Surveillance

If the country has an eradication or control program for Aedes aegypti, baseline information on population densities in the affected or adjacent areas should be available. Wherever there is no control program, the distribution of the vector should be mapped, its favorable larval habitats should be determined and adult population densities should be studied by determining land rates and the distribution and examination of oviposition traps.

Initiation of new or additional control activities should be related to the prevalence of Aedes aegypti or virus activity in the immediate and nearby areas. The epidemiologist should be responsible for determining the presence or likelihood of the introduction of diseases.

Recruitment and training of new vector control personnel may be problematic and depends in part on the budgetary priorities of the government. Even with assistance from external sources, tremendous strain is placed on budgets during natural disasters. If paid personnel are unavailable during emergencies, military recruits, school children, boy scouts and other volunteers may be used.

Backup availability of laboratories is necessary for identification of mosquitoes. This may be available from an Aedes aegypti or malaria control service. Universities and other research institutions might have a professional entomologist or student biologists capable of making taxonomic identification. All surveillance programs must have maps, office space, and clerical and other technical and administrative assistants to organize and evaluate field data. Information from the program should be made available to vector control and epidemiological personnel as soon as possible.

Estimates of Larval Populations

Even though adult populations are the most important of the populations of Aedes aegypti, when personnel trained to make species identification are available, the surveillance of larval populations is easier and more reliable than that of adults. The systematic collection of larvae serves to determine the presence, distribution and relative abundance of Aedes aegypti.

Estimates are usually limited to those obtained when the frequency of Aedes aegypti larvae in water-filled containers in the vicinity of occupied buildings is assessed. Larvae can, however, be found in containers in vacant lots and along roadsides.

Surveys can be rapidly performed in several manners. Biases in information gained can be minimized if: (1) blocks to be surveyed are selected by a randomized procedure and then all houses on the selected blocks are investigated, (2) every third (or some other ordinal number) house is systematically investigated, and (3) fifty or more houses per subdivision are placed randomly in the sample.

One or more collectors, who search for and examine all water-filled containers in the vicinity of occupied buildings, take part in surveys for larvae. During the initial survey, the species composition should be determined through the collection for laboratory identification of one larva from each positive container ("single-larva-per-container survey"). Simple inspection or "visual larvae surveys," may suffice thereafter. Each individual who participates in the survey will need the following:

(1) Forms and pencils
(2) A flashlight
(3) A small mirror
(4) A dipper (if possible made of white enamel)
(5) A squeeze-bulb syringe or medicine dropper for transferring larvae
(6) A tea strainer to remove larvae from debris or dark water
(7) Collection bottles and a wax or grease marking pencil
(8) A container or bag to carry the instruments and equipment.

The results of the surveys are usually expressed in terms of one or more of the following indices:

(1) The House Index (or premises Index), in which the percentage of the houses examined and found positive for Aedes aegypti larvae is reported
(2) The Container Index (or receptacle Index), in which the percentage of the water-holding containers that were examined and found positive for Aedes aegypti larvae is reported
(3) The Breteau Index, in which the total number of containers with Aedes aegypti larvae is described per hundred houses.

Criteria for interpreting the probability of the transmission of yellow fever by Aedes aegypti, from the results of such surveys, have been published in the World Health Organization Weekly Epidemiology Record (49, 1971:493500). Urban transmission of yellow fever is unlikely if the Breteau Index is less than five, the House Index less than four and the Container Index less than three. But where these figures are, respectively, greater than fifty, thirty-five and twenty, there is a high risk of transmitted yellow fever. Comparable criteria have not yet been established for dengue, but a similar interpretation of the indices may also be valid. In both cases, the indices of Aedes aegypti, the transmission of virus and the level of immunity in a population are related.

Estimates of Adult Populations

Adult surveillance is particularly appropriate in areas where it is necessary to quickly assess the effect of an emergency adulticiding operation. There are both direct and indirect methods of sampling adult Aedes aegypti populations. The direct methods vary in their degree of difficulty in interpretation of data. Three direct methods may be employed: resting, landing and sheet drop collections.

Resting collections may be recommended over landing counts, especially if dengue has been reported to a substantial extent in the area. Resting collections consist of search for adults in bedrooms, and other rooms in houses, garages and outbuildings. They may also be performed in yards, cemeteries, tires and junkyards. The adults are captured with small vials, hand sweep nets, or mouth or battery powered aspirators. The adults usually rest in shaded places and dark corners on walls, clothing, or mosquito nets, and under tables, chairs, or beds. Aedes aegypti can be found resting throughout the day so there is no restriction on time of day in which they may be collected. The collector should spend a certain standardized period of time, such as 20 minutes, in each house. This allows density to be expressed in catch-per-house and catch-per-man-hour. Mosquitoes are identified by species and sex. Collection stations can be selected at random, or they can be located at predefined sites. It should be remembered that collecting depletes the population and thus, that the same house should not be sampled every day.

