Mosquito-borne 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 is transmitted to humans, horses, birds, and other animals by the bite of an infected mosquito during blood feeding. A 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.

To learn more about risk, symptoms and possible vaccines, click HERE.

View the latest West Nile virus statistics HERE.

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.

Where it’s Found

The virus can be imported into an area by wild birds that may be infected elsewhere. These birds show no symptoms, as they serve as reservoirs of the virus. The infected birds are then fed on by local mosquitoes that can pass the virus on to humans through future bites.

The virus cannot be transmitted directly from person-to-person or from birds to people. Most infections are mild and are characterized by headache, fever and nausea. Individuals with weaker immune systems, such as infants and the elderly, are more likely to experience severe illness. Case fatality rates range from 3-30%.



There is no specific treatment for SLE. Antibiotics are not effective against viruses, and no effective antiviral drugs have been discovered.


Since 1964, 4,478 human cases have been reported in the U.S., with an average of 130 cases per year. Southern California experienced its worst outbreak of human SLE cases from 1984-1986. The combined counties of Los Angeles, Orange, Riverside, and San Diego reported 26 confirmed human cases in 1984. In Los Angeles County, 16 confirmed human cases occurred, seven within the District, with two cases resulting in death. In 1985, three confirmed cases were reported, and only one occurred within the District, followed by three more cases in 1986. SLE is endemic to Los Angeles County.

In 2016, SLE was detected in multiple counties in California — Los Angeles, Kern, Orange, Riverside and San Bernardino.

According to the CDC, there have been reports of a correlation between microcephaly and zika virus, which explains the advisories for women who are pregnant or will be pregnant to avoid mosquito bites. Get the latest information from CDC.

Watch for:

  • Fever
  • Rash
  • Joint Pain
  • Red eyes
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 mosquitoes and Asian tiger mosquitoes can transmit dengue; both of these mosquitoes are present in Los Angeles County.

Symptoms include:

  • high fever
  • severe muscle / joint pain
  • mild bleeding

There is no vaccine for dengue. Read more about dengue fever on CDC’s website.

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.

Yellow fever mosquitoes and Asian tiger mosquitoes can transmit chikungunya; both of these mosquitoes are present in Los Angeles County.

Symptoms include:

  • Painful joint / muscle pain that can last for several months
  • Fever

There is no vaccine for chikungunya. Read more about Chikungunya on CDC’s website.

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. In the early stages of infection, dogs and cats appear healthy but can cause damage to internal organs such as the heart, liver, and kidneys. Symptoms are not evident until the advanced stage of the disease. Pets with massive infections can die from failure of the affected organs. The highest incidence of canine heartworm occurs in working and sporting dogs. Dogs and cats that live in areas where heartworm occurs should be regularly checked by a veterinarian. Tests are available to determine if a pet has a heartworm infection. Medications are available from veterinarians to prevent a heartworm infection.

Western equine encephalomyelitis is a viral disease that is transmitted to horses and humans by infected Culex species mosquitoes. The virus invades the central nervous system, including the spinal cord and brain, causing illness similar to SLE. WEE is known to occur in Los Angeles County.

Malaria is no longer an issue for Californians, thanks to vector control. Malaria is an infectious disease of the red blood cells, is caused by a microscopic protozoan (Plasmodium) entering the red blood cells of a human through the bite of an infected anophele mosquito. Symptoms of malaria include fever and flu-like illness. It may cause anemia and jaundice due to the loss of red blood cells.

For most people, symptoms begin 10 days to 4 weeks after infection. Two kinds of malaria, P. vivax and P. ovale can relapse. When these parasites come out of hibernation and begin invading red blood cells, the person will become sick. Malaria can be prevented by taking prophylactic drugs and cured with prescription drugs.

The World Health Organization estimates that world-wide 300-500 million cases of malaria occur and more than 1 million people die of malaria each year. P. falciparum is responsible for many fatalities. About 1,200 cases of malaria are diagnosed in the United States each year.

Most cases in the U.S. occur in immigrants and travelers returning from malarial areas, mostly from sub-Saharan Africa and the Indian subcontinent. Malaria is not endemic to Los Angeles County.

Learn how to stay mosquito-free and reduce your risk of getting sick