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
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 has one generation each
season, and 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, 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.
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, a mosquito found in
wetlands. They lay their eggs in rafts directly on the water's surface. The mosquito is an
aggressive biter, strong flier, and feeds during the evening or night. This species usually has
one generation each season, and over-winters as larvae attached to the stems of cattails or
similar aquatic plants.
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 the mosquito which carries the disease, Anopheles quadrimaculatus, is still found in
Dengue and Yellow Fever are both 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.
Canine (Dog) Heartworm is a disease affecting dogs that is transmitted by mosquitoes. It is
caused by a roundworm, Dirofilaria imitus. The Culex pipiens and possibly Aedes vexans are the
vectors in our area. The disease is ongoing and best controlled by prevention. Veterinarians
typically prescribe drugs to prevent the roundworm larvae development in dogs.
HIV in Mosquitoes
Mosquito-borne viruses must be able to multiply and infect the salivary glands of the mosquito.
This does not occur when HIV is ingested by mosquitoes. Studies have concluded that
mosquitoes are not a factor in HIV transmission.