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Mosquito-borne Diseases
Mosquitoes cause more human suffering than any other organism with over one million people dying from mosquito-borne diseases every year. Not only can mosquitoes carry diseases that afflict humans, but they also transmit several diseases and parasites that dogs and horses are very susceptible to. These include dog heartworm, West Nile virus (WN) and Eastern equine encephalitis (EEE). In addition, mosquito bites can cause severe skin irritation through an allergic reaction to the mosquito's saliva - this is what causes the red bump and itching. Mosquito vectored diseases include protozoan diseases, i.e., malaria, filarial diseases such as dog heartworm, and viruses such as dengue, encephalitis and yellow fever.

The malaria parasite (plasmodium) transmission by female Anopheles mosquitoes is an ancient disease originating in Africa probably. The term malaria is derived from the Italian, (mal-aria) or "bad air" because it was thought to come on the wind from swamps and rivers. Scientists conducted much research on the disease during the late1880s and early 1900s. Approximately 40% of the world's population is susceptible to malaria, mostly in the tropical and sub-tropical areas of the world. It was by and large eradicated in the temperate area of the world during the 20th century with the advent of DDT and other organochlorine and organophosphate mosquito control insecticides. However, more than one million deaths and over 300 million cases are still reported annually in the world. It is reported that malaria kills one child every 40 seconds. In the United States malaria affected colonization along the eastern shore and wasn’t effectively controlled until the 1940s when the Anopheles mosquitoes were controlled. A resurgence occurred during the 1960s and early 70s in the United States due to returning military personnel from Vietnam. Anopheles quadrimaculatus was the primary vector of the Plasmodium vivax (protozoa) in the United States (Foote and Cook 1959). Antimalarial drugs have been available for more than 50 years and recently scientists in Britain and the United States has cracked the code of the malaria parasite genome, a step that may help boost the campaign against the disease.

Dog heartworm (Dirofilaria immitis) can be a life-threatening disease for canines. The disease is caused by a roundworm. Dogs and sometimes other animals such as cats, foxes and raccoons are infected with the worm through the bite of a mosquito carrying the larvae of the worm. It is dependent on both the mammal and the mosquito to fulfill its lifecycle. The young worms called microfilaria circulate in the blood stream of the dog. These worms must infect a mosquito in order to complete their lifecycle. Mosquitoes become infected when they blood feed on the sick dog. Once inside the mosquito the microfilaria leave the gut of the mosquito and live in the body of the insect, where they develop for 2-3 weeks. After transforming twice in one mosquito the third stage infective larvae move to the mosquito's mouthparts, where they will be able to infect an animal. When the mosquito blood feeds, the infective larvae are deposited on the surface of the skin. The larvae enter the skin through the wound caused by the mosquito bite. The worms burrow into the skin where they remain for 3-4 months. If the worms have infected an unsuitable host such as a human the worms usually die at this point. The disease in dogs and cats cannot be eliminated but it can be controlled or prevented with pills and/or injections. Some risk is present when treating dogs infected with heartworms but death is rare – prevention is best. Of course good residual mosquito control practices reduce the treat of mosquito transmission. Until the late sixties, the disease was restricted to southern and eastern coastal regions of the United States. Now, however, cases have been reported in all 50 states and in several provinces of Canada.

Arthropod-borne viruses (arboviruses) are the most diverse, numerous and serious diseases transmitted to susceptible vertebrate hosts by mosquitoes and other blood-feeding arthropods. All arboviral encephalitides are zoonotic, being maintained in complex life cycles involving a nonhuman primary vertebrate host and a primary arthropod vector. These cycles usually remain undetected until humans encroach on a natural focus, or the virus escapes this focus via a secondary vector or vertebrate host as the result of some ecologic change. Humans and domestic animals can develop clinical illness but usually are "dead-end" hosts because they do not produce significant viremia, and do not contribute to the transmission cycle There are several virus agents of encephalitis in the United States: West Nile virus (WN), eastern equine encephalitis (EEE), western equine encephalitis (WEE), St. Louis encephalitis (SLE), La Crosse (LAC) encephalitis, dengue and yellow fever all of which are transmitted by mosquitoes. Another virus, Powassan, is a minor cause of encephalitis in the northern United States, and is transmitted by ticks. A new Powassan-like virus has recently been isolated from deer ticks. Encephalitis is global, for example; in Asia about 50,000 cases of Japanese encephalitis (JE) are reported annually.

