Mosquitoes...                     West Nile virus info click here!

Twenty-three species of mosquitoes are found in Contra Costa County. Each species differs in behavior, habitat, ability to transmit disease and tendency to bite humans. For this reason we ask homeowners reporting mosquito problems to provide a sample of the mosquitoes that have been bothering them; identifying the species helps us to locate the source of the problem.  In addition to causing annoyance and discomfort, mosquitoes have the potential to transmit disease causing agents including encephalitis viruses, malaria and dog heartworm, all of which occur in Contra Costa County.



                          


Mosquito Biology...
Mosquitoes have four life stages, egg, larva, pupa and adult, the first three of which require standing water for their development. Only adult females feed on blood, in order to obtain protein for their developing eggs. Both females and males feed on sugars from plants as an energy source.

Mosquito Surveillance...
The goal of our program is to prevent mosquito problems before they happen. We monitor mosquito populations throughout the county by regularly inspecting all bodies of water known to harbor mosquito larvae. We also monitor adult mosquito populations through the use of special traps located throughout the county. Weekly mosquito counts at each location are compared with historical averages in order to locate problem areas where control efforts should be concentrated and avoid unnecessary treatments elsewhere.

Mosquito Control...
Our mosquito control program is based on the principle of integrated pest management (IPM). It is most effective to control the larval stages since they are concentrated in a smaller area and cannot fly away. Larval control is achieved through source reduction (elimination of standing water), and through the use of safe, effective biological and chemical control methods chosen specifically to minimize their impact on the environment. Biological control agents include mosquitofish, which feed on the larvae, and bacterial insecticides like BTI which affect only mosquito larvae. Chemical control agents include insect growth regulators and larvicidal oils. Research published by the University of California has demonstrated that high adult populations of certain mosquito species increase the risk of disease transmission to people. Spraying adult mosquitoes is occasionally necessary to protect public health when mosquito populations are unusually high or mosquito-borne diseases have been detected. We currently use pyrethrins  (plant-derived insecticides)  and pyrethroids (synthetic versions of pyrethrins), combined with piperonyl butoxide (a chemical that enhances the effectiveness of other materials) for adult mosquito control.

  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 on to 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 Health Services 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...
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 preventive medication is available from your veterinarian

Other Mosquito-Borne Diseases...
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 travelers 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, any time. Our comprehensive mosquito surveillance and control program is aimed at predicting and preventing similar disease outbreaks in Contra Costa County.

09/04/09