|Emergency Vector Control after Natural Disaster (PAHO)|
|Part II: Control measures for specific vectors|
|Chapter 7: Culex quinquefasciatus and other pest mosquitoes|
There will be little available baseline information about pest mosquito densities unless it has been collected because of a tourist industry or in a municipal mosquito control program. The pest mosquito population density immediately after a natural disaster may be low as in the case of Aedes aegypti. Changes in the environment which occur both during and after a disaster may, however, favor rapid population increases.
Mosquitoes that breed in certain habitats may increase in number within one month after a disaster. These habitats are salt marshes, bogs, fresh-water swamps, mangrove swamps, sewage effluents, semi-permanent ponds, woodland pools, artificial containers, ditches, irrigation wastes, water impoundments, rice fields, and natural containers such as tree holes, rock holes, and crab holes. Although problem species will differ greatly from area to area, they should not go unidentified. Surveillance, if meaningful, must include knowledge of the species and habitats of all mosquitoes encountered.
Checklists of the mosquitoes and other biting insects that have been collected in the area may exist in universities or libraries. Basic information about flight range, host preferences, life cycle, larval habitats, adult resting places and specific control methodology, may be obtained from the literature. Topographical maps and aerial photographs assist in locating potential problems, as they do in the case of Aedes aegypti. In areas where there has been some type of environmental management such as diking for purposes of mosquito control, reconnaissance flights provide valuable information about the condition of the control method. (Maps and photographs can also be used to locate sampling stations for use in evaluating population densities.) If topographical maps or photographs do not exist or do not produce the information required, taking simple photographs and making maps while in flight can help in orientation and location of breeding places. Such flights can also be of service for planning chemical control and drainage operations.
Culex quinquefasciatus breeds in highly polluted water. Rapid increases in populations might occur where pit latrines have been constructed as a temporary measure after a disaster has disrupted normal sewerage systems. Refugee camps should, therefore, be examined regarding the placement of latrines, open dumping of garbage in flooded areas, and the existence of any other standing water (especially artificial containers) that might become polluted.
Population sampling can generally be accomplished at weekly intervals in marshes, swamps and impounded water habitats. Biweekly collections of Culex quinquefasciatus may be necessary. It must be emphasized, however, that these mosquitoes are unlikely to be a medical problem and sampling should be considered only if staff and resources are already available. Control of most Culex quinquefasciatus habitats can be effected without a great amount of surveillance. Complaints from the refugee or resettlement sites may provide sufficient surveillance.
Estimates of Larval Populations
In initial surveys, the collector must assume that wherever there is standing water there are mosquito breeding sites. As the collector becomes familiar with the area, species and preferred larval habitats, the observations can be refined. Entomologists can assist in identification of the specimens.
Larvae are collected with a white enamel dipper, white enamel pans or other water containers or a siphon. Flashlights or mirrors can be used to illuminate breeding places and to locate water and evidence of breeding. Medicine droppers and other types of pipettes can be used to transfer larvae from the collecting device to vials and bottles. For convenience, dips are usually done in multiples of ten, and inspections are made either weekly or every two weeks.
Estimates of the Adult Population
Adult surveys must be designed to show a relationship between breeding sites and the human population. Changes in population densities observed through one or more collecting methods can be used to determine the need for adult control measures, their effect, the extent of the mosquito problem and the possible arboviral trouble spots. As with all surveys, it is essential to have maps of the area, well designed forms for collecting data, use of standard collecting methods and sites, and a well organized and trained staff. Under some conditions, it is possible to use the adult collections to attempt to isolate arbovirus. This should be considered only if there is an indication of Venezuelan, eastern, western, or St. Louis encephalitis or other arbovirus activity in the vicinity of the disaster. There must be a laboratory that can handle the isolation attempts, as well as the necessary field and laboratory entomological equipment, and trained staff available to carry out this type of work.
There are a number of methods that can be used to collect mosquitoes. The following is a listing of possible methods:
(1) For landing/biting collections in which either animals or man are used:
(a) Determine the biting habits of the vectors
(b) Standardize the time of day and the location of collection, the length of collecting period, and the type of bait used
(c) When a vector-borne disease problem is known or the population densities are high, employ landing rates.
(2) For window traps (used if time and staff permit):
(a) Use either entry or exit traps (the latter are used in some
malaria entomological evaluations)
(b) Employ entry traps (non-blooded mosquitoes only) for virus isolation attempts
(c) Operate during the night when the bait is in the shelter
(d) Standardize the number of collections per unit of time.
(3) For animal bait traps:
(a) Use traps of sufficient size and strength to hold the bait
animal comfortably and to permit easy entry and removal
(b) Operate at night and standardize the time at which the bait is placed in the trap and removed (even early morning temperatures can be uncomfortable for a caged animal)
(c) If the specimens collected are to be used for virus isolation attempts, use traps that are designed to separate the caught mosquitoes from the bait
(d) If a carbon dioxide trap is also desired, use smaller bait traps like the Lard Can Trap designed by Bellamy and Reeves (Mosq. News 12: 256258).
(4) For light traps:
(a) Choose from several types, the two most popular of which in
the Americas are the New Jersey and the Centers for Disease Control miniature
light traps, or modifications of them, and if there is no electricity, consider
use of battery models
(b) Remember that light traps attract both males and females from considerable distances, and that collection with the CDC light trap can be enhanced with a supply of carbon dioxide or by hanging dry ice nearby
(c) When locating or positioning light traps, take care not to place them in competition with other light sources
(d) Remember that collections can be made of live mosquitoes by attaching a mesh bag, and of dead mosquitoes by attaching a killing jar; and that live collections can be used for virus work
(e) Run traps on a schedule and, preferably, at established sites
(f) Take care in handling and separating material, since many other insects are also attracted to light traps.
(5) In regard to natural and artificial resting stations, it is useful to know that:
(a) Many mosquitoes (especially some species of Anopheles and
Culex) seek out dark, cool humid places to rest during the day
(b) It may take some searching to find natural resting places for different mosquitoes; buildings, especially unscreened ones that shelter man and animals, bridges, culverts, caves, and hollow trees, may serve as resting sites; collections can be made with an aspirator
(c) Artificial resting boxes have been used with some success, but during emergencies that follow disasters their use might not have to be considered
(d) Large mechanical aspirators and vacuum devices may be used for collecting mosquitoes that are resting in vegetation.
(6) Use of the sheet drop technique, described in Chapter 5, is questionable in disaster situations.
The interpretation of larval and adult surveys depends upon the baseline data that are available and the types of vector-borne diseases found in the disaster stricken area. Pest mosquitoes will cause a great amount of discomfort in many postdisaster situations. However, priorities have to be weighed before funds and staff are committed to controlling them.