Mosquito Surveillance and Disease Monitoring

General:  Successes in mosquito control often hinge upon information, which accurately reflects the local abundances of different mosquito species in a variety of habitats.  The activities of many mosquito abatement programs rely heavily on their knowledge of local larval breeding sources and timing and location of adult mosquito hatches.  Awareness of these sources and their characteristics allows TCMA to target larval mosquitoes at the appropriate time of season for most effective control as well as prepare for controlling adult mosquitoes.  This system of surveying and cataloging larval and adult mosquito populations permits an overview of mosquito production, and evaluation of the success of different control techniques, and makes possible a cost-effective review of the personnel and resources required for each task. 

Larval Surveillance:  Teton County is home to 38 species of mosquitoes that occupy a diversity of aquatic environments, including flood-irrigated pasture, seasonal snowmelt pools, and naturally flooded stream banks and marshes.  Teton County Mosquito Abatement surveys thousands of sites each year for larval mosquito activity.  Beginning in March, when early season mosquito larvae hatch (often under the cover of ice) and continuing into November, mosquito control workers visit known and suspected mosquito breeding habitats to collect information and take control measures, when necessary.  At each site, small samples of water are collected with a simple, standardized sampling tool familiarly known as a “dipper”.  The amount of mosquito per dip is averaged and the habitat is described for future index.  In characterizing each site, mosquito control workers record such information as water temperature, type and abundance of vegetation as well as the amount of light and shade.  Sample larvae from a site are returned to the TCMA laboratory where they are identified, and this information along with the complete site survey is entered daily into a GIS mapping program.  Knowing when and where each species of mosquitoes is emerging allows us to more accurately predict future hatches and target similar environments where mosquito breeding is suspected.

Adult surveillance:  In addition to larval surveillance, TCMA maintains a network of mosquito traps, which give us an indication of the relative abundance of the different mosquito species occurring throughout Teton County.  Adult mosquitoes are identified, and knowing which species of adults are present in a certain area can give us clues where to look for future larvicide treatments.  Also, when surveys for adult mosquitoes indicate a large enough population, control using adulticide sprays may be warranted, if conditions permit.

Because of the different effects of mosquito attractants between species as well as other practical constraints, TCMA employs four different types of adult mosquito traps:

New Jersey Light Traps:  These permanently mounted traps use incandescent light to attract mosquitoes and a 110V-powered fan for collection.  They have been used by mosquito control agencies since the early 1930's and are useful for making comparisons with historical data.  They are limited however, since not all mosquito species are equally attracted to light.  Additionally, they are limited to operation between dusk and dawn and are less effective when nighttime temperatures and mosquito activity is lower.

Gravid Traps:  These traps use stagnant water and collection fan and net to capture gravid female mosquitoes that are seeking suitable habitat to deposit their eggs.  They are useful because they attract some species of mosquitoes that are more commonly associated with transmitting West Nile Virus.  They do not attract any host-seeking mosquitoes or gravid mosquitoes, which deposit their eggs on drying earth or clearer water.

Centers for Disease Control CO₂-baited Light Traps:  These highly portable traps rely on a small incandescent light and a plume of CO₂ provided by melting dry ice to lure host-seeking mosquitoes.  They can be very effective collecting host-seeking mosquitoes at any time during the day but they require attention for their maintenance.

Mosquito Magnets:   Mosquito Magnets also use a CO₂ plume to attract mosquitoes, but because they use liquid propane tanks as a CO₂ supply, they require less maintenance.  They have limited portability due to their larger size. 

Disease monitoring:  One of the more surveillance projects is the monitoring of field-collected adult mosquitoes for mosquito-borne viruses. We routinely test samples collected with for adult mosquito surveillance for West Nile Virus.  Presently, we monitor for viral activity using the Rapid Analyte Measurement Platform (RAMP) system, which uses a qualitative immunochromatography assay to detect WNV.  In disease monitoring, our attention is focused on those species of mosquitoes that most commonly transmit WNV between the primary disease reservoirs as well as those associated with bridging transmission to humans.