VIRGINIA MOSQUITO CONTROL ASSOCIATION
MOSQUITO TRANSMITTED DISEASES
Encephalitis Viruses and Heartworm
EASTERN EQUINE ENCEPHALITIS:
Eastern Equine Encephalitis (EEE) is generally the worst strain or type encephalitis. The mortality rate is 30 to 60 percent in humans and even higher in horses. It is especially bad in children. Fortunately, large and widespread outbreaks are uncommon; the twenty-four years between 1961 and 1985 produced only 99 confirmed human cases in the United states.
EEE occurs in late Summer and early Fall in parts of Canada, Northcentral U.S., along the Eastern and Gulf coastal States, and some down into Central and South America.
The life cycle of EEE: The virus circulates in the wild bird populations by Culiseta melanura and other bird-feeding mosquitoes. It is believed to be transmitted to humans, horses and other mammals by Bridge Vectors or mosquitoes that bite birds and mammals. There are 23 species that are believed to be capable of transmitting EEE to either birds, reptiles or mammals. Some believe that the strongest Bridge Vectors involved in the infection of man or horse with EEE are Ochlerotatus sollicitans, Coquillettidia perturbans, Oc. canadensis, Aedes vexans and Anopheles crucians.
There is a vaccine that will prevent EEE in horses.
The latest human EEE cases confirmed in Virginia occurred in Portsmouth, VA in 1998...it was fatal and another fatal case in Glouchester, VA during the 2003 season.
ST. LOUIS ENCEPHALITIS:
St. Louis Encephalitis (SLE) usually produces 3 to 20 percent mortality rate and most cases occur in late summer. It is worse in older people. It can occur in most of U.S. and may be in large epidemics.
The St. Louis area had an epidemic in 1933 with a total of 1,095 cases and over 200 deaths. In 1975-76 more than 2,000 cases were reported in thirty States. We in Virginia have had a number of cases over the years...one case in 1975...3 cases in Richmond in 1976...one case in Richmond in 1977...one case in 1990...the latest was one case in the Petersburg, Va. area in 1997.
SLE is transmitted by Culex quinquefasciatus in central and southeastern U.S. and by Culex nigripalpus in southeastern U.S.
LACROSSE ENCEPHALITIS:
The LaCrosse Encephalitis (LAC) is one of the California group encephalitis viruses that affects primarily children. The mortality rate is less than one percent but can cause seizures and may retard development up to five years.
The states of Minnesota, Wisconsin, Indiana, Ohio and West Virginia have the most reported cases but the western region of Virginia and some other southern states also have cases.
Most cases occur during July, August and September. LAC is transmitted to humans by Aedes triseriatus a tree hole and man-made container breeding mosquito. The adult female Ae. triseriatus may transfer LAC to her offspring through the egg. Amplification of the virus also takes place through an Ae. triseriatus and chipmunk and/or squirrel cycle.
WEST NILE FEVER, WEST NILE VIRUS or WEST NILE ENCEPHALITIS:
The West Nile Virus (WNV) was identified for the first time in U.S. in New York City in 1999. The virus caused encephalitis in 62 people and numerous horses in the New York City area, resulting in seven human, ten equine and many bird deaths. The virus continued to spread in 2000 and at the end of the year had been found in twelve states and the District of Columbia. Seven dead crows tested positive for WNV in Virginia and one in N.C. The West Nile virus will probably eventually spread throughout the eastern United States or maybe the entire nation.
The mortality rate in the U.S. has been about 5 to 10 percent. It has been compared to SLE in that it is more serious to older patients.
The limited two year observation of WNV in the U.S. has provided some of the following information:
- Theory...WNV entered the U.S. either by an infected bird that was brought in or migrated to the New York City area or a human infected by the virus in another country traveled to New York area. (The bird theory is believed to be the strongest)
- The WNV has been isolated in 40 mosquito species world wide...It has been isolated in 20 species in the U.S. (these 20 species are listed near the bottom of the page "CASES OF MOSQUITO TRANSMITTED DISEASES IN VA.")
- The WNV in the United States causes a bird disease and is transmitted by mosquitoes. Birds appear to be the reservoir animal for the virus, which is transmitted by the bite of a mosquito from bird to bird and to other dead-end hosts, such as humans and horses. There is no evidence to indicate that the disease is transmitted from one animal to another through direct contact, except crows roosting together and perhaps birds of prey feeding on infected birds.
- 21 Mosquito Species that have tested positive for WNV: Aedes albopictus, Ae. cinereus, Ae. vexans, Anopheles punctipennis, An. quadrimaculatus, Coquillettidia perturbans, Culex pipiens, Cx. restuans, Cx. nigripalpus, Cx. quinquefasciatus, Cx. salinarius Culiseta melanura, Ochlerotatus canadensis, Oc. cantator, Oc. japonicus, Oc. sollicitans, Oc. trivittatus, Oc. signifera, Psorophora columbiae, Ps. ferox and Uranotaenia sapphirina ...Oc. japonicus is a relatively new species to the United States but is spreading fast. It has gone from Long Island, N.Y. area to Virginia in just three years.
HEARTWORM DISEASE:
The heartworm disease is caused by Dirofilaria immitis, worm-like parasites that are better known as heartworms. The disease may occur in your pet dog or cat. It is a very serious disease. Heartworms interfere with the normal flow of the blood from the right side of the heart to the vessels serving the lungs. If left untreated, heartworm disease can reduce your pet's quality of life, cause congestive failure of the heart and other organs, and ultimately lead to death.
The life cycle of the heartworm.
The tiny immature heartworms are transmitted through the bite of a mosquito. At frist, the parasite move into your pet's tissues, eventually they occupy the heart and blood stream. When other mosquitoes bite your pet they pick up these new immature worms and transmit them to the next dog or cat that they bite.
Symptoms
Your pet may become listless, tire easily after exercise, develop an occasional to persistent cough, and become anemic. In advanced cases, pets often suffer congestive heart failure, complcations may develop in the liver and kidneys. The blood supply to the lungs and other major organs may become blocked.
Treatment
The treatment of heartworms is expensive. The best method is preventive medicine to prevent an infection. You should take your pet to a veterinarian for examination and recommendation for the proper preventative medicine.