The steady progress of malaria-eradication programs initiated and carried out during the past 15 years has now slowed down. Malaria has retreated from the periphery of its global distribution but shows few signs of disappearance from the central area of its highest endemicity in tropical countries which form most of the developing world. Areas where eradication of this disease is not planned in the immediate future and where malaria control has made little impact on the high incidence of this infection are inhabited by 357 million people.

Much work has been done during the past decade on the integration of entomological studies with an epidemiological approach, but there are still large gaps in our knowledge of some basic factors of transmission of malaria in nature.

The division of Anopheles into “primary” and “secondary” vectors of human malaria is uncertain and often arbitrary. It appears that the best criterion of such an epidemiological rank of the local vector depends on the results of well-conducted malaria-eradication operations. Bearing this in mind some 50 species of Anopheles could be tabulated as major vectors of human plasmodia, either throughout the whole range of their geographical distribution or a large portion of it.

Field application of some theoretical results of the study of dynamics of malaria is far from satisfactory because of the inadequacy of basic data on the physiology and bionomics of Anopheles. Moreover, the sampling procedures applied to local anopheline populations are often biased and occasionally misleading. There are considerable difficulties in assessing quantitatively the impact of control measures based on the amount of transmission.

The results of malaria-eradication programs or pilot projects were appraised in 12 epidemiological zones covering most of the world where the infection was or still is prevalent. It appears that the effects of residual insecticide campaigns were more rapid and complete in temperate climates, in countries at the periphery of the geographical distribution of the relevant vector, or on islands.

Physiological resistance to DDT or to cyclodiene derivatives, and especially to both groups of insecticides, was often, but not always, the major obstacle to malaria eradication.

A survey of the present situation revealed that resistance to chlorinated hydrocarbons was recorded in 26 vector species of Anopheles; double resistance to DDT and dieldrin/benzene hexachloride (HCH) was found in 11 species of malaria vectors. Resistance to dieldrin usually precludes any further use of this insecticide or of HCH; on the other hand, resistance to DDT is usually of a moderate degree, and its further use may still be of some value. In many programs the irritant or deterrent effect of DDT was responsible for disappointing results, while in some it might have been an asset when malaria was of a very unstable character. In some parts of the world the vectors have pronounced exophilic and exophagic habits and thus evade the toxic action of residual insecticides. Other technical or operational causes of nonresponse of vectors are related to the mobility or distribution of the human population, to the type of dwellings, to cultural patterns, or to socioeconomic factors.

The definition of “problem areas” hitherto applicable only to areas of malaria-eradication programs where the transmission was not interrupted in spite of well-conducted operations should be extended also to those parts of the world (e.g. tropical Africa) where pilot projects failed to interrupt transmission.

Concurrently with the applied study of the type and degree of physiological resistance in Anopheles, cooperation between research institutes and field workers produced a considerable body of knowledge on genetics of insecticide resistance and on cytotaxonomy. The latter is of particular value for the separation of cryptic species (e.g. A. gambiae A, B, and C).

The obstacle of physiological resistance can be met by the development of new residual insecticides, by the use of larvicides whenever applicable, by mass drug administration, and by integrated methods of biological control.

One of the problems that the world may soon be facing is the possible spread of Anopheles vectors of malaria from the area of their normal geographical distribution to other areas. Some of these events may arise in consequence of large hydro-engineering projects; others may be due to the increased speed and volume of international air communications. The quadrilateral of the South Pacific east of the New Hebrides is at present free of Anopheles and may be particularly vulnerable.