The rise and fall of Anopheles arabiensis (Diptera: Culicidae) in a Tanzanian village

Abstract

The continual recruitment of new individuals makes it difficult to study both the survival of multivoltine mosquitoes, and the size of the infectious reservoir in natural populations of malaria vectors. During long-term surveillance of a population of Anopheles gambiae Giles sensu lato in a Tanzanian village by daily light trapping, a temporary dry spell resulted in the cessation of recruitment for a period of 33 days, and a decline in numbers of A. arabiensis Patton caught from over 2000 to less than 10 in a sentinel house. Traps placed elsewhere in the village indicated similar proportionate declines although numbers caught varied according to location. A survival rate of 83% per day was estimated from the rate of population decline. Survival was unrelated to the size of the mosquitoes. The infectious reservoir (the chance of a mosquito acquiring an infection) was estimated to be 2% per feed. The exploitation of fortuitous events which temporarily eliminate a single stage in the life cycle has general applicability in the study of the bionomics of multivoltine insects.

Rapid assessment of malaria risk using entomological techniques: taking an epidemiological snapshot

“…Thus, one purpose of the RA is to identify transmission 'hot spots' that should be prioritized for interventions. For example, in the Ifakara region of Tanzania, although the 'average' villagers may receive a mean of 300 infective bites per year, some villagers receive 10 times less than that and some, who live in the more remote areas, 10 times more, this heterogeneity being a direct consequence of variation in mosquito biting-rates ( Figure 1) (Charlwood et al 1995a;1995b).…”

“…Using a sentinel light trap in an area of high mosquito density allows immediate mapping and comparison of spatially heterogeneous data. Reprinted from Charlwood et al (1995a;1995b) with permission from CABI publishing in consistent and comparable ways, there is also the exciting possibility that information can be collated from diverse ecological and epidemiological settings, contributing the much-needed fine-scale resolution for national-, regional-and continental-scale estimates of malaria risk (such as Mapping Malaria Risk in Africa) (Le Sueur et al 1997). …”

“…Each trapping or collecting method and subsequent handling of the mosquitoes can be used to sample a definable subpopulation of the local mosquitoes (left column), and these are selected according the specific aims of the RA. For example, indoor resting catches will provide the full range of information necessary to assess mosquito population structure, association with human hosts, and essential malaria information, and up to six of these collections could be performed by a field worker per day but also in age structure, depending upon the season, and the RA should take into account the possibility that a population is stable, recruiting or in decline (Figure 1) as this greatly influences the sporozoite rate and EIR (Figure 4) (Charlwood et al 1995a). …”

“…Nevertheless, relative gradients in mosquito densities between areas do not change overmuch (Figure 1) and ratios between collection sites at any one time will reflect all other collections. When coupled to meteorological data and compared to published material, it should be possible to place the RA 'snapshot' into a reasonably predictable temporal context (Charlwood et al 1995a).…”

“…Estimates of survival rate from parity rates of samples collected over a short period of time are only applicable if the population is in a steady state (i.e. when recruitment and death rates are equal) (Charlwood et al 1995a) and here, setting the RA into a temporal context is important. Alternatives to parous-rate estimations (e.g.…”