Abstract
BackgroundThe number of Anopheles arabiensis (Diptera: Culicidae) and Anopheles pharoensis caught by human and cattle baits was investigated experimentally in the Arba Minch district of southern Ethiopia to determine if attraction to humans, indoors or outdoors, was affected by the presence or absence of cattle.MethodsField studies were made of the effect of a surrounding ring (10 m radius) of 20 cattle on the numbers of mosquitoes collected by human-baited sampling methods (i) inside or (ii) outside a hut.ResultsThe numbers of An. arabiensis caught outdoors by a human landing catch (HLC) with or without a ring of cattle were not significantly different (2 × 2 Latin square comparisons: means = 24.8 and 37.2 mosquitoes/night, respectively; n = 12, P > 0.22, Tukey HSD), whereas, the numbers of An. pharoensis caught were significantly reduced (44%) by a ring of cattle (4.9 vs. 8.7; n = 12, P < 0.05). The catch of An. arabiensis in human-baited traps (HBT) was 25 times greater than in cattle-baited traps (CBT) (34.0 vs. 1.3, n = 24; P < 0.001) whereas, for An. pharoensis there was no significant difference. Furthermore, HBT and CBT catches were unaffected by a ring of cattle (4 × 4 Latin square comparison) for either An. arabiensis (n = 48; P > 0.999) or An. pharoensis (n = 48, P > 0.870). The HLC catches indoors vs. outdoors were not significantly different for either An. arabiensis or An. pharoensis (n = 12, P > 0.969), but for An. arabiensis only, the indoor catch was reduced significantly by 49% when the hut was surrounded by cattle (Tukey HSD, n = 12, P > 0.01).ConclusionsOutdoors, a preponderance of cattle (20:1, cattle:humans) does not provide any material zooprophylactic effect against biting by An. arabiensis. For a human indoors, the presence of cattle outdoors nearly halved the catch. Unfortunately, this level of reduction would not have an appreciable impact on malaria incidence in an area with typically > 1 infective bite/person/night. For An. pharoensis, cattle significantly reduced the human catch indoors and outdoors, but still only by about half. These results suggest that even for traditional pastoralist communities of East Africa, the presence of large numbers of cattle does not confer effective zooprophylaxis against malaria transmitted by An. arabiensis or An. pharoensis.
Highlights
The number of Anopheles arabiensis (Diptera: Culicidae) and Anopheles pharoensis caught by human and cattle baits was investigated experimentally in the Arba Minch district of southern Ethiopia to determine if attraction to humans, indoors or outdoors, was affected by the presence or absence of cattle
Do malaria vectors fly past cattle in response to a human bait? Overall, for Experiment 1, the catch of An. arabiensis in human landing catch (HLC) was reduced with cattle present, but the effect was not significant, whereas for An. pharoensis, the catch was significantly less (44%) with a ring of cattle than without (4.9 vs. 8.7 mosquitoes/night; n = 12, P < 0.05, Figure 2A)
Taken together with the results of Tirados et al [2] the present study shows that even though An. arabiensis is highly anthropophilic, it frequently lands on cattle, which means that wherever cattle are present, there is a good chance the mosquitoes will be killed by insecticide-treated cattle (ITC)-treated animals outdoors, where the cattle:human ratio is high
Summary
The number of Anopheles arabiensis (Diptera: Culicidae) and Anopheles pharoensis caught by human and cattle baits was investigated experimentally in the Arba Minch district of southern Ethiopia to determine if attraction to humans, indoors or outdoors, was affected by the presence or absence of cattle. Anopheles arabiensis (Diptera: Culicidae), second only to An. gambiae s.s. in its malarial vectorial capacity, is generally described as the ‘less anthropophilic’ or ‘more opportunistic’ and more ‘exophagic’ of the two species, in eastern Africa [1,2,3] This suggests that the presence of cattle within human settlements could divert malaria vectors away from humans, thereby reducing malaria transmission by passive zooprophylaxis. If there is a high underlying entomological inoculation rate (EIR) of, say, 1 infective bite/person/ night, halving this to, say, 0.5 bites/person/night will not provide any material reduction in the incidence of malaria [2] Terms such as anthropophily and exophagy do not describe specific behaviours per se, but, rather, are generally descriptive behavioural types inferred from the observed origin of bloodmeals. The high proportion of human bloodmeals in the cattle camp begs the question: what is the behavioural mechanism that leads An. arabiensis to obtain such a biased feeding pattern?
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