Mosquitoes are a problem throughout Ohio. At least 58 different species of mosquitoes are found in the state. Fortunately, only a few species cause annoyance. Nevertheless, their presence affects most everyone. Mosquitoes, whether in the home, yard, park or elsewhere, interfere with work and leisure activities. Some mosquitoes attack pets and even farm animals, causing weight loss and decreased milk production. Others transmit diseases such as malaria to humans, encephalitis to humans and horses, and heartworm to dogs.
Most control is usually directed for relief from bites and the irritating reaction that follows. However, disease transmission is also of importance. From 1979 to 1997, 257 cases of imported malaria were reported in Ohio. All cases were acquired in other countries. Without adequate human surveillance and treatment of these imported cases, malaria could become a serious problem in Ohio because the vector mosquitoes occur here. Three types of encephalitis virus are transmitted by mosquitoes in Ohio: LaCrosse encephalitis (LAC) (formally called California), St. Louis encephalitis (SLE) and Eastern equine encephalitis (EEE). From 1963 through 1997, 829 human cases of LAC were diagnosed in Ohio, including six deaths. LAC is transmitted by Aedes mosquitoes, with animals of the squirrel family being the normal hosts. Aside from acquiring the virus from feeding on infected rodents LAC can also be transmitted transovarially, that is, it can be passed from an infected female to her offspring.
St. Louis encephalitis (SLE) is transmitted by Culex mosquitoes. From 1964 through 1997, there were 441 human cases of SLE diagnosed in Ohio, with 32 deaths. In 1975, 416 cases of SLE occurred, including 29 deaths, in the worst epidemic of SLE in the state's history. Birds are the normal vertebrate hosts for both SLE and EEE.
Eastern equine encephalitis (EEE) is a very rare disease, however, it is the most severe form of encephalitis. The human case fatality rate is about 30 percent and many of those who recover suffer some form of permanent mental impairment. A human case has never been diagnosed in Ohio. Prior to 1991, Ohio had only recorded one case of EEE. This was a fatal equine case. During August and September of 1991, however, EEE was implicated in over 40 equine fatalities occurring in the Killbuck Creek valley of Holmes County and Wayne County, Ohio. EEE was confirmed in 17 of these fatalities. EEE is most common in the Atlantic coast states. Michigan has been the only Midwestern state to have recurring outbreaks. Humans and horses are both considered deadend hosts. Neither circulate sufficient levels of virus to infect additional mosquitoes. Coquillettidia perturbans and Culiseta melanura are the primary mosquitoes implicated in EEE outbreaks.
Canine heartworm is a serious and potentially fatal disease of dogs. The heartworm is a nematode (Dirofilaria immitis) that is transmitted to healthy dogs by several species of mosquitoes. Adult heartworms, which reach a length of 6 to 24 inches, are found in the heart and pulmonary arteries of the dog. They will cause death if left untreated. To reduce heartworm infections, confine dogs indoors during periods of heavy mosquito activity, reduce local mosquito populations or consult a veterinarian for preventative drug treatment.
Despite national and statewide educational campaigns on AIDS, many people are concerned that mosquitoes can transmit the virus (HIV). Evidence gathered by the Centers for Disease Control indicates that arthropod transmission of HIV is not occurring in the continental United States or other parts of the world. Their research indicates that the arthropodborne transmission of HIV is virtually impossible because the amount of blood that could be ingested and carried by the mosquito is too small to hold an infective dose of HIV. If you would like further information on this subject, contact the Vectorborne Disease Unit of the Ohio Department of Health.