Vector Borne Disease Control-Mosquitoes

Fite the Bite Brochure (pdf)
Mosquitos and the Diseases they Carry (Slide Show)

Adult mosquitoes are small, fragile insects with slender bodies, one pair of wings, and three pairs of long, slender legs. They vary in length from 3/16 to ½ inch. Only the female mosquito bites, the male does not. The female feeds mainly on blood, but can suck plant juices. Male mosquitoes feed only on plant nectar. Mosquitoes are a type of fly, but they differ from other flies in that they have a long proboscis specially designed for piercing, and scales on the veins of their wings. Crane flies, Mayflies, and other flies are sometimes mistaken for mosquitoes.

Mosquitoes need stagnant water in which to breed. The larvae drown in moving water. The mosquito breeding season starts sometime in May, and ends sometime in September or October, depending on the weather. Mosquitoes cannot survive freezing temperatures, and are killed by a hard frost. They survive the winter mainly as eggs, although some adult mosquitos have been found to survive the winter in sheltered locations. Mosquitoes breed, and lay eggs throughout the warm months.

Mosquitoes go through a complete metamorphosis. After hatching from an egg, mosquitoes go through a larval stage, and a pupal stage before turning into adults. Provided conditions are favorable, the eggs hatch into larvae in two days, or a few hours, depending on the species.

Mosquito larvae about 1/8 to 3/16 inch long, and resemble a small caterpillar or worm. The larvae live under water, but need to breath air. The larvae breath through a short tube which they put through the surface of the water. The larvae normally stay near the surface, but if disturbed will dive deeper into the water with a wriggling motion. The larvae are called "wrigglers" because of the wriggling motion.

After six to eight days the larvae turn into pupae, which look like little brown comas, and are called "tumblers" because of the motion they make when disturbed. The adult mosquito emerges from the pupae in about two days, and will be ready to feed in a day or two. It takes a total of ten to twelve days for the mosquito to become an adult once the egg hatches.

Different species of mosquitoes have different characteristics and breeding habits. Some species have a flight range of less than one half mile, whereas others may range for many miles. Some species live longer than other species, but the adults live two to three weeks on average.

Mosquitoes can transmit a number of diseases through their bite, including dengue fever, encephalitis, malaria, yellow fever, and West Nile Virus (WNV). The mosquito borne diseases which have been found to occur in Ohio are malaria, LaCrosse encephalitis (LAC), St. Louis encephalitis (SLE), Eastern equine encephalitis (EEE), and West Nile Virus.

Two hundred and forty-one cases of malaria were reported in Ohio from 1979 to 1993. All of the cases were acquired in other countries, but malaria could establish itself in Ohio without adequate surveillance and treatment of the disease. Malaria is caused by a very small blood-borne parasite. The symptoms of malaria are periodic fevers followed by chills and shaking.

Encephalitis is an inflamation of the brain caused by a virus. Seven hundred and fifty-seven human cases of LAC were diagnosed in Ohio from 1963 through 1993 with five deaths. Animals of the squirrel family are the normal hosts. There were 441 human cases of SLE diagnosed in Ohio from 1664 through 1993 with 32 deaths. Birds are the normal vertebrate hosts for SLE and EEE.

Eastern equine encephalitis is a very rare disease. However, it is the most severe form on encephalitis. No human case has ever been diagnosed in Ohio. Seventeen equine fatalities occurred in Ohio during 1991, and one equine fatality occurred prior to 1991.

The mosquito-borne disease which has become of greatest concern to citizens is West Nile Virus. WNV is now endemic in Clermont County, that is, it can be found in all parts of the county, and it is here to stay. Nationwide, 43 deaths occurred from WNV in 2011. In contrast, an average of 36,000 people die each year in the United States from influenza.

WNV is spread to people from the bite of an infected mosquito. Most people who are bitten by an infected mosquito will not develop symptoms of WNV. A few people  will develop mild flu-like symptoms such as a slight fever, headache, body aches, and sometimes a skin rash, or swollen lymph glands. Symptoms usually occur 15 days after being bitten by an infected mosquito. A very small number of those people affected by the virus  will develop a severe infection with symptoms which may include the rapid onset of a high fever, a severe headache, neck stiffness, nausea or vomiting, confusion, muscle weakness or paralysis, seizures, and coma. These are symptoms of encephalitis. In rare cases, WNV can result in death.

Everyone exposed to a mosquito that carries the WNV is susceptible, but people at greatest risk are those older than 50. Those who are immune-compromised may also be at greater risk.

In order to diagnose a WNV infection a doctor will need to test either blood or cerebrospinal fluid from a spinal tap for antibodies to the virus. There is no use in testing for the virus if the victim has not developed severe, or life-threatening symptoms of the disease since there is no specific treatment for WNV, and no vaccine for WNV is currently available for humans. There are no antibiotics or antiviral medications that can be used in the treatment of WNV. All care is supportive, which means that one can only try to alleviate the symptoms of the disease.

The virus is not spread by person-to-person contact. It is possible to contract the virus via blood transfusion, by organ transplant from an infected person, through breast feeding, and through the uterus. One cannot contract the virus by eating the meat of an infected animal. There is no evidence that people can get the disease by handling infected animals. However, hunters should wear latex or rubber gloves when field dressing, or butchering game, since infectious agents can enter through open cuts and sores.

WNV is mainly a disease of birds. Over 80 species of birds have been identified with the virus, but not all birds are susceptible to the virus. Crows and blue jays are particularly susceptible to the virus, and suffer a high mortality rate from the virus. Crows and blue jays happen to be in the same family of birds, but there is no known reason why crows and blue jays are more susceptible than others birds. Raptors (hawks and owls) are also more susceptible.

