Onchocerciasis elimination from Africa: One step in Northern Sudan.

Onchocerciasis is one of the two filarial helminth "neglected tropical diseases" (the other being lymphatic filariasis) that has been targeted for geographically local elimination followed by global eradication. The last known areas of Onchocerca volvulus transmission in the Americas have recently been reported to be eliminated. In contrast, achieving metrics for interruption of O. volvulus transmission in Africa, thus removing the requirement for continued monitoring and mass drug administration (MDA) with ivermectin, has been more challenging. To date, transmission cessation of O. volvulus has been validated only in the Mount Elgon region of eastern Uganda. Annual and biannual ivermectin MDA was delivered in this endemic focus from 1994 to 2011, in combination with sustained vector control aimed at reducing the local larval Simulium neavei vectors that have a phoretic association with freshwater crabs. Subsequent to this accomplishment, the World Health Organization (WHO) updated in 2016 the criteria for stopping MDA as a result of transmission interruption. The technical procedures and the corresponding cutoff values to signify transmission interruption included the following: 1) screening pools of black flies by polymerase chain reaction for the DNA repeat sequence Ov150; minimal elimination value is < 1/2,000 Ov150-positive flies and 2) serologic screening of school-aged children < 10 years of age for immunoglobulin G4 antibodies to the Ov16 antigen; elimination value is antibody prevalence < 0.1%. In this issue of the American Journal of Tropical Medicine and Hygiene, Zarroug and others describe results of a 3-year post-MDA treatment survey that confirm elimination of O. volvulus transmission by these criteria in Abu Hamed, a geographically isolated endemic focus in northern Sudan inhabited by approximately 120,000 people. Of the 5,266 children tested for Ov16 antibody, one 9-year-old child was positive. This child had never traveled outside her home village and had a negative skin snip for Ov150 DNA, indicating that she probably did not have a patent infection with skin microfilariae, but had previously been exposed to O. volvulus infