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Mosquitoes and Disease

Some species of mosquitoes are involved in the transmission of important disease-causing agents (pathogens). In California, these diseases include encephalitis viruses, malaria and dog heartworm. These pathogens are specifically adapted to transmission by mosquitoes. Most pathogens of humans, such as measles, hepatitis or HIV (AIDS) viruses cannot develop in mosquitoes. Western Equine Encephalomyelitis (WEE), Saint Louis Encephalitis (SLE), and West Nile Virus (WNV) are viruses transmitted by mosquitoes which can cause a serious and potentially fatal inflammation of the brain in horses and humans. Wild birds serve as a reservoir host. Mosquitoes can pick up the virus by biting infected birds and later transmit it to humans and horses, which are “dead end hosts” (cannot pass the virus onto others).

As part of a Statewide Arbovirus Surveillance Program, the District maintains five flocks of sentinel chickens, located near Hercules, Martinez, Oakley, Walnut Creek and Knightsen, to monitor for mosquito-borne encephalitis viruses. Blood samples are taken every two weeks from May through October and submitted to the California Department of Public Health for testing. Chickens are not harmed by the virus. In addition, mosquitoes collected in our traps are submitted for testing. Detection of the virus leads to increased control operations in the affected area. The goal of our mosquito control operations is to keep adult mosquito populations below the level where transmission of disease to humans becomes likely. WEE was detected in sentinel chickens in Contra Costa County in 1993, 1994, and 1997, although no human cases were reported.


Dog heartworm is a roundworm infection transmitted to dogs (and sometimes cats) by the Western tree hole mosquito. This mosquito develops in water-filled holes in oaks and other trees and is abundant from March through June. It is a very difficult mosquito to control since it is impossible to check every tree in the county for larvae. Filling in tree holes on your property with sand or other material, and eliminating clogged rain gutters and water-filled containers can help to reduce the problem. Coyotes and foxes are reservoir hosts for dog heartworm. Once the adult worms become established in the heart chambers of a dog they are very difficult to eliminate and can cause circulatory problems and eventually kill the animal. Fortunately a preventative medication is available from your veterinarian.


Other mosquito-transmitted diseases occasionally found in our area include malaria, dengue fever, and yellow fever. Although these diseases are not currently established in our county, infected travellers occasionally return from trips abroad and could transmit these diseases to others through vector mosquitoes. We work together with the State and County health departments to reduce the likelihood of local transmission. The rapid spread of West Nile Virus across North America illustrates that new mosquito-borne diseases can be introduced anywhere, anytime. Our comprehensive mosquito surveillance and control program is aimed at predicting and preventing similar disease outbreaks in Contra Costa County.