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Mosquito Borne Diseases and Their Transmission Methods

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The encephalitis viruses cause concern to the people and horses in Texas: West Nile virus (WNV), eastern equine encephalitis virus (EEEV), western equine encephalitis virus (WEEV), and St. Louis encephalitis virus (SLEV). These viruses are normally transmitted from bird to mosquito to bird and sometimes from bird to mosquito to human. EEEV, WEEV, and WNV can also be transmitted from bird to mosquito to horse. When the incidence of any of these viruses increases in the bird populations, it becomes more likely that people and horses will be infected.

WNV: West Nile virus threatens birds (wild and domestic), horses, and people. The vector of WNV in urban Texas areas is the southern house mosquito, Culex quinquefasciatus. Two forms of the virus can affect humans: West Nile fever (WNF) and West Nile neuroinvasive disease (WNND).

EEE virus: A mosquito that breeds in fresh- water swamps, Culiseta melanura, is typically involved in the bird-to-mosquito-to-bird cycle of eastern equine encephalitis virus. Because this mosquito rarely bites humans or horses, other mosquitoes, such as Coquilletidia, probably play a role as a “bridge vector” in transmitting EEE virus to humans and horses.

WEE virus: The main mosquito vectors for the western equine encephalitis virus, particularly west of the Mississippi River, are Culex tarsalis and Aedes dorsalis. Other insects such as the swallow bug (Cimicidae) may also serve as overwintering hosts of the WEE virus.

SLE virus: The primary urban vectors of the St. Louis encephalitis virus are the northern house mosquito (Culex p. pipiens) and the southern house mosquito (Culex p. quinquefas- ciatus), with the latter species causing concern in Texas. The chief vector in rural areas of the western United States is Culex tarsalis; in Florida and potentially in Texas, it is Culex nigripalpus. SLEV is becoming less common in the United States, and research has suggested that the presence of WNV has contributed to this displacement.

VEE and CE viruses: The Venezuelan equine encephalitis (VEE) virus complex and the California encephalitis (CE) virus complex also have been recorded in Texas. The dark rice field mosquito, Psorophora columbiae, is the only mosquito that has been confirmed to transmit VEE from horse to mosquito to horse in Texas. The tree-hole mosquito, Aedes triseriatus, is the primary vector of CE, and tree squirrels are the primary reservoirs. The La Crosse type of California encephalitis is the strain that occurs most often in humans, but only a few cases have been confirmed in Texas.

Zika virus

Zika virus is transmitted to humans primarily by the bite of Aedes aegypti and Aedes albopictus mosquitoes. Unlike WNV, humans (or wild primates) are the primary reservoir for the virus. In addition, the Zika virus may be spread through sexual transmission, blood transfusion, mother to unborn child, and during birth. Most infections, however, occur as the result of the bite of an infected mosquito.

Chikungunya virus

The Chikungunya (CHIK) virus is transmitted primarily by Aedes aegypti and Aedes albopictus mosquitoes. International travelers brought imported cases (patients contracting the disease while out of the country) of the CHIK virus to the United States in 2014. Florida has been the only state to have reported cases acquired locally through the bite of mosquitoes.


Another virus-caused disease transmitted by mosquitoes is dengue, or breakbone fever. The more serious manifestations of this disease are called dengue hemorrhagic fever and dengue shock syndrome. It is transmitted from infected humans to susceptible humans by mosquitoes. In Texas, the mosquito primarily responsible for transmitting dengue to people is Aedes aegypti. Mosquitoes obtain dengue virus from the blood of infected humans during the period from the day before the person has a fever through the third or

Yellow fever

Historically, yellow fever is one of the most feared epidemic diseases in the United States because the mortality rate in humans can reach 85 percent. Although the last case originating in the United States occurred in 1911, it is still regarded as so dangerous that U.S. law requires that cases of yellow fever be reported immediately. In the classical “urban type” of yellow fever, epidemics are the result of transmission from human to mosquito to human. The virus is spread by the yellow fever mosquito, Aedes aegypti.


Although malaria had disappeared as a significant problem in the United States by the early 1950s, it is still one of the most serious communicable diseases affecting people worldwide. The parasites are transmitted by Anopheles mosquitoes. Although at least 16 species of Anopheles occur in the United States, only two species are known to be significant vectors of malaria: Anopheles freeborni and Anopheles quadrimaculatus.

Dog heartworm

Dog heartworm is caused by a mosquitoborne filarial worm (a threadlike parasite) called Dirofilaria immitis. D. immitis is normally transmitted from dog to mosquito to dog. We do not know definitely what the mosquito vectors of dog heartworm are in Texas, but several mosquito species are suspected from the genera Culex, Aedes, and Anopheles.

Mosquitoes can also transmit D. immitis to people, where the parasite usually migrates to the lungs and less often to the heart. It causes a condition known as eosinophilia. Some people also have allergic reactions. Fortunately, human cases of D. immitis are rare. Although dog heartworm is fairly wide- spread in the United States, it is most prevalent along the Atlantic and Gulf Coasts from Massachusetts to Texas and up the Mississippi River Valley to Minnesota.


Information obtained from article written by Sonja L. Swiger, Assistant Professor and Extension Entomologist
Entitled Mosquitoes and Diseases They Transmit

The Texas A&M University System (Texas A&M Agrilife Extension)

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