Dry season occurrence of Anopheles mosquitoes and implications in Jabi Tehnan District, West Gojjam Zone, Ethiopia

Malaria vectors undergo egg, larval, pupal and adult stages in their life time. Their egg, larval and pupal stages are limited to water bodies and have very small spatial dispersion. In dry seasons, the number and size of larval habitats is generally believed to reduce significantly and contribute to a low population of malaria-transmitting adult Anopheles [1‐3]. The narrow spatial dispersion and low population size of the immature vectors make them more amenable to environmental management [4, 5]. Thus, larval habitat management can best be implemented in dry seasons in the fight against malaria vectors. Even so, Anopheles species exploit a variety of breeding habitats that vary considerably in size, altitude, vegetation cover and topography [1, 6, 7]. In a narrow topographic area, breeding habitats can have enormous variability in their anopheline productivity [7‐9]. The majority of vectors may emerge from prolific habitats, which could account only for a small proportion of the habitats. Thus, effective larval interventions can be achieved through targeting productive habitats during dry seasons [6].

After emergence from pupal stage, malaria-transmitting adult female Anopheles undertake mating with their male counterparts and find human blood meal to nourish their eggs. They imbibe blood mainly during night hours inside residential houses and rest in door until their blood meal is digested and eggs are matured. Accordingly, insecticide-impregnated mosquito nets and residual insecticide sprays are employed inside houses to prevent and kill indoor biting and indoor resting vectors, respectively. In dry seasons, the adult vector population may reduce significantly owing to the reduction in breeding habitats [2, 3, 10]. Even so, some may survive due to factors, such as behavioural adaptation and insecticide resistance [4, 11]. They may prefer some housing conditions over others [12] and show a pattern of clustering in which case a great majority of them could occur inside smaller proportion of the houses built near their breeding habitats.

Streams were the most prolific breeding habitats of Anopheles followed by ponds and swamps. The great majority of larvae inhabiting these habitats were An. gambiae, followed by An. cinereus and An. chrysti. In Ethiopia, Anopheles arabiensis, a sibling species in the An. gambiae complex, is widely distributed and remains a major malaria vector throughout the country. Previous studies reported occurrence of An. arabiensis and other species of Anopheles larvae along streams in a central highland area [18] and in the middle course of the rift valley [20] of Ethiopia. Studies from other east African countries also support this observation [21‐23].

Abundance of An. gambiae larvae along streams and also in ponds and swamps during dry seasons entails a strengthened vector control strategy in the habitats against aquatic stages of anophelines during the seasons. Management of the streams during dry seasons can reduce the vector populations significantly as the aquatic stages (eggs, larvae and pupae) of mosquitoes remain confined within relatively small habitats and cannot readily escape control measures [6, 24‐26]. Disruption of the aquatic stages of mosquitoes during the dry seasons would decrease the size of an already small population, and may delay an explosive build-up of the vector populations towards the end of dry seasons, during the onset of the rainy seasons and following the rains in inland villages [10, 19, 24, 27]. It can also serve as an alternative control strategy to malaria transmitting vectors that are resistant to the available insecticide based control tools (such as LLINs and IRS) [28‐30] and hence can be highly effective in controlling malaria transmission in Jabi Tehnan district if used and also integrated in the current malaria vector control package [4]. As the streams are used for washing clothes and as sources of drinking water for cattle (personal observation during the study), chemical based larval intervention cannot be employed. Instead, temporary and scheduled manipulation of the habitats such as straightening, flashing and changing their course can be employed.

Indoor spray catches revealed occurrence of adults of An. gambiae, An. chrysti, An. demeilloni, An. cinereus and An. coustani in a decreasing order of density. Anopheles gambiae, which comprises An. arabiensis and Anopheles amharicus in Ethiopia, was the most frequently caught mosquito. Most previous studies reported An. arabiensis from different parts of the country and is the main vector of malaria, while An. amharicus (formerly Anopheles quadriannulatus species B) was described to occur in the country [30‐33]. An. arabiensis is most frequently reported to occur in dry seasons than any other species in the An. gambiae [18, 25, 34]. Hence, the An. gambiae catches can presumably be considered as An. arabiensis and could be considered the primary vector responsible for the transmission of Plasmodium falciparum and Plasmodium vivax malaria in the area during the seasons [13, 14]. However, it remains important to incriminate An. cinereus, An. chrysti, An. demeilloni and An. coustani for their vectorial role in the area.

The houses from where adult mosquitoes surveyed, both located within 1 km radius from a nearby permanent breeding habitat, were observed to have similar density of indoor resting An. gambiae. This could partly be explained by the similar housing conditions and the indoor occurrence of the blood meal sources readily in every house during the night hours. However, this observation differs from previous studies in which case the indoor density of Anopheles varied significantly among houses within the same village [12, 25, 35, 36]. The indoor occurrence of the vectors could maintain transmission of the malaria during the dry season and serve as sources for increased transmission intensity in the villages at the time of vector population built up [27]. In the area, the domestic animals and humans share the same house in the night creating favourable blood meal source for the mosquitoes. Species of the An. gambiae complex feed during night hours and An. arabiensis a species in the complex feeds on humans and cattle alternatively [17]. This may suggest the possibility of indoor feeding and resting activity of the An. gambiae in Jabi Tehnan District.

The occurrence of adult anopheline mosquitoes inside residential houses indicates the need for proper utilization of existing malaria vector control tools namely the insecticide treated mosquito nets and indoor residual spraying. The use of improved housing conditions remains a top priority in view of the increasing abundance of insecticide resistant malaria vectors and also sustaining the low level transmission of malaria in the district.

It was not possible to carry out immature and adult Anopheles survey during every month of the year and hence describe the peak and low Anopheles density season(s). As a result it was not possible to clearly describe occurrence of the mosquitoes and the respective prolific breeding habitats during other seasons. It was not also possible to test the sporozoite infection status of the mosquitoes due to the shortage of reagents.

The streams were prolific breeding habitats of Anopheles followed by ponds and swamps during dry seasons in Jiga Yehlmidar, Mender Meter and Wongie Berkegn villages, Jabi Tehnan District, West Gojjam Zone, Ethiopia. A high population of indoor resting Anopheles gambiae, followed by An. cinereus, An. chrysti, An. demeilloni and An. coustani were caught in the villages during the seasons. This implies the need to carry out management of breeding habitats, improved housing and scaled-up use of insecticide (LLINs and IRS) based adult vector control tools, during dry seasons, to decrease the population of malaria transmitting Anopheles mosquitoes in the villages.