Synoptic Epidemiological Studies of Malaria in El Salvador

Summary

A detailed description is given of the field and recording methods used in 12-month Synoptic Studies of some problem and comparison malariaeradication areas in El Salvador, with the hope that it may serve as a guide to a simple, inexpensive, and efficiently productive operational approach to the evaluation of factors underlying the persistence of malaria so that appropriate remedial measures may be instituted.

In six bimonthly 2-week visits to each of eight areas data were gathered by two teams of epidemiological aides under the direction of an epidemiologist and a medical entomologist, utilizing about 14 different procedures. Prompt tabulation and summarization enabled concurrent evaluation of the field work. Epidemiological data included daily minimum-maximum temperatures and morning and evening relative humidities; repeated censuses of houses to include new and temporary shelters and spraying history; quantitative survey of sprayable indoor surfaces (relative amounts of openings, impervious and porous materials); repeated censuses of inhabitants and malaria prevalence (by age and sex) through 20% random parasitemia surveys and active case detection by the team and passive case detection by the malaria notification posts and investigation and classification of malaria cases (autochthonous vs. imported). Entomological studies (involving two-thirds of available man-hours exclusive of travel) consisted of collections by aspirator of adult anophelines on inside walls from 8 a.m.–noon, on human bait in and around houses from 6:30–8:30 p.m. and 6 p.m.–6 a.m., and in and around cattle corrals at night (to determine activity and relative abundance of species); limited sampling of larvae locally present; and DDT-susceptibility and excito-repellency tests of Anopheles albimanus. Data previously gathered by the malaria eradication service and in an intensive PAHO field study of two coastal villages facilitated the planning of the studies.

A. albimanus comprised about 98% of the adult and 65% of the larval anophelines collected. Its biting activity was usually, but not always, highest in the first half of the rainy season and was not highly correlated with malaria incidence. Biting occurred both indoors and, somewhat more frequently, outdoors, and this species also predominated in cattle-corral collections. A. pseudopunctipennis comprised 35% of the larvae collected, but adults of this and five other species were found only occasionally. No taxonomic studies were undertaken. A. albimanus was highly resistant to DDT in four of the study areas and highly, if not quite completely, susceptible in the other four, without significant seasonal variation. All populations exhibited marked excito-repellency to DDT, with a suggestion that two resistant populations may be losing their excito-repellency (which would render DDT useless).

Six Malariological Zones can be recognized in El Salvador (total area 8164 sq. miles, population originally at risk 1,820,000 with A. albimanus the only vector), heavy, moderate or minimal transmission being each associated with either DDT-resistance or -susceptibility: 1. heavy with DDT-resistance; 2. heavy with DDT-susceptibility (in the region in which mass administration of chloroquine-primaquine has minimized transmission); 3. moderate with DDT-resistance; 4. moderate with DDT-susceptibility (assumed to be present but not encountered); 5. minimal with DDT-resistance; and 6. minimal with DDT-susceptibility.

Suspension of DDT-spraying is recommended because its limited value does not now justify its high cost and old residues are still exerting appreciable effect. Mass administration of sugarcoated chloroquine-primaquine pills should be extended to the entire problem area, fortnightly in Zones 1 and 2; initially monthly in Zone 3; and expectantly in Zones 4, 5, and 6. Provision should be made to control epidemic outbreaks by appropriate means.