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

Moses N Katabarwa,Annet Khainza,James Katamanywa,Rolf Garms,Peace Habomugisha,Thomas R Unnasch,Edson Byamukama,Ephraim Tukesiga,Frank O Richards,Edridah Tukahebwa,Christine Nahabwe,David Oguttu,Monica Ngabirano,Thomson Lakwo

doi:10.4269/ajtmh.16-0181

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.

Highlights

Before 1970, the focus of Imaramagambo was still a sparsely populated jungle where onchocerciasis was largely unknown, as exemplified by its absence on the list of onchocerciasis foci in Uganda.[1,2,3] as more people migrated to the area to exploit its fertile agricultural land, onchocerciasis began to emerge as a public health problem
In an unpublished working paper by Angus McCrae, it was generally stated that S. neavei transmits onchocerciasis in a continuous range along the eastern slopes of the western Rift Valley escarpment, draining into Lakes Edward, George, and Albert, where the Simulium flies were universally prevalent along rivers with a gallery forest ecology.[8]
The objective of the surveys was to ascertain whether the freshwater crab P. aloysiisabaudiae, which serves as the obligate phoretic host of the larvae and pupae of the vector S. neavei, still occurred in the focus

Summary

Introduction

Before 1970, the focus of Imaramagambo was still a sparsely populated jungle where onchocerciasis was largely unknown, as exemplified by its absence on the list of onchocerciasis foci in Uganda.[1,2,3] as more people migrated to the area to exploit its fertile agricultural land, onchocerciasis began to emerge as a public health problem. The Imaramagambo onchocerciasis focus stretches from southwest of Lake Edward, from the Kigezi game reserve south toward the Bwindi impenetrable forest[5,6] and KashoyaKitomi, east of Lake George.[7] The focus includes portions of the Imaramagambo and Kalinzu natural forests. In an unpublished working paper by Angus McCrae, it was generally stated that S. neavei transmits onchocerciasis in a continuous range along the eastern slopes of the western Rift Valley escarpment, draining into Lakes Edward, George, and Albert, where the Simulium flies were universally prevalent along rivers with a gallery forest ecology.[8] In a letter dated October 14, 1966, Angus McCrae, former senior entomologist, Ministry of Health Uganda, explicitly wrote, “In the upper Nchwera in the Kalinzu Forest, the crabs appeared to be P. aloysiisabaudiae of the Rwenzori type, and not of the Ishasha system type with scarlet intersegmental membranes of the chelae.”

Methods

Results

Conclusion