Los Angeles County West Vector & Vector-Borne Disease Control District

Mosquito-Borne Disease Surveillance

    Collecting adult mosquitoes can provide several important pieces of information.   When traps are set at specific locations over a period of time, or in response to service requests, increases in the mosquito population can be detected.  Once these mosquitoes are identified, control measures can then be more easily directed.   Knowing what species of mosquito is breeding can help Vector Control Technicians find the breeding source and take the appropriate control measures.  After identification by District Vector Ecologists, these mosquitoes can also be tested for the presence of disease.  The detection of virus in a mosquito which feeds on humans indicates a true potential for human disease, and immediate control measures can be implemented.  Trap collections not only determine where control measures are needed, but also determine the effectiveness of control measures which are in place.

Mosquito-Borne Diseases

    Several of the 48 known species of mosquitoes in California can carry disease under the right conditions.  When a female mosquito takes an animal blood meal, which she uses as nourishment for her developing eggs, she may transmit certain disease causing organisms to humans and other animals.  These organisms are taken with blood from other infected humans and animals.  The mosquito completes the cycle when she bites the next susceptible host, causing infection.  The two most important diseases affecting humans worldwide are encephalitis and malaria.

    In Los Angeles County, mosquitoes that are capable of transmitting St. Louis Encephalitis (SLE) and Western Equine Encephalomyelitis (WEE) are pooled and sent to the State Viral and Rickettsial Laboratory for testing.

    Currently, the District uses three types of traps to monitor mosquito populations.

New Jersey Light Trap

  
    The New Jersey Light Trap continues to be the most widely used adult mosquito trap in California.  It is an old trap by design and is most effective in rural environments where there is little competing light sources.  Adult mosquitoes are attracted to the all metal trap by a 25-watt incandescent light bulb mounted beneath a wide conical top.   Mosquitoes attracted to the light are drawn into the trap by a downward blowing fan through a screened funnel into a quart killing jar containing an adulticide.   Mosquitoes collected from the New Jersey Light Trap cannot be tested for viruses.   The District rarely uses New Jersey Light Traps for mosquito surveillance because this type of trap performs very poorly in urbanized environments with many competing light sources.  The District primarily uses New Jersey Light Traps for monitoring midge populations.

CDC-Type CO2-Baited Trap

    This trap is used to selectively sample host-seeking females attracted to the trap by the sublimation of dry ice into carbon dioxide (CO2) which simulates the exhaled respiratory gases of birds or mammals.  The trap consists of a central 3" diameter plastic cylinder housing a 6V DC motor and a 4" fan blade, a collection net attached to the bottom of the cylinder, a 6V battery power source, and an insulated container with 2-5 pounds of dry ice.  CDC-type traps utilized by the District also incorporate a mini-light source which help attract mosquitoes as well.   Mosquitoes attracted to the trap are drawn in through the top of the trap and forced downward by the fan into the collection net.  Live-trapped females can be counted and tested for mosquito-borne arboviruses. 

Gravid Trap

   
    This trap selectively samples gravid (ready to deposit eggs) female house mosquitoes (Cx. quinquefasciatus) that are seeking suitable oviposition sites.  The gravid trap incorporates three components: a base reservoir filled with oviposition attractant (hay or manure infusion), a vertically-directed suction apparatus and a top mounted collection carton.  The intake orifice of the suction apparatus is positioned one inch above the surface of the oviposition attractant.  Gravid females attracted by the infusion descend into the base reservoir where they are swept into the suction apparatus and directed upward into the collection carton. 

Sentinel Chicken Surveillance

    Sentinel chicken serology is performed by placing chickens in an area over an extended period of time and testing their blood for the presence of antibodies to SLE and WEE viruses.  The District maintains 15 flocks of chickens located strategically throughout the District. 

    Thewpe1.jpg (6868 bytes) chickens are bled once every two weeks during the months of May through October.   Blood samples are processed and tested in the District's laboratory.  Prior to equipping the District's laboratory, the blood samples were sent to the State Viral and Rickettsial Laboratory.  By performing these tests "in-house", the District is able to sample more often and obtain results within 48 to 72 hours.  Therefore, the presence of viral activity is identified earlier.  Confirmation samples will be sent to the State Laboratory for quality control.  The results obtained from these laboratory tests are used to increase inspections and control measures in areas where viral activity is present.

    It is important to note that the chickens are well cared for at all times.  Only a very small amount of blood is taken from each chicken every other week.  The chickens represent a critical element of the District's surveillance program and help to prevent any transmission of SLE and WEE to the human population.

strips.gif (23078 bytes)Once a small amount of blood is collected, the blood is transferred to a numbered filter paper strip.  This filter paper strip is later used for laboratory testing.

 

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