Animals that are bitten by an infected mosquito can contract WNV, but there have been no reported deaths of dogs and cats from the virus. Animals cannot contract the virus by eating another animal, or by eating a bird infected with the virus. Horses are particularly susceptible to the virus.

The best way to protect oneself from WNV and other mosquito-borne diseases is to avoid being bitten by mosquitoes. Avoid mosquito infested areas. Mosquitoes prefer cool, shady areas, and can’t take much direct sunlight. Stay indoors when mosquitoes are most active, which is at dusk and at dawn. Wear light colored clothing, as mosquitoes are attracted by dark clothing. Wear long-sleeved shirts, and long slacks. Avoid perfumes, colognes, and scented soaps as these may attract mosquitoes.

Chemical repellants containing DEET (N,N diethly-meta-toluamide) as the active ingredient are effective in repelling mosquitoes. If a repellant with DEET is used, products that contain 30 percent or less of DEET are recommended for adults, and products that contain 15 percent or less are recommended for children. Repellants with DEET last about four hours, and should be applied immediately before going out of doors. Avoid applying repellents if staying indoors.

Read and follow label directions in using products containing DEET. Use repellants sparingly, and in the weakest concentration that does the job, especially on children. The higher the percent of active ingredients in a mosquito repellant, the more pesticide is absorbed into the body. Children should not be allowed to apply repellents themselves. Avoid applying repellents to the mouth or hands of children.

In outdoor areas, aerosol bombs, smoke pots, and citrinella candles all have limited use. Citrinella plants are of no use for repelling mosquitoes. Sonic repellers do not work. "Bug zappers" and "mosquito magnets" attract mosquitoes into the yard. If these devices are used, they should be placed at the farthest distance from people.

Mosquito populations can be controlled with adulticides, with larvicides, and by the elimination of breeding sites. An adulticide is a pesticide which kills only the adult mosquitoes. A larvicide is a pesticide which kills only the larvae.

Adulticiding is accomplished by spraying or misting an area with a pesticide. Adulticiding is generally not recommended, and is not performed by Public Health. Pesticides can have an adverse affect upon the environment, and have potential health risks which may outweigh the potential benefits. Adulticiding is effective for only a few hours, and must be performed daily. Spraying programs are extremely expensive, and Public Health does not have the resources to implement a spraying program.

Larviciding is a much more effective means of mosquito control, and is recommended by Public Health wherever the elimination of breeding sites is not practical. Larviciding can be performed at relatively low cost, with virtually no adverse environmental impact, or adverse health effects.

The larvicide recommended by Public Health for home use is called "Mosquito Dunks". It can be purchased at hardware stores and garden stores either as a small, doughnut-like cake, or in a granular form. It is non-toxic, and contains a bacteria which is lethal only to mosquito and black fly larvae. The cakes or granules are placed in water where mosquitoes are breeding. One cake will treat up to one hundred square feet of surface area, and will last for thirty days, if it isn’t washed out during a rain.

Public Healthdoes not perform larviciding, and does not distribute "Mosquito Dunks" or other pesticides. Purchasing larvicides, and larviciding is the responsibility of the individual property owner.

The elimination of breeding sites is the method of mosquito control most highly recommended by Public Health, and should be employed wherever it is practical. If a swimming pool has become a breeding site for mosquitoes, then it is better to drain it than to treat it. Swimming pools have not been found to be major breeding sites in Clermont County, however. A neglected swimming pool is unsightly, but may not necessarily have become a breeding place for mosquitoes. If the water in a swimming pool has turned green it is due to the presence of algae, which is harmless. Frogs living in a swimming pool do not create a health nuisance, and are actually beneficial since they eat mosquitos.

There are many swampy areas in Clermont County, especially in the northeastern portion of the county, and it is often impractical to improve the drainage in these areas. Pools of water found on wooded lots have not been found to be favorable breeding sites for mosquitos. Such pools generally contain clear, clean water. Mosquitoes prefer dirty water in which to breed, that is, water with a high organic matter content. Muddy pools of water on construction sites, and pools of water contaminated with sewage are particularly favorable breeding sites, as mosquito larvae feed on bacteria, and small bits of organic matter in the water.

The most favorable breeding sites for mosquitoes are those which are manmade, especially in urban or suburban areas. The best way of reducing the number of mosquitoes around the home, and in the neighborhood is by eliminating these manmade breeding sites. Check around the home for anything which could be holding water, such as tin cans, plastic containers, flower pots, roof gutters, tires, wheelbarrows, tarps, and children’s toys.

Containers which are holding water should be emptied, and either discarded, or turned over. Tires should be emptied of water, and stored so that water doesn’t accumulate in them. If roof gutters are holding water or are clogged, then they should be cleaned. Water in bird baths should be changed at least once a week. Ornamental pools should not be allowed to stagnate, and swimming pools should be tended regularly.

The elimination of breeding sites is an individual responsibility, and it may take a community effort. No government agency can accomplish the task alone.

Clermont County Public Health regulates mosquito breeding sites under the Public Health Nuisance Regulation. Public Health will order the elimination or treatment of breeding sites where they have been identified on private property. Mosquito larvae must be observed in stagnant water before Public Health can take any action. The elimination or treatment of breeding sites is the responsibility of the owner of the property on which the breeding sites are located, regardless of the source of the water. Public Health does not have the legal authority to order someone other than the property owner to either treat, or eliminate standing water on any property.

Resources:

Ohio Department of Health

Centers for Disease Control and Prevention