Are herders protected by their herds? An experimental analysis of zooprophylaxis against the malaria vector Anopheles arabiensis

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.

Blood meal sources and entomological inoculation rates of anophelines along a highland altitudinal transect in south-central Ethiopia

“…In addition, it had the highest mixed human and bovine blood index among the five anopheline species. An experimental study in southern Ethiopia [44] documented similar number of An. pharoensis catches both in human- and cattle-baited traps.…”

“…Thus, it can be suggested that An. pharoensis of south-central Ethiopia may have a moderately opportunistic feeding behaviour probably due to its similar responsiveness to cattle and human host cues [44]. This tendency of the mosquito to feed on humans increases its vectorial capacity.…”

Variation in host preferences of malaria mosquitoes is mediated by skin bacterial volatiles

“…Anopheles arabiensis is more zoophilic than An . gambiae and has a preference for biting cattle and other warm‐blooded animals, although in some areas it has been found to be anthropophilic (Costantini et al ., ; Mutero et al ., ; Tirados et al ., , ; Mahande et al ., ). Anopheles arabiensis rarely takes bloodmeals from chickens (Githeko et al ., ; Jaleta et al ., ); however, chicken odour was found to be either repellent (Jaleta et al ., ) or attractive (Busula et al ., ), depending on the set‐up.…”

A dynamic model of some malaria-transmitting anopheline mosquitoes of the Afrotropical region. I. Model description and sensitivity analysis

“…It is also possible that An. arabiensis uses cattle for navigation [147]. Over time, such differences might lead to dominance by one species.…”