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Other Mosquito Borne Diseases

St. Louis Encephalitis (SLE)
As with West Nile Virus (WNV), SLE is caused by a virus that cycles between mosquitoes and birds, with occasional spill-over into humans and other mammals. Humans are considered a dead-end host for the virus, which cannot be transmitted from person to person. Unlike WNV, the virus has little effect on birds that are infected. SLE affects the central nervous system causing an inflammation of the brain. Most human infections are subclinical (mild and not diagnosed) exhibiting flu-like symptoms. More severe cases can exhibit high fever, nausea, headache, personality changes, paralysis, and in 2-20% of the severe cases, death. The elderly are most susceptible to SLE. An epidemic of SLE within Illinois occurred in 1975, with a few cases reported in the 1980's and 1990's. The mosquitoes that transmit (vector) SLE are the same as WNV, with description under that category.

Other Encephalitis Viruses

LaCrosse Encephalitis (LAC) is caused by a virus that cycles between mosquitoes and small mammals as chipmunks and ground squirrels. Humans are considered a dead-end host. LAC is endemic in Illinois with 5-15 cases per year, primarily occurring in local foci in central and northwestern regions of the state. The majority of LAC cases are mild and subclinical. Less than 1% fatality occurs in cases severe enough to be diagnosed. Children under the age of 16 are most susceptible to this virus. As with other mosquito-borne encephalitis, LAC cannot be transmitted from person to person. The eastern tree hole mosquito, Ochlerotatus triseriatus (formerly Aedes triseriatus) is the vector of this disease. This mosquito normally develops in water filled rot cavities in trees (tree holes), but has adapted well to many man-made habitats as discarded tires, buckets, and other artificial containers. The Ochlerotatus triseriatus adult mosquito lays its eggs singly on the inside wall of the tree hole or artificial container just above the waterline. The adult mosquito is an aggressive biter with feeding all day long, and generally stays within the vicinity of its source. The Ochlerotatus triseriatus over-winters in the egg stage.

Western Equine Encephalitis (WEE) is similar to SLE, cycling between birds and mosquitoes with rare spill-over to the human population. WEE primarily affects horses, and is typically found west of the Mississippi River, but has also been found in Illinois. Human cases severe enough to be diagnosed can have a 2-5% fatality. The primary vector of WEE is Culex tarsalis, and possibly Culex pipiens. The Culex tarsalis, like other Culex spp., lay eggs in rafts directly on the water's surface. The mosquito can be found in sunlit sources with high organic content as ditches and artificial containers. The mosquito seldom travels more than 1 mile from its source, however has been known to travel up to 10 miles. This species has continuous generations each season, and over-winters as adults. Culex tarsalis is uncommon in our area, being far more common further West in the United States.

Eastern Equine Encephalitis (EEE) is another virus that cycles between birds and mosquitoes with horses and humans as dead-end hosts. Human infections of EEE can range from mild to severe, with 50-75% of diagnosed severe cases resulting in death. Fortunately, human cases of EEE are rare with only 150 cases throughout the United States from 1964-1998. No human cases of EEE have been reported in Illinois, although the virus has been found regularly in bird populations. The primary vector of EEE is Coquillettidia perturbans (also known as the cattail mosquito), a mosquito found in wetlands. They lay their eggs in rafts on the water's surface on or near aquatic plants. The mosquito is an aggressive biter, strong flier, and feeds during the evening or night. This species usually has one generation each season, over-winters as larvae, and attach to the stems of cattails or similar aquatic plants to obtain oxygen.

Other Mosquito-Borne Diseases

Malaria is a disease caused by a protozoan parasite transmitted from person to person via the mosquito. At one time, malaria was prevalent in Illinois, with concentration in the southern part of the state. Mosquito control efforts in the 1920's have eliminated the risk of malaria in Illinois, although a mosquito which carries the disease, Anopheles quadrimaculatus, is still commonly found in the area.

Dengue, Yellow Fever, and Chikungunya are diseases caused by viruses that are transmitted by mosquitoes. They are common in the Caribbean, South America, Asia, and Africa, but are unlikely to occur in Illinois, as the Aedes aegypti's and Aedes albopictus's range does not reach our area.

Dog Heartworm is a disease affecting dogs that is transmitted by mosquitoes. It is caused by a roundworm, Dirofilaria immitis. The Culex pipiens and Aedes vexans are common vectors in our area, among many other species. The disease is ongoing and best controlled by prevention. Veterinarians prescribe drugs to prevent the roundworm larvae development in dogs.

Mosquito-borne viruses must be able to multiply and infect the salivary glands of the mosquito. This does not occur when HIV and many other human viruses are ingested by mosquitoes.