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close this bookEmergency Vector Control after Natural Disaster (PAHO)
close this folderPart II: Control measures for specific vectors
close this folderChapter 6: Anopheline vectors of malaria
View the document(introductory text...)
View the documentSurveillance of Malaria
View the documentAnopheline Control
View the documentEvaluation of Control Measures

Surveillance of Malaria

Malaria surveillance can be directed toward the detection of human cases or toward changes in the mosquito population. In malaria control programs, case detection is of greater priority.

Epidemiological Surveillance of Human Cases

Most malaria programs entail both active and passive case detection. These activities include the taking of blood slides by either voluntary collaborators or program staff members, who make house-to-house visits following established procedures, and at clinics, hospitals and health offices. The information should be studied that is yielded from the yearly and monthly blood slides made in all areas directly or indirectly influenced by the disaster.

Once the eradication program of a country is in the maintenance phase, there are certain circumstances in which there is great potential for reestablishment of transmission. The threat of transmission is posed when there are a large number of imported cases of malaria, suitable environmental conditions and a relatively high abundance of anopheline vectors.

Areas within the range of disasters of potential risk of malaria transmission can be delineated according to the above factors. Malaria surveillance should be upgraded after a disaster. If there is relocation of populations or changes in community life and activities, this may involve considerable reorganization. Voluntary collaborators should be on the lookout for a sudden rise in the number of fever cases. All government and private medical facilities should also be alerted, and activities undertaken in the field and in laboratories should be evaluated.

An alert of this type is always capable of overtaxing the capacities of laboratory facilities. When there is great concern that a malaria epidemic may occur, there should be an attempt to increase the size of laboratory staff or that of any other laboratory with qualified technicians, such as those of medical schools, hospitals and private clinics. It should be noted, however, that redirection of slides will cause additional logistical problems for the epidemiologist who is gathering statistics.

The epidemiological program may face a number of other problems. Census data and maps may be inaccurate because of the movement of families after a natural disaster. Officials in the malaria control program should establish surveillance systems in new settlements which allow them to correct such inaccuracies.

Officials of the epidemiological surveillance service should know which malaria parasites are present in a community, and should monitor any changes in prevalence. An evaluation should be made of the changes in risk, and to indicate the existence of areas or populations in which complimentary control measures are required. Control tactics encompassing chemoprophylaxis, case detection and treatment, and vector control activities should be developed. An appropriate monitoring system should exist in which staff members are alerted to necessary changes regarding the timing of their strategy. Both the stocks of antimalarial drugs and the ordering of new supplies should receive periodic review.

In the interval between the onset of disaster and the period of possible increase in malaria infections, the director of the malaria control program should ensure the complete reestablishment of the full surveillance operation, with voluntary collaborators, field staff and health services. This will entail the provisioning of adequate supplies of antimalarial drugs for both prophylaxis and case treatment.

Epidemiological and entomological vigilance should be intensified and pertinent data should be displayed on a large, schematic map which usually facilitates the assessment of areas that require priority attention. The extent and the distribution of both confirmed and suspected cases of malaria should be shown. The major agricultural growing area, and the areas of high-risk of disease transmission should be delineated according to three factors. These are the size and distribution of vector populations, increases of larval breeding sites, and the presence of potential disease reservoirs.

The epidemiologist should meet with members of the program's vector control and entomology staff to study the increases that may have occurred in malaria infections and changes in vector population densities.

Entomological Surveillance

Entomological surveillance in malaria control programs has not historically received adequate attention. The entomological surveillance system will thus probably be less effective after a natural disaster than the epidemiological system. However, vector control personnel may have been involved in the evaluation of antivectorial measures and may have valuable information about the insecticides and the vectors. Specifically, they may know about the state of insecticide susceptibility, duration of residual effect, and spraying cycles. Concerning the vectors, they may have information about the delimitation of seasonal and geographic aspects of vector influence, the habitats and behavior of primary and secondary vectors, and the vectorial capacity of these mosquitoes.

The foundation of most malaria control programs lies in the activity of vector control personnel. Consequently, it is members of the vector control staff who will have maps, be able to provide up-to-date information about insecticide treatments, and have a thorough knowledge of the communities. They will also know the epidemiological situations (attack, consolidation, and maintenance) of various areas, and thus they make plans for emergency control accordingly.

If there are entomological and vector control personnel available in the malaria program, a postdisaster survey should be taken in the suspect endemic area. The survey should entail gathering the following information from those potential risk areas:

(1) Location of larval site sampling stations, classified according to future productivity of vector species and plotted on contour maps

(2) Adult mosquito densities determined by:

(a) human and/or animal collections
(b) resting and/or pyrethrum knockdown collections
(c) light traps
(d) other methods of collection that can be undertaken if there is available staff and time and equipment.

(3) Anopheline species determined to be involved in the area and the possible flight ranges from the various breeding sites

(4) History of insecticide treatment, and the results of insecticide susceptibility tests as well as bioassays of the walls of structures that have been treated recently with insecticide.

Once these initial surveys have been completed, permanent study sites can be located in which the monitoring of larval and adult anopheline densities can be continued. Meteorological events in the areas, especially of rainfall, should be recorded. Vector densities should then be compared with changes in these events. The type of agriculture, and human and domestic animal movement into or away from the risk areas should be noted.

Night bait collection of anophelines will be necessary because of their biting habits. This will require overtime work and additional transportation costs for the entomology teams.

In the section of Chapter 5 entitled "Surveillance," a detailed discussion on larval and adult collection methods is presented. Specific points to be considered in anopheline surveys include the following facts:

(1) Not all of the species of anopheline mosquitoes are vectors of malaria
(2) Various anopheline species may have different host preferences
(3) The biting times of the different anopheline species vary
(4) Not all of the anophelines enter light traps
(5) Some anophelines are endophilic, and some are exophilic, while others are both
(6) Flight ranges of the various anophelines are not the same.

A comparison of the findings of geographical reconnaissance and preliminary surveys, and routine surveys in the risk area should give the entomologist information with which to assist the vector control specialists in planning control activities. The basic information to be obtained includes knowledge of the following:

(1) The vectors that are present in the area, and their breeding sites
(2) The seasonal variations and relative densities of the vectors
(3) T he vector's host preferences and feeding, flight and resting habits
(4) Susceptibility of vectors to insecticide
(5) The extent of man-host contact
(6) The presence or absence of active malaria transmission
(7) The proper application and current residual effect of insecticides in dwellings, which may be difficult to evaluate if vector control staff members have not kept records of the previous spraying, dates and chemicals applied
(8) Alternate insecticides that are in stock or can be ordered
(9) The local geographical, meteorological and hydrological conditions that determine breeding season and sites.