Landing counts are made on humans. Thus, a collector can collect either from his own body or that of a second person, if collectors are working in pairs. Trials should be made before initiating this type of survey and the methods to be used should be standardized because individuals vary in attractiveness to mosquitoes. It is recommended that a twenty-minute collecting period be made in each house, between the hours of 0900 to 1100, and that the results be expressed in terms of catch per man hours.

The following items of equipment are required for adult surveys:

(1) Forms and pencils
(2) Flashlight
(3) vials
(4) Aspirator
(5) Sweep net.

If a knockdown aerosol is available, the sheet drop technique is a quick and easy method of obtaining a representative sample of adult mosquitoes inside dwellings. In this technique, a white sheet is spread out on the floor and over the furniture of an occupied room, and then the occupants are asked to leave for approximately fifteen minutes. The room, made as airtight as possible, is sprayed with the knockdown aerosol. After having left the room for ten minutes, the collector enters again to collect all the mosquitoes that have dropped on the sheet.

It is important to make certain that collection procedures are standardized; otherwise, results that should not be compared will cause incorrect conclusions to be drawn about changes in populations. Collections at a simple location should be performed by the same individuals, in a uniform manner, and at the same time of day. The number of houses to be sampled can be decided on-site.

Ovitraps provide an indirect method of assessing the presence and size of the adult Aedes aegypti population after a natural disaster. This method is particularly good for detecting the presence of Aedes aegypti where the density is so low that larvae are difficult to find. Ovitraps cannot be used to effectively measure adult population densities, but if they are used routinely they can indicate changes in the population.

Ovitraps consist of black-enamelled, one-pint glass jars (of 130 mm height and 75 mm diameter). Almost any wide-mouthed glass jar with a glossy black ceramic paint on the outside can be used. Tin cans, beer cans or bamboo pots can be substituted for glass jars, but the same type of container must be used throughout the study. Clean water is added to a depth of two to three cm and a paddle is clipped vertically to the inside of the jar. The paddle should be made with an absorbent material; porous hardboard is recommended, but heavy cardboard, heavy velour paper or cloth can be used. When it has absorbed water, the paddle is an attractive surface on which mosquitoes deposit eggs. The size of the paddle should be standard, for example, 2 cm by 13 cm. The rough surface of the paddle should face the center of the jar. Paddles are usually changed at intervals of five or seven days; however, the exact schedule that should be used depends upon the number of positives identified per collection.

Ovitrap placement

Courtesy Dr. M. Giglioli, Cayman Islands

To obtain the full benefit of ovitrap surveys, the area should be fairly extensively covered. Maps should be consulted for deciding locations in which to place the traps, and visits should be made to the area for selecting the sites. Transects or grids should be used in the survey. It is recommended that sites should be from one to two hundred meters apart, and that the traps are placed within thirty meters of the grid line.

Points to be considered are:

(1) The traps should be placed at or near ground level because females usually fly near the ground
(2) Traps should be visible to the female mosquitoes that fly over them
(3) Traps should not be placed where they will fill by rain
(4) All traps should be placed where children, cats, dogs and other small animals do not have access to them
(5) Ovitraps should be located in partial or total shade, in adult resting places such as shrubbery or junk. Placing them in the rear of a house is preferable to placing them in the front
(6) Females tend to prefer tires to ovitraps, so ovitraps should not be located in tire yards or near other locations where tires are piled.

When the paddles are collected, the water should be changed and the inside of the jar should be wiped clean. Ovitrap jars should be assigned numbers or otherwise marked with an identifying code. Paddles should be dated and should also be marked according to the code. If collectors miss a site, the date will help the laboratory technician in recording information. When transported to the laboratory, paddles may be placed in a plastic bag or wrapped in toilet tissue or other soft paper. Some workers have designed carrying cases similar to microscope slide boxes for the transporting of paddles.

The occurrence, distribution and changes in population density of Aedes aegypti in an area are revealed by the presence of its eggs on ovitrap paddles. All mosquito eggs found on the paddle might not, however, be Aedes aegypti.

If there is a question concerning accuracy of identification of the eggs, they should be hatched so that the larvae can be identified. Some workers recommend counting all of the eggs on the paddle; however, for disaster followup the mere recording of the presence or absence of Aedes aegypti eggs should be sufficient.