Successful interruption of the transmission of Onchocerca volvulus in Mpamba-Nkusi focus, Kibaale district, mid-western Uganda

Abstract

Background: The Mpamba-Nkusi onchocerciasis focus is situated in the mid-western part of Uganda. It has an area of 300 km² and used to have Simulium neavei as the vector which develops in a phoretic association on freshwater crabs. Ground larviciding with temephos (Abate EC500) was initiated in June 2002. All the 330 communities in this focus have undergone annual treatment with ivermectin since 1995 and were later shifted to semi-annual treatment in 2009.

Objective: To establish the impact of mass drug administration in combination with larviciding on the interruption of O. volvulus transmission.

Design: Longitudinal study

Setting: Rural areas in Mpamba-Nkusi focus, Kibaale district.

Subjects: Individuals five years and above living in the focus.

Interventions: Annual and semi-annual treatment with ivermectin supplemented by vector elimination were used. Epidemiological, entomological and serological assessments were conducted.

Results: Freshwater crabs (n = 14,391) caught from monitoring sites (n = 41) since 2008 were negative for immature stages of S. neavei. The S.neavei population was reduced following trial and initiation of ground larviciding. No adult S. neavei has been caught in the focus for over five years. Parasitological examination of individuals residing in the focus revealed a microfilaria (mf) prevalence of 0.3% (95% CI 0 – 0.65%; n = 732) in 2012. Serological assays testing for Onchocerca volvulus antibodies conducted on 3351 children <15 years in 2009 indicated point prevalence of 0.6% , (95% CI, 0.3-0.8%) while in 2012 another survey conducted among 3,407 children, only 1/3407 (0.03%, 95% CI, 0-0.09%) individual was positive for O. volvulus antibodies.

Conclusions: Epidemiological and entomological findings suggest that interruption of transmission has been achieved.