Mosquito & Mosquito-Borne Disease Surveillance

Mosquito surveillance 

Modern pest management requires surveillance data in order to plan and evaluate control work. Field surveys are the foundation of an effective program. Data on the mosquitoes with the greatest potential to adversely affect public health and comfort, including data on mosquito density and distribution, are essential in order to plan and conduct effective control measures. 

In Maryland, the species occurring in the greatest abundance, or that have the greatest impact on human comfort, economic growth and public health are Ochlerotatus sollicitans, Ochlerotatus taeniorhynchus, Aedes albopictus, Aedes vexans, Ochlerotatus canadensis, Anopheles crucians/bradleyi, Anopheles quadrimaculatus, Culex pipiens, Culex salinarius, Coquillettidia perturbans and Psorophora columbiae.

Several Aedes and Ochlerotatusspp. and Coq. perturbans are implicated in the transmission of eastern equine encephalitis to humans, equines and ratites. 

Dog heartworm in canines is the most common mosquito-transmitted disease in Maryland, with several vector species. Culex species are important vectors of West Nile encephalitis and St. Louis encephalitis. 

Aedes albopictus, popularly known as the Asian tiger mosquito, is a recent introduction to the Maryland mosquito fauna. It was introduced into the United States from Asia by the international used-tire trade and has spread throughout much of the country via interstate tire shipments. This species has become a major pest in several urban areas of Maryland and is a vector of West Nile encephalitis. 

The newest addition to the mosquito fauna of Maryland is Aedes japonicus, first found in Frederick County in 2000. Ae. japonicus is also a beneficiary of the international tire trade and an excellent vector of disease.

Larval surveillance

Larval surveillance requires extensive logistical preparation due to the size and remoteness of many breeding areas. The largest expanse of breeding area exists in the southern Eastern Shore region of Maryland. In general, mosquito larval habitat is most widespread in the coastal plain. High-level aerial photography and satellite imagery are used for rural areas to locate potential breeding areas based on soil and plant community associations. In urban environments, Ae. albopictus and Cx. pipiens are found in obscure breeding sites in yard to yard searches.

Potential breeding areas are subject to "ground truthing" to confirm the location of the sites and measure the presence of mosquito larvae. Confirmed breeding areas are mapped on United States Coast and Geodetic topographic maps (scale 1:24,000), or larger scale city maps, and used as a reference for future surveillance. Frequent and regular inspections of breeding areas are carried out from March through September to determine larval density and species composition. 

Larval surveillance data are used to guide the course of the control program and are the most important field data available to the mosquito control manager. This information is used in the decision-making process of whether or not to initiate control efforts, what type of control to employ and in evaluating the effectiveness of the control.

Identifying & prioritizing breeding sites

Breeding areas that consistently produce large numbers of important pest or vector species and are located within flight range of a community are considered as potential sites for control. 

Several Aedes and Ochlerotatus spp. are capable of long distance (up to 20 miles or more) dispersal flights. Other species, such as Ae. albopictus, Oc. canadensis, and Coq. perturbans are weaker fliers (1-5 miles). Culexand Anopheles mosquitoes have a much more restricted flight range than do most Aedes and Ochlerotatus mosquitoes and breeding areas for the former species more than two miles from a community are not considered for control. 

All breeding sites within a community, or close to the perimeter of a community, are designated as primary control sites for control of mosquito larvae.

How weather affects breeding sites

Meteorological conditions significantly affect breeding sites. 

A prolonged drought will evaporate the surface water from a wetland and temporarily halt mosquito larval production. However, a drought will also eliminate the population of mosquito predators and exacerbate mosquito breeding, particularly in areas producing salt marsh Ochlerotatus species, when surface water returns. 

A summer with frequent rainfall and/or flooding tides will maintain a higher water level, which usually favors larval production of Anopheles and Culex species, but may also boost the population of predators and eliminate mosquito production. 

Therefore, a good larval surveillance program cannot be static. It must be dynamic and flexible to accommodate fluctuations in weather and land use patterns.

Adult mosquito surveillance

Surveillance for adult mosquitoes is conducted using traps and landing rate counts. Traps are an easy, relatively low cost way to obtain an index of the number of mosquitoes in an area, but, data are subject to wide variation. Trap data provide a good historical record, if trap locations are constant from year to year. Trap collections are time-consuming to process and the information on mosquito abundance is often delayed for several days after the catch is made and of reduced value from the standpoint of operational control. For arboviral surveillance, traps utilize carbon dioxide as an attractant to increase trap capture rates..

In Maryland, a trap designed by the Communicable Disease Center (CDC) is the trap type generally used to monitor pest and vector adult mosquitoes. The CDC trap is battery-powered and easily portable. For pest surveillance, the trap uses no source of attraction other than a small light. A collection of more than 10 anthropophagous (human biting) female mosquitoes per night of trap operation is considered to be the level which causes discomfort and/or complaints from the majority of people. The light trap action threshold for ground spraying of adult mosquitoes is 10 per trap night. The action threshold to suppress pest populations of adult mosquitoes by aerial spraying (application of insecticide by an aircraft) is a light trap collection of 100 female mosquitoes.

Landing rate counts

Landing rate counts provide immediate indicators of adult mosquito activity. The counts are taken for a short period of time at specific, predetermined locations. Inspectors serve as "bait" to attract mosquitoes which attempt to blood feed. A count is made of the number of mosquitoes landing on the readily visible portions of the inspectors' bodies, below the waist, during a two minute interval. 

Landing rate counts for salt marsh Ochlerotatus species and Ae. albopictus can be taken during daylight because these species will actively attempt to blood feed at that time. However, most other species, particularly Anopheles spp. and Culex spp., must be sampled during twilight periods, or at night, because of their nocturnal activity. 

The action threshold for landing rate counts to justify ground spraying for the control of adult mosquitoes is 3 mosquitoes in 2 minutes. The action threshold for aerial spraying is 12 mosquitoes per minute.

Mosquito-borne disease surveillance

MDA and the Maryland Department of Health (MDH) collaborate to conduct a surveillance program to detect mosquito- borne viruses of public health concern. 

The program monitors the occurrence of Zika Virus, eastern equine encephalitis (EEE) virus, West Nile virus (WNV), and St. Louis encephalitis (SLE) and other viruses in mosquitoes, wild birds, domestic animals and humans. These viruses are maintained in nature in mosquito cycles. 

Isolation of viral presence in the mosquito cycle provides an early warning of virus transmission and is cause for increasing public awareness campaigns to reduce risk of disease and to take proactive steps for mosquito control to further reduce the risk to humans, domestic animals and zoo animals. 

West Nile virus has affected every region of Maryland and is now considered as being endemic throughout the state.