The behaviour of Homo sapiens, the forgotten factor in the transmission of tropical disease.

The behaviour of the pathogens responsible for tropical disease and the behaviour of the hosts other than man are both studied in great detail, but the behaviour of man, the third component in these cycles of transmission, is for the most part totally and inexplicably disregarded. Even when the pathogens are actively brought to us through the agency of an arthropod host, we too often ease the passage of the vectors either by unthinkingly providing facilities for their breeding or by neglecting the simple steps that can be taken to prevent their feeding on us. The problem resolves itself into two parts, (i) the collection and collation of relevant data on human behaviour, and (ii) the taking of steps to change this behaviour. Part two has recently been greatly facilitated by the development of radio transmission via artificial satellite. While WHO is now making a start on both these aspects it is doing so at a relatively low level. Instead, the two phases of this new approach should be given top priority even if it means large scale reorganization of relevant university departments and even of WHO itself. We have, after all, had almost 100 years to try out the old methods and, as far as the Third World is concerned, they have for the most part failed.

Diuresis in newly emerged, unfed mosquitoes. II. The basic pattern in relation to escape from the water, preparation for mature flight, mating and the first blood meal.

A study of 2000 newly emerged adult mosquitoes (Aedes aegypti) shows that the rate of excretion is high at the time of eclosion but falls rapidly over the next few minutes. In both sexes, however, the high rate of excretion is regained during a second peak in the middle of day 1. It remains high for about 6 h and then falls in a remarkably steady manner to reach a low level plain, which persists at least till the end of day 3. In females this plain is not reached till some 8 h after the onset in males. This steady low level of activity, which is higher in females than in males, is here regarded as the basic rate of excretion. The rapid loss of fluid in the newly emerged adult enables the insect to make its first teneral flight, escaping from the surface of the water within minutes of eclosion. The second peak of excretion occurs within the period of rest during the first day of adult life when muscles needed for swimming are being replaced by those needed for flight. The low rate of excretion which follows on day 2 and is continued on day 3 signals the establishment of full flight activity covering mating and, in the female, the first blood meal. During the blood meal and immediately afterwards the rate of excretion is already known to increase significantly; for a truer measure of this increase the amounts should now have the female basic rate, as here defined, deducted from it.

Diuresis in newly emerged, unfed mosquitoes. I. Fluid loss in normal females and males during the first 20 hours of adult life.

Unfed mosquitoes (Aedes aegypti) decapitated soon after emergence from the pupa survive better than those decapitated later. In a test involving more than 700 females and more than 600 males, 80% of the mosquitoes decapitated during the first hour of adult life survived a period of 45 h post emergence (p.e.), whereas about 80% of those decapitated at 12 h or later failed to survive the same period. There was a linear fall in the rate of survival between 1 and 10 h p.e., after which the rate fluctuated between 10 and 30%. The mechanism accounting for this phenomenon seemed most likely to be loss of fluid under the control of a diuretic hormone released from the head, early decapitation protecting the insects from excess loss of fluid. To test this explantation the rate of excretion was measured in normal unfed mosquitoes at regular intervals during the first 20 h of adult life. In females the rate of excretion was shown to rise from 10 nl min-1 during the first hour of adult life to each 27 nl min-1 12 h later. In males the rate of excretion fell from 7 to 5 nl min-1 during the first 3 h before rising to 12 nl min-1 by 9 h p.e. The rate of excretion decreased in both sexes from 15-16 h onwards. Newly emerged mosquitoes thus increase their preparedness for flight by loss of water through enhanced excretion during the first day.

The influence of the brain hormone on retention of blood in the mid-gut of the mosquito Aedes aegypti (L.). III. The involvement of the ovaries and ecdysone.

Most female mosquitoes require a blood-meal in order to produce mature oöcytes. An egg development neurosecretory hormone (EDNH), which is produced in the medial neurosecretory cells (m.n.c.) of the brain and stored in the corpus cardiacum, is released into the haemolymph following the ingestion of blood and is essential for the promotion of ovarian development to maturity. It has been shown that a factor from the m.n.c., presumably EDNH, is necessary if the blood-meal is to be retained in the mid-gut until the oöcytes approach maturity. The present paper shows that retention is not a direct result of the action of EDNH, but is dependent on the ovaries and may well involve ecdysone. Removal of the ovaries before a blood-meal leads to early haem-defaecation, but delay can be restored by injection of ecdysterone. Sub-threshold feeders and mosquitoes decapitated immediately after the intake of blood, each of which would be expected to eliminate the blood-meal early, also show a delay in the onset of haem-defaecation when injected with ecdysterone. Further, both in ovariectomized insects and sub-threshold feeders the time of onset of haem-defaecation is associated with the dose of ecdysterone given.

Breeding places and seasonal incidence of Aedes aegypti, as assessed by the single-larva survey method.

The single-larva survey method was employed to study the breeding places and seasonal incidence of Aedes aegypti in Dar es Salaam, Tanzania. From May 1968 to May 1969, 28 462 containers of water-located in approximately equal numbers indoors and outdoors-were investigated. The highest frequency of breeding (8.0%) of A. aegypti was observed in tires and motor parts. Drums, barrels, water-pots, and other receptacles left outdoors showed a higher frequency (3.1%) than those kept indoors (0.6%). Metal containers were infested to a greater extent than those made of mud, wood, or other materials; 2.5% of coconut shells, snail shells, etc. and 1.3% of tree holes, plant axils, and cut bamboos were infested. The seasonal prevalence, expressed as a container index, closely followed and paralleled the fluctuations in rainfall. The value of this survey method for both ecological studies and practical control purposes is discussed.

Diel periodicity in the landing of Aedes aegypti on man.

The dynamics of transmission of disease agents by vectors depends, in part, on the probability of host-vector contact, which can vary with fluctuations of both host and vector. As important as seasonal variations is 24-hour periodicity in activity. Periodicity in the landing of males and females of Aedes aegypti on man has been assessed by means of catches of 15 hours or longer, with several persons as a bait. The assessments were made in a suburban area of Tanzania and continued throughout one year. Activity was observed to be almost entirely diurnal and diphasic. Whereas the detailed activity pattern of males agreed closely with that found elsewhere in East Africa, that of the females was unusual on account of the symmetry of the morning and afternoon peaks. Possible causes of differences among studies are discussed.