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Mosquito-Borne Diseases

Disease Threats in Los Angeles County

To many people’s surprise, mosquitoes can transmit viruses to people and animals living in Los Angeles County. Our disease surveillance program, breeding source control efforts, and public education efforts help reduce the risk of mosquito-borne diseases.

Unfortunately, it is nearly impossible to get rid of every single mosquito since there are so many standing water sources, like buckets or plant saucers, in our communities. Most of the time, mosquito bites result in an itchy, allergic reaction. However, there are cases of people being infected with a mosquito-borne disease. Here are some of the diseases that affect LA County residents in our District.

West Nile Virus

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West Nile virus is transmitted to humans, horses, birds, and other animals by the bite of an infected Culex sp. mosquito during blood feeding. A Culex mosquito is infected by biting a bird that carries the virus. The virus is not spread through person-to-person contact but can be passed through blood transfusions.
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St. Louis encephalitis (SLE)

St. Louis encephalitis is a serious viral disease that is spread by infected Culex species mosquitoes. It is one of several mosquito-borne virus diseases that can affect the central nervous system and cause severe complications and death.

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Zika Virus

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Zika is spread mostly by the bite of the invasive Aedes species mosquitoes. These mosquitoes bite primarily during the day, indoors and outdoors. Zika can be in your body for months. This means, men, women and children are susceptible.



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Dengue fever is only transmitted by mosquitoes from one infected person to another. Unlike West Nile virus, dengue viruses do not need birds or another host to transfer between people. Yellow fever (Aedes albopictus) mosquitoes and Asian tiger mosquitoes (Aedes aegypti) can transmit dengue; both of these mosquitoes are present in Los Angeles County.



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Chikungunya (Chik-gun-GOON-ya) is a painful, but rarely fatal mosquito-transmitted virus. Since 2013, the disease has spread very quickly through South America, the Caribbean, and Mexico. In 2014, there were confirmed reports of U.S. travelers coming back with the virus.


Canine Heartworm

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Heartworm is a filarial worm disease of dogs and cats that is transmitted by the Western treehole mosquito and Australian backyard mosquito. The disease is prolonged and usually fatal due to worm congestion in the heart chambers and vessels. Heartworm symptoms vary with each individual case. Dogs and cats that live in areas where heartworm occurs should be regularly checked by a veterinarian. Medications are available from veterinarians to prevent a heartworm infection.

Currently, there is no local outbreak or transmission of Zika, Dengue, or Chikunguyna in Los Angeles County.