Malaria in migrant agricultural workers in western Ethiopia: entomological assessment of malaria transmission risk

Abstract

Background: Ethiopia has made great strides in malaria control over the last two decades. However, this progress has not been uniform and one concern has been reported high rates of malaria transmission in large agricultural development areas in western Ethiopia. Improved vector control is one way this transmission might be addressed, but little is known about malaria vectors in this part of the country.

Methods: To better understand the vector species involved in malaria transmission and their behaviour, human landing collections were conducted in Dangur woreda, Benishangul-Gumuz, between July and December 2017. This period encompasses the months with the highest rain and the peak mosquito population. Mosquitoes were identified to species and tested for the presence of Plasmodium sporozoites.

Results: The predominant species of the Anopheles collected was Anopheles arabiensis (1,733; i.e. 61.3 % of the entire Anopheles), which was also the only species identified with sporozoites (Plasmodium falciparum and Plasmodium vivax). Anopheles arabiensis was collected as early in the evening as 18:00 h-19:00 h, and host-seeking continued until 5:00 h-6:00 h. Nearly equal numbers were collected indoors and outdoors. The calculated entomological inoculation rate for An. arabiensis for the study period was 1.41 infectious bites per month. More An. arabiensis were collected inside and outside worker's shelters than in fields where workers were working at night.

Conclusions: Anopheles arabiensis is likely to be the primary vector of malaria in the agricultural development areas studied. High rates of human biting took place inside and outdoor near workers' residential housing. Improved and targeted vector control in this area might considerably reduce malaria transmission.

Keywords: Agricultural development; Anopheles arabiensis; Ethiopia; Malaria; Migrant workers.

Fig. 1

Map of districts in Ethiopia illustrating the location of Benshangul-Gumuz and the location of the sites in this study (red circle). Map produced using ArcMap v. 10.5

Fig. 2

Tukey box-plots showing median and interquartile ranges of the number of An. arabiensis collected in indoor and outdoor human landing catches (n = 96) at workers’ shelters on small farms (< 100 ha), Dangur woreda, Ethiopia, 2017

Fig. 3

Tukey box-plots showing median and interquartile ranges of the number of An. arabiensis collected in indoor and outdoor human landing catches (n = 96) at workers’ shelters on large farms (> 100 ha), Dangur woreda, Ethiopia, 2017

Fig. 4

Directly measured hourly biting rate (bites per person per hour) indoors (solid grey line) and outdoors (dashed black line) in worker camps in eight farms in Dangur district (96 indoor collections, 96 outdoor collections) show 48 % of directly measured biting occurred indoors. Lines representing human biting rates are overlayed on top of bar charts representing the mean proportion of humans outdoors, indoors and awake, and indoors and asleep each hour of the evening. Approach derived from Monroe et al. [20]

Fig. 5

Human and An. arabiensis data were combined to derive human adjusted biting rates for ITN unprotected and protected individuals. (A) Behaviour-adjusted biting rates per person per hour for an unprotected individual are presented. The percentage of vector bites occurring indoors for unprotected individuals is 75 %, and the percentage of bites occurring while asleep indoors for an unprotected individual is 54 %. (B) Behaviour-adjusted estimates of An. arabiensis bites per hour per person in the study areas if ITNs were used, using the typical start and end times shown in the study area. The percentage of vector bites that would be prevented by using an ITN is 51 %. Approach and calculations derived from Monroe et al., 2020 [20]