Studies on Salt-Water and Fresh-Water Anopheles gambiae on the East African Coast

The brackish water form of A. gambiae on the East African coast—and probably in Mauritius—is not the same as A. melas of West Africa.In salt-water gambiae a variable proportion of the females have an additional dark band on the palps, resembling 4-banded melas, but the remainder are indistinguishable from typical gambiae.Eggs and larvae of salt-water gambiae show no morphological differences from those of fresh-water gambiae, thereby differing from A. melas of West Africa.Larvae of the two forms show a clear-cut difference in reaction to sudden changes in salinity, and a simple test has been worked out whereby wild-caught females can be accurately identified by the reactions of their progeny.This physiological test has formed the basis of all work in comparing the incidence, habits, and infectivity of salt and fresh-water gambiae in Dar-es-Salaam.Exposed to equal chances of infection in the same village during 1947 and 1948, fresh-water gambiae had a sporozoite rate of 9·4 per cent. while that of salt-water gambiae was 0·8 per cent.About 4 per cent, of both forms were infected with filaria larvae, but monthly figures showed that infection rates in salt-water gambiae may rise to 22 per cent.Fresh-water gambiae show little tendency to leave African houses at dawn after feeding, whereas in salt-water gambiae over one-third of freshly blood-fed females leave the house at dawn.In fresh-water gambiae many half-gravid females leave the shelter of the house at dusk on the night after the blood feed. There is no marked difference in infectivity between those which leave the hut and those which remain indoors at this stage.Blood-fed and gravid females of fresh-water gambiae, funestus, and salt-water gambiae have been found in outdoor resting places, gravid females predominating in the case of the first two.Although larvae of salt-water gambiae can complete their development in pure sea water, in nature increasing salinity becomes a limiting factor before it reaches that of sea water, continuous breeding being no longer possible at salinities over 83 per cent. sea water.Salinity as a limiting factor explains the rather restricted breeding of salt-water gambiae on the coast, and suggests that certain coastal fresh-water swamps at Dar-es-Salaam could be cleared of all Anopheline breeding by salinifying with sea water.