Disease Surveillance

The District prevents and controls three vectors: mosquitoes, black flies, and midges. It is important to control mosquitoes to reduce their potential as a nuisance and carrier (vector) of diseases. Diseases, which are of concern in Southern California, are St. Louis encephalitis (SLE), Western Equine encephalomyelitis (WEE), West Nile virus (WNv), malaria, and heartworm to dogs and cats. Several species of mosquitoes occurring within the District boundaries can transmit these debilitating and sometimes fatal diseases to people and animals.

Mosquitoes Known to Occur Within the Greater Los Angeles County Vector Control District

SCIENTIFIC NAME	COMMON NAME	HABITAT	MEDICAL IMPORTANACE
Aedes albopictus	Asian Tiger Mosquito	Lucky bamboo plants in nurseries and man-made containers	Potential vector for dengue fever, WNv,and other encephalitis viruses
Anopheles franciscanus	none	Shallow sunlit pools with algae	Not known to carry disease in California
Anopheles hermsi	Western Malaria Mosquito	Clear pools with matted algae	Malaria vector
Culex erythrothorax	Tule Mosquito	Ponds, lakes, wildlife refuges, and marshes with tules and cattails	Potential vector for WNv
Culex stigmatosoma	Banded Foul Water Mosquito	Polluted water (e.g., industrial and agricultural wastes); prefers to bite birds	Secondary SLE vector
Culex quinquefasciatus	Southern House Mosquito	Polluted water (e.g., septic tanks, dairy drains, catch basins, and underground storm drains)	Vector of WNv; secondary for SLE and WEE
Culex tarsalis	Western encephalitis Mosquito	Agricultural, commercial, man-made or natural sources	Principal SLE, WEE, and WNv vector
Culex thriambus	none	Foothill riparian habitats in sunlit pools along streams and other water courses.	Potential vector for WNv
Culex restuans	none	Found in foul water	Potential vector for WNv
Culiseta incidens inornata particeps	Cool Weather Mosquitoes	Fresh and brackish waters and containers	Not known to carry disease in California
Ochlerotatus sierrensis	Western treehole mosquito	Treeholes (particularly oak), tires, and containers	Canine heartworm vector
Ochlerotatus washinoi	Woodland pond Mosquito	Occurs in floodwater habitats	Not known to carry disease in California

*SLE-St. Louis encephalitis

*WEE-Western Equine encephalomyelitis

*WNv-West Nile virus

Mosquito-Borne Diseases In Los Angeles County

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. 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.

Western Equine encephalomyelitis (WEE): 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 to Los Angeles County.

West Nile virus (WNv): West Nile virus is transmitted to humans, horses, birds, and other animals by the bite of infected mosquito during blood feeding. A mosquito is infected by biting a bird that carries the virus. The virus is not spread by person-to-person contact or directly from birds to humans. Most mosquitoes do not carry the virus, and most people bitten by a mosquito may not be exposed to the virus. Fewer than one out of 150 people who are bitten by an infected mosquito get severely ill. In most cases people who are infected do not become sick or show symptoms. The virus can, in rare cases, cause encephalitis and death. The mortality rate is 3-15% and varies depending on the age and condition of the health of the victim. The elderly are most at risk for severe cases of the disease. There is no specific treatment for WNv. In a serious case, an individual may be hospitalized to ensure good supportive care. A WNv vaccine is available for horses.

Malaria: Malaria, 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 yearly 300-500 million cases of malaria occur and more than 1 million people die of malaria. 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.

Heartworm: Heartworm is a filarial worm disease of dogs and cats that is transmitted by the western treehole 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.

Disease Surveillance

The disease surveillance program serves as an early warning system in the detection of mosquito-borne viruses that can infect people and animals. Testing for the presence of virus/pathogens in mosquitoes, sentinel chickens, and wild birds helps identify the disease transmission cycle before human cases occur.

