Mosquito Bourne Diseases



West Nile virus (WNV) emerged from its origins in 1937 in Africa (Uganda) into Europe, the Middle East, west and central Asia and associated islands. It is a Flavivirus (family Flaviviridae) with more than 70 identified viruses. Serologically, it is a Japanese encephalitis virus antigenic complex similar to St. Louis, Japanese and Murray Valley encephalitis viruses. Similar to other encephalitises, it is cycled between birds and mosquitoes and transmitted to mammals (including horses) and man by infected mosquitoes. WNV might be described in one of four illnesses: West Nile Fever might be the least severe in characterized by fever, headache, tiredness and aches or a rash. Sort of like the “flu”. This might last a few days or several weeks. The other types are grouped as “neuroinvasive disease” which affects the nervous system; West Nile encephalitis which affects the brain and West Nile meningitis (meningoencephalitis) which is an inflammation of the brain and membrane around it. (CDC)














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 none at all to a mild flu-like illness with fever, headache, and sore throat. More serious 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. Georgia (08-25-03) reported 2 human EEE cases, three mosquito pools positive and 55 horses cases. Alabama has 2 positive human cases, three mosquito pools and 32 horse infections. Florida has reported EEE activity in 52 of 67 counties and two human cases. Over 200 horses were confirmed with EEE in 50 counties during 2003.
The death is the only human EEE case reported so far in 2003 in Virginia. In Virginia, there have been at least 7 cases of EEE in horses in 2003. Eight cases have been reported nationwide with 4 resulting in deaths. EEE activity has been low thus far in 2004.


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.


Malaria is an ancient disease probably originating in Africa. The malaria parasite (plasmodium) is transmitted by female Anopheles mosquitoes. 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 1880s 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 three million deaths and 300 – 500 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 have cracked the code of the malaria parasite genome, a step that may help boost the campaign against the disease.


Yellow fever, which has a 400-year history, occurs only in tropical areas of Africa and the Americas. It has both an urban and jungle cycle. 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.



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 victims 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. 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; still 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.



Dengue is a serious arboviral disease of the Americas, Asia and Africa and PAHO. Although it has a low mortality, dengue has 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. This year in El Salvador there have been 879 cases of Dengue or dengue hemorrhagic fever (DHF) and 4 deaths. In 2003 Dengue was reported in Bangladesh, India and in the Americas as well as other areas. Brazil reported over 350,000 cases and 42 deaths (ProMed 11-27-03).

In 2004 Venezuela has reported more than 11, 600 cases classic dengue fever and over 700 cases of DHF. Indonesia dengue outbreak has caused over 600 deaths and more than 54,000 cases. Laredo (Texas) reported the 1st confirmed case of dengue in Texas this year. In 1999, Laredo and Nuevo Laredo had an outbreak of almost a 100 cases. Sri Lankan has reported 24 deaths and 4348 cases this year.

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