Volume 14, Issue 1
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



The authors, in collaboration with the staff of CNEP, Mexico, carried out an investigation of persistent malaria transmission in a coastal area of the State of Oaxaca from May through August 1962. The area had been thoroughly sprayed with DDT for five years, and spraying continues. In a detailed survey of malaria cases and housing in ten villages, it was found that malaria-positive houses tended to be in isolated positions, to be smaller than average, to have more inhabitants and to show a greater discontinuity in, and lack of, walls than the negative ones.

A study of malaria cases detected by various methods among 29,216 inhabitants of the area over a period of 30 months showed an annual incidence (including relapses) of 1.32% in this population. The incidence was substantially higher in the second half of each year than in the first, and there was a general upward trend between 1960 and 1962. DDT was applied at average intervals of six months, but spraying within the area was distributed randomly through the year. Counting of the cases detected when the DDT deposits were less and more than three months old, respectively, showed an increase from 2.46% to 3.49% in the proportion of slides positive when the insecticide deposits were old, although only very small amounts of unsprayed interior surface were then present.

The vector anophelines found biting man were and . Both bit human baits freely outside and inside the sprayed houses. Although the relative incidence of indoor biting was somewhat reduced in the weeks following spraying, the habit of the people of sleeping nearly always indoors meant that the bulk of contact between them and the vectors must occur within the houses. The vectors were found susceptible to DDT (with some indication of incipient resistance in ), while the small numbers caught in window-traps were shown to leave the houses soon after biting and to have a low 24-hour mortality rate thereafter. The two vectors are considered to be highly irritable by DDT and to escape easily from the treated houses. During the investigation it was observed that malaria transmission could take place in spite of low vector densities.


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