The Imaramagambo Onchocerciasis Focus in Southwestern Uganda: Interruption of Transmission After Disappearance of the Vector Simulium neavei and Its Associated Freshwater Crabs

Abstract

It was not until early 1990s that, when the Imaramagambo focus of southwest Uganda was mapped, mass treatment with a single annual dose of ivermectin for onchocerciaisis control commenced. However, comprehensive investigations on its transmission were launched after a nationwide policy for onchocerciasis elimination in 2007. Entomological surveys throughout the focus from 2007 to 2015 have yielded few or no freshwater crabs (Potamonautes aloysiisabaudiae), which serve as the obligate phoretic host of the larvae and pupae of the vector Simulium neavei No S. neavei flies have been observed or collected since 2007. Skin snips (microscopy) from 294 individuals in 2008 were negative for skin microfilariae, and of the 462 persons analyzed by polymerase chain reaction skin snip poolscreen in 2009, only five (1.08%) persons were indicated as infected with onchocerciasis. All five of the positive persons were at least 40 years old. Serosurvey results showed negative exposure among 3,332 children in 2012 and 3,108 children in 2015. Both were within the upper bound of the 95% confidence interval of the prevalence estimate of 0.06%, which confirmed the elimination of onchocerciasis. Treatment coverage in Imaramagambo was generally poor, and transmission interruption of onchocerciasis could not be attributed solely to annual mass treatment with ivermectin. There was sufficient evidence to believe that the possible disappearance of the S. neavei flies, presumed to have been the main vector, may have hastened the demise of onchocerciasis in this focus.

Figure 1.

Map of Uganda showing the onchocerciasis foci of Uganda and progress toward onchocerciasis elimination.

Figure 2.

The Imaramagambo onchocerciasis focus in southwestern Uganda in Bushenyi and Mitooma districts associated with the Kalinzu and Maramagambo forest reserves (FR) to the West and the Kashoya-Kitomi FR to the northeast. = the sites where few Potamonautes aloysiisabaudiae were collected during surveys in 2008, 2009, and 2012 (cf. Table 2); = the sites where there were no crabs (cf. Table 2); = rivers in the Kashoya-Kitomi focus where P. aloysiisabaudiae still occurs; = red line encircles the area where 212 communities were under annual treatment.

Figure 3.

Map of Imaramagambo onchocerciasis focus showing sites where blood spots and skin snips were collected and where crab trapping and fly catching were done.