The mosquito literature from Benishangul-Gumuz is extremely limited and we did not find any literature related to the malaria vectors present in Dangur woreda. In this study we identified eight species of Anopheles, with An. arabiensis, the major malaria vector in Ethiopia, as the most prevalent. Additionally, An. arabiensis was the only one found to have Plasmodium circumsporozoite protein in the head and thorax, indicating that this is likely to be the most important vector in agricultural areas in Dangur woreda. However, An. coustani and An. pharoensis were also collected in small numbers and these species have been found to be capable of transmitting Plasmodium parasites in laboratory and field studies [20–21].
The biting times of An. arabiensis present a challenge for the protection of migrant workers from infectious bites. An. arabiensis were found biting indoors and outdoors at nearly all time points, indicating an equal risk for workers staying indoors and outdoors, similar to elsewhere in Ethiopia . Furthermore, biting started as early as the 18:00 h-19:00 h time period and reached a first peak between 19:00 and 22:00. Tadesse et al. (associated manuscript) found that at least half of workers went to bed by 21:30, but this would leave them exposed to the first peak for 2.5 hours before sleep. Additionally, LLIN use by migrant workers was found to be quite low, as they often left their nets with their families when they came to the farms for work. One limitation of this study is that the HLCs were completed by 6:00 h, whereas mosquito host-seeking may have continued on beyond this time point, posing further risk for workers in the early morning. Higher mosquito densities in the large farms suggest that more farming activities and hence higher workers’ population might enhance the population of the vectors in those farms. Janko et al.  reported a positive correlation of agriculture coverage and the density of biting An. gambiae s.l. For example, they indicated that a 15% increase in agricultural cover was associated with increased probabilities of An. gambiae s.l. mosquitoes biting indoors (11.3% (95% UI -15.3 to 25.6) – 19.7% (-12.1 to 35.9). This range was obtained based on factors such as season (month of surveillance), temperature, and precipitation.
The lower mosquito vector density in the fields compared to outdoors near shelters might be due to higher attraction to places where the host population was higher.
We observed seasonal variations in the numbers of mosquitoes collected that were generally similar to those in other areas in Ethiopia, with an increase in mosquito populations during the main rainy season, and a decrease in population size as the rains end towards the end of the year [24–25]. It is important to notice that the peak of the agricultural activities and hence demand for workers occurs during the time of the year with the largest populations of An. arabiensis.
Finally, the detection of sporozoites in An. arabiensis allowed us to calculate their. This was found to be 1.41 infectious bites per month in the study area during the 6 months that monitoring was conducted. This figure is similar to EIR found in other parts of Ethiopia. Massebo et al.  found a yearly EIR of 17.1 in Chano, in the Southern Nations, Nationalities, and People’s Region. In both locations annual EIR is well above 1, the threshold below which Beier et al.  estimated EIRs needed to drop below for substation reductions of malaria prevalence. However, EIRs in highly malarious areas where vector control has had an impact can often reach much higher levels, indicating that improved vector control and treatment for malaria could have an important impact on transmission in these farms. Therefore, it might be very useful for the national malaria control strategy to focus on universal coverage of bednet distribution, including for the mobile and migrant populations and improvement of other vector control measures such as IRS and environmental management in such areas of the country.
Insecticide resistance was not assessed as a part of this study. Further work should be done to evaluate the insecticide susceptibility of An. arabiensis in this area. As IRS is not currently an option for protection of many of the workers due to the poor quality of structures or incomplete walls, the susceptibility testing should focus on the evaluation of pyrethroids, chlorfenapyr, and the synergism of pyrethroid susceptibility with piperonyl butoxide.
There were some limitations to our study. Collections were only made over a six-month period, and not over the whole year. While the six months chosen were the primary season for malaria transmission, there may be risks of malaria transmission outside these months. Also, collections in the fields were conducted from September until December, whereas collections in the worker’s shelter areas were conducted from July to December. An additional limitation of this study was that the mosquito collections ended at 06:00, and there may be some mosquito biting after this time. The ELISA reactions were considered positive at twice the value of the mean optical density of negative controls and were not re-boiled when positive. Finally, the current recommendations for analysis of human exposure do not take into account mosquito response to human behavior. Outdoor biting mosquitoes that do not find humans outdoor may to move indoors to feed on humans, and thus we may have underestimated the risk, for both net users, and, to a greater extent, non-users.
In conclusion, we consider An. arabiensis to be the most likely primary vector of malaria in the agricultural development areas surveyed. While the EIRs do not indicate high rates of transmission, the low use of vector control interventions and lack of access for treatment result in a real risk of malaria for workers staying in these locations. Improved malaria prevention and treatment could have a valuable impact on worker heath and productivity in these areas. However, it seems likely that the behavior of An. arabiensis, particularly outdoor biting and wide range of biting times will also pose challenges to implementing effective vector control.