Dengue is a serious disease of Asia and Africa. It has a low mortality with very uncomfortable symptoms and has become more serious, both in frequency and mortality, in recent years. Aedes aegypti and Ae. albopictus are the vectors of dengue. The spread of dengue throughout the world can be directly attributed to the proliferation and adaptation of these mosquitoes. Over the last 16 years dengue has become more common, for example; in south Texas 55 cases were reported in 1999 causing one death. More recently, Hawaii recorded 85 cases of dengue during 2001.

Yellow fever, which has a 400-year history, occurs only in tropical areas of Africa and the Americas. It is a rare illness of travelers anymore because most countries have regulations and requirements for yellow fever vaccination that must be met prior to entering the country . However, over the past decade it has become more prevalent. In 2002 one fatal yellow fever death occurred in the United States in an unvaccinated traveler returning from a fishing trip to the Amazon.

EEE is spread to horses and humans by infected mosquitoes. It is among the most serious of a group of mosquito-borne arboviruses that can affect the central nervous system and cause severe complications and even death. EEE is found in North America, Central and South America, and the Caribbean. It has a complex life cycle involving birds and a specific type of mosquitoes including several Culex species and Culiseta melanura. These mosquitoes feed on infected birds and become carriers of the disease and then feed on humans, horses and other mammals. Symptoms may range from no symptoms to a mild flu-like illness with fever, headache, and sore throat. Infections of the central nervous system lead to a sudden fever and severe headache followed quickly by seizures and coma. About half of these patients die from the disease. Of those who survive, many suffer permanent brain damage and require lifetime institutional care. There is no specific treatment. A vaccine is available for horses, but not humans.

SLE transmitted from birds to man and other mammals by infected mosquitoes (mainly some Culex species) SLE is found throughout the United States, but most often along the Gulf of Mexico, especially Florida. Major SLE epidemics occurred in Florida in 1959, 1961, 1962, 1977, and 1990. The elderly and very young are more susceptible than those between 20 and 50. Symptoms are similar to those seen in EEE and like EEE, there is no vaccine.

LAC is much less common than EEE or SLE, but occurs in all 13 states east of the Mississippi, particularly in the Appalachian region. It was reported first in 1963 in LaCrosse, Wisconsin and the vector is thought to be a specific type of woodland mosquito (Aedes triseriatus) called the tree-hole mosquito with small mammals the usual warm-blooded host. It occurs in children younger than 16 and once again there is no vaccine for LaCrosse encephalitis.

WEE was first recognized in 1930 in a horse in California. It is found west of the Mississippi including parts of Canada and Mexico. The primary vector is Culex tarsalis and birds are the most important vertebrate hosts with small mammals playing a minor role. Unlike LAC it is nonspecific in humans and since 1964 fewer than 1000 cases have been reported As with EEE a vaccine is available for horses against WEE but not for humans.

West Nile virus (WN) emerged from its origins in 1937 in Africa into Europe, the Middle East, west and central Asia and associated islands. Similar to the other encephalitis it is cycled between birds and mosquitoes and transmitted to mammals (including horses) and man by infected mosquitoes. While over 25 species of mosquitoes have tested positive for WN transmission, the Cules pipiens group seem the most common species associated with infecting people and horses. It first appeared in North America in 1999 in New York with 62 confirmed cases and 7 human deaths. Nine horses died in New York in 1999. In 2001 66 cases (10 deaths) were reported in 10 states. It occurred in birds or horses in 27 states and Washington D.C., Canada and the Caribbean. There were 733 horse cases in 2001 in 19 states with Florida reporting 66% of the cases and approximately 33% were fatal. In 2001 more than 1.4 million mosquitoes were tested for WN. As of June 2002, 14 states and Canada have reported positive bird or horse cases in 2002, but no human cases have thus far been reported. It is a central nervous system infection similar to EEE. There is a vaccine for horses. Up to date information from a variety of resources from county, state, federal and international sources may be found at.

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