maryland communities. Patricia Ferrao, entomologist with the MD Department of Agriculture, indicated that although testing of adult mosquitoes has been done in Southern Maryland, so far no mosquitoes have tested positive for West Nile Virus in the State of Maryland. ("West Nile virus may be more widespread this year", Susan Craton, The Maryland Independent, April 27, 2001.)

Mosquito Spraying

The West Nile virus can be transmitted by mosquito species other than the Asian Tiger. However, the all-day feeding habits of the Asian Tiger mosquito may increase the risk of spreading West Nile virus. The Maryland mosquito spraying program is not the best against the Asian Tiger mosquito. Spraying is usually done at sunset or after dark for native mosquitoes. Asian Tiger mosquitoes are virtually absent during these hours. Cy Lesser, Maryland Department of Agriculture admitted Maryland's spray program is relatively ineffective against the Asian Tiger Mosquito. The concern expressed by Lesser, Chief of the Maryland Department of Agriculture's Mosquito Control Section, was that the Asian Tiger Mosquito is active during the day and "you virtually can't find these mosquitoes at night". ("Virus-bearing mosquitoes invade region", Nancy Bromley McConaty, The Maryland Independent, June, 2000.)

Help Reduce Mosquito Populations

The Maryland Department of Agriculture recommends that homeowners and gardeners help to reduce breeding places for the Asian Tiger mosquito and other mosquito species as well. Homeowners are encouraged to keep gutterspouts clean to allow proper drainage. They are asked to check for trapped water in such items as toys, tarps, tires, and drained pools. Pet dishes and water troughs should be flushed at least twice a week. Even small amounts of water permit the Asian Tiger mosquito to breed.

Gardeners are particularly asked by Maryland public health officials to flush plant container trays regularly, sweeping mosquito larvae over the rim onto the ground. Unused planters should be drained and dried. Other water sources for flower beds and gardens should be checked to prevent the smallest amounts of standing water.