|Emergency Vector Control after Natural Disaster (PAHO)|
|Part II: Control measures for specific vectors|
|Chapter 5: Aedes aegypti|
Evaluation of emergency control measures is often done poorly or is completely ignored because of inadequate planning and lack of resources and trained staff. If an epidemic is curtailed, or fails to develop, the operation is essentially considered to have been successful.
This practice should not be overly criticized; emergency operations should not be delayed because there has been no opportunity to evaluate them. However, it is always advisable to evaluate control measures, since proper evaluation can save valuable time and money during all stages of emergency operations, and can contribute to the guidelines to be used in future emergencies.
Evaluating entomological control measures provides important information that can be used as the basis for deciding when and in what exact areas insecticides should be applied. It also allows the effectiveness of the insecticides, and of the overall program to be assessed. Evaluation of the procedures used and of the quality and amount of work performed will point to failures and ways in which failures can be remedied.
The same information is needed about the success of control measures taken after natural disasters and other emergencies. Thus, if procedures have already been adopted for evaluating existent vector control programs in the area, their use can be extended to the evaluation of the entomological control measures taken during the emergency.
The population sampling methods that have been described above in regard to surveillance can also be used for the evaluation of chemical control measures. Larval surveys in which the House, Container and Breteau Indices are used, may yield indication of pretreatment and posttreatment changes in the size of larval populations. These surveys are especially helpful if larvicides have been applied. To some degree they can also be used to determine the extent of public acceptance of the treatment, if the presence or absence of sand granules or miniquets is noted. However, in evaluating emergency adulticidal action, larval surveys will show little or no immediate response.
Space spray operations by either ultra-low volume applications or thermal fogs should lower the adult population immediately. Pretreatment and posttreatment comparisons of adult resting or landing collections will show not only the immediate effect of treatment on the population at the end of twenty-four hours; if comparison is made after treatment at two or three day intervals, the results can be used to schedule additional insecticide applications. Similar timed collections made in an untreated area will show the effect of climatic changes upon changes in population densities, or other unrelated fluctuations which may be taking place simultaneously. If there is a trained technician available, it is worthwhile, but not essential, that the technician dissect the organisms to obtain pretreatment and posttreatment parous rates.
Ovitraps can also reflect immediate changes in the adult female population. If sufficient numbers are used, they may reveal population recovery and indicate where there have been misses or weakness in coverage.
Use of bioassays, of insectary-reared Aedes aegypti or other species, is a valuable method of evaluation. Wild-caught mosquitoes may be used, but if they are it will be necessary to adjust the sample size to compensate for lack of uniform age of the sample. Three- or four-day-old, blood-fed females are usually used for adult bioassays, and third or early fourth instar larvae are used for the larval bioassays. The latter are, however, of limited value for the evaluation of space spray applications.
Adult bioassays are performed by placing thirty to one hundred adults in a cage. Excellent, reusable cages can be constructed with galvanized screen wire although it is also possible to use inexpensive, disposable cages that can be made with paper cups, cardboard tubes, or wire frames covered with a fine mesh fabric such as tulle. The cages should be placed at thirty to one hundred meter intervals across the path of an aerial swath, or at right angles to the path of ground equipment, in the direction of the spray. One hour after exposure, the cages are collected and the insects are transferred to clean holding cages. There they are given food and held for a 24 hr. mortality count. It is usually true that the closer the source of spray, the higher is the mortality rate. Mortality rates should be plotted against site of the cage. The results should give indirect indication of mortality rates of the natural populations, and of swath width, unsatisfactory coverage and other breakdowns in application.
Immediately following a natural disaster, or during a vector-borne disease epidemic, the possibility of insecticide resistance is sometimes forgotten. The World Health Organization has kits for testing the susceptibility of adult and larval mosquitoes to insecticides. If these kits are not available, field bioassays of various available insecticides can be performed. In the Americas, there is resistance of Aedes aegypti to organochlorine insecticides, and in certain areas tolerance to some of the organophosphate insecticides may exist. Even where there are no routine vector control programs, the use of agricultural and household pesticides may increase the potential for resistance development.
When ultra-low volume equipment is used, it may be necessary to calibrate dosage and determine the droplet size. The technical brochures provided by the equipment or insecticide supplier should contain information about these procedures.