A study of the age-composition of populations of Anopheles gambiae Giles and A. funestus Giles in North-Eastern Tanzania

Abstract

Polovodova's technique for determining the physiological age of mosquitos was used in a study in 1962–64 of the age-composition of populations of Anopheles gambiae Giles and A. funestus Giles resting in houses in two areas of Tanzania. One area was around Muheza, 25 miles from the coast, where the climate is humid and equable, and the other was around Gonja, 80 miles inland, where hardly any rain falls for five months of the year.It was found that the age-composition was almost identical in populations of A. gambiae and A. funestus at Muheza, about 20 and 23 per cent., respectively, being 3-parous and older and 1 per cent. 7-parous and older in both species. At Gonja, the population of A. gambiae was much younger, 14 per cent, being 3-parous and older and only 0·3 per cent. 7-parous and older. The oldest mosquitos found at Muheza included one 12-parous female of A. gambiae and one female of A. funestus believed to have laid eggs 14 times. No examples of A. gambiae older than 8-parous were found at Gonja.Dissections to determine the condition of the ovariolar sacs in A. gambiae at Gonja showed that in 87 per cent, of freshly fed parous females an interval of at least 24 hours had occurred since oviposition. At Muheza, 72 per cent, of individuals of this species in the cool season and 52 per cent, in the hot season were in a similar condition.Marking and recapturing experiments were carried out with females of A. gambiae in order to be able to correlate calendar age with physiological age. The oldest recaptured was 34 days old and was found to have laid eggs 10 times. From data on 60 recaptures, it was concluded that, although there was some irregularity, the first gonotrophic cycle lasted 3–4 days and later cycles 3 days.Age-specific sporozoite rates in A. gambiae rose from 4·1 per cent, for 3-parous to 32 per cent, for 7-parous and older females, and in A. funestus from 3·2 per cent, for 3-parous to 30 per cent, for 7-parous and older females. Most of the infected 3-parous females were gravid, indicating that few were infective at the beginning of the fourth cycle. On this account it was concluded that some 80 per cent, of malaria infections were transmitted in the fifth, sixth and seventh cycles.Analysis of the distribution of age-groups indicated that both A. gambiae and A. funestus showed a deficiency of nulliparous females, presumably because greater numbers of this group rested outside houses. From the second to seventh cycles the proportions of successive age-groups in both species at Muheza declined regularly at a rate corresponding to a mortality of 37·8 per cent, per cycle for A. gambiae and 38·6 per cent, for A. funestus, or 14·6 and 15·0 per cent, per day, respectively. Beyond this age the mortality was considerably higher. At Gonja, the population of A. gambiae declined at a rate corresponding to a mortality of 51·5 per cent, per cycle for the second to sixth cycles, or 20·9 per cent, per day. Above this age, the mortality was estimated to be higher still.From the regression of infectivity on age it was estimated that 6·8 and 6·1 per cent, of A. gambiae and A. funestus, respectively, became infected at each blood-meal.These findings are discussed in the light of current epidemiological theory.

Dry season refugia of malaria-transmitting mosquitoes in a dry savannah zone of east Africa.

“…A major factor determining the vectorial capacity of a mosquito species is the female survival rate. 19 This can be estimated in a number of ways 20 including capture-recapture experiments or dissection of the ovaries. The gonotrophic age of subsamples of An.…”

“…19 Estimates of survival rates either from the capture-recapture experiments or by gonotrophic age grading were obtained by regression of the numbers in each age class with age. 19 Two models were tested: where (t) and S(t) are the mortality and the survival functions respectively and ␣ and ␤ are constants. The parameters of the models were estimated using maximum likelihood methods, by maximizing the Poisson log likelihood…”

“…Population densities vary according to season, more so than those described from other areas of Tanzania. [4][5][6] Typically at the start of the long or during the short rains, the number of An. arabiensis collected in houses, especially in those bordering the valley, undergoes explosive growth.…”

A second chance to tackle African malaria vector mosquitoes that avoid houses and don't take drugs

“…However, asexual blood stages, and the infectious sexual-stage gametocytes they give rise to, enable temporary escape from the human body as sporogonic stages living in the bodies of mosquitoes that may survive for up to 14 feeding cycles, equivalent to at approximately one and a half months. 52 The fact that sporogonic stage infections can persist longer than single complete doses of therapeutic drugs enables P. falciparum populations to partially evade such exclusively human-directed chemical attack for the simple reason that mosquitoes do not take drugs. Although the benefits of controlling reservoirs of human parasite infection with therapeutic drugs are intuitive, appealing, and well established, only limited impact can be expected where high transmission levels persist 18,46,50,53 and the dangers of applying strong selection pressure for drug resistance upon robust parasite populations are also obvious.…”

Anopheline Mosquitoes and the Malaria Scourge

“…gambiae survive for more than three or four gonotrophic cycles [36]. Although a small percentage was found to survive for over ten cycles [37].…”