Mosquito Testing

Disease surveillance is conducted to detect St. Louis encephalitis (SLE), Western equine encephalomyelitis (WEE) and West Nile virus (WNv) transmission with District boundaries. Initial surveillance begins by setting mosquito traps to collect adult mosquitoes for identification. Because they require a blood meal to produce eggs and involved in the disease transmission cycle, only female mosquitoes are submitted for virus testing. Ten to 50 live anesthetized female mosquitoes are deposited into vials to comprise a mosquito pool. These pools are stored at minus 70 degrees Celsius and shipped to the University of Davis Arbovirus Research Unit (DARU) for testing.

New Jersey Light Traps

The New Jersey light trap (NJLT) is exclusivly used to estimate mosquito population densities. A NJLT uses an incandescent light source and fan to attract and then suck mosquitoes into a container. NJLT’s are not used for disease surveillance. Of these traps, 25 were located throughout the District. In March of 2001, several non-productive trap sites were discontinued because they were not capturing sufficient numbers of mosquitoes due to competing light sources in urban areas. Specimens are collected from the traps weekly during mosquito season and biweekly during the winter.

EVS/CO2 Baited Traps

The encephalitis vector surveillance (EVS) traps use dry ice (CO2) as a bait to attract host-seeking female mosquitoes. EVS traps are used mainly to determine whether SLE, WEE or WN virus occurs within a localized area. During mosquito season, traps are set over night once a week between early spring through late summer at several fixed locations throughout the District to capture mosquitoes particularly Cx. tarsalis and Cx. quinquefasciatus, which are the vectors of SLE, WEE, and WNv .

Reiter Traps

The Reiter trap uses a hay infusion media to attract gravid (pregnant) blood-fed Cx. quinquefasciatus mosquitoes. Reiter traps are also important in collecting mosquitoes for virus testing, because the blood-fed mosquitoes are more likely to contain virus. Like EVS, these traps are placed late in the afternoon and collected the following morning. They are situated in close proximity to the EVS trap sites.

Sentinel Chicken Testing

The sentinel chicken flocks also serve as an early warning indicator system for detection of SLE, WEE, and WN viruses transmitted by mosquitoes that can potentially infect people and animals. Blood samples are taken bi-weekly from the sentinel chickens and analyzed by the state health department’s Viral and Rickettsial Disease Lab to determine the presence of virus antibodies, indicating whether mosquitoes in the area have transmitted virus. This early detection allows the opportunity to prevent the occurrence of human infections. The Vector Ecologists maintain seven chicken flocks throughout the District year-round.

Wild Bird Testing

Many wild birds species, such as sparrows and finches are reservoir hosts for the same mosquito-borne viruses that can infect people. For this reason, they play an important role in the District’s disease surveillance program. Blood samples from captured free-ranging wild birds are taken bi-weekly to obtain sera samples and analyzed by the University of Davis Arbovirus Research Unit for the presence of SLE, WEE, and WNv antibodies. The Vector Ecologists also band each captured bird, record biological data, and then release the bird unharmed. Six wild bird traps are maintained year-round in suspect encephalitis transmission foci within District boundaries.

Dead Bird Surveillance Program

Wild birds are the animals from which mosqutioes primarily acquire West Nile virus. Although many birds that are infected with WNv will not appear ill, WNv can cause serious illness and death in some birds. The most severe illnesses are seen among the corvids (including crows, jays, ravens, and magpies). Dead American Crows are the most commonly reported bird infected with WNv.

If you find a bird that has recently died, particularly a crow or other corvid, please call the California Department of Health Services hotline at 1-877-WNV-BIRD. Not all birds will qualify for testing; however they will record all reports of dead birds. To move or discard a carcass, wear rubber or latex gloves. If gloves are not available, use a plastic bag turned inside-out over your hand and invert the bag to surround the bird. Seal and discard in the trash if the bird will not be picked-up for testing.