Bites before and after bedtime can carry a high risk of human malaria infection

Abstract

Background: Understanding biting distribution of potentially infectious (parous) mosquitoes at various hours of the night would be useful in establishing the likely impact of bed nets on malaria transmission. Bed nets are highly effective at preventing biting by older malaria vectors, which occurs when most people are in bed. However, this behaviour is likely to vary across ecological settings and among mosquito populations.

Methods: Field experiments were conducted in Minepa village within Kilombero Valley. Two outdoor catching stations located approximately 50 m from each other were established for mosquito collection. On each experimental night, mosquitoes were collected using human landing catch (HLC) by a single adult male at each station from 18:00 to 07:00 h. To compare the distribution of mosquito biting and the composition of their age structure, mosquitoes were sorted and recorded according to the hour they were collected. A sub-sample of Anopheles arabiensis was dissected to determine their parity status. Insectary-reared An. arabiensis within the semi-field system (SFS) with known age were also released in the SFS (10 m × 20 m) and recaptured hourly using HLC to determine the effect of parity on biting distribution.

Results: Overall, there was no statistical association between the parity status and the biting time of An. arabiensis either in the field or in the SFS (P ≥ 0.05). The wild and insectary-reared An. arabiensis were observed to exhibit different hourly biting patterns.

Conclusion: The study has shown that mosquito biting time phenotype is not influenced by their parity status. These findings imply that the risk of human exposure to potentially infectious bites is equally distributed throughout the night, thus supplementary measures to protect people against bites in evening and morning are desirable.

Keywords: Biting time; Distribution; Malaria; Mosquito; Mosquito age; Parity; Transmission.

Fig. 1

Images of an insectary-reared female Anopheles arabiensis in the semi-field. Fed on glucose solution containing 2 g/l synthetic blue food colour (a) or rhodamine B (b). Blue food colour was only visible in the abdomen (a), but rhodamine B was visible throughout in the thorax and abdomen

Fig. 2

Distribution of biting times for wild Anopheles arabiensis and Anopheles funestus in Kilombero Valley, Tanzania. The dashed line represents Anopheles arabiensis and the continuous line represents Anopheles funestus

Fig. 3

Biting activity of wild Anopheles arabiensis in the field compared with insectary-reared Anopheles arabiensis in the semi-field system. The dashed line represents the proportion of An. arabiensis that were captured biting at each hour of the night in the field and continuous line represents An. arabiensis that were recaptured biting at each hour of the night in the semi-field system

Fig. 4

The proportion of parous Anopheles arabiensis that were sampled biting across different hours of the night. a Represents parous rate that were captured in the field; b represents those that were released and recaptured in the semi-field system. Data points represent absolute proportion of parous biting at each hour and Bars represent the 95% confidence interval. X axis represents hour of the night

Fig. 5

The proportion of hourly biting of the parous Anopheles funestus. X axis represent hour of the night

Fig. 6

Distribution of the proportion of parous Anopheles arabiensis that were recaptured biting at each night of the study in the semi-field system. Data points represent absolute proportion of parous biting at each night of the study and Bars represent the 95% confidence interval. X axis represents night of the study