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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.
The
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.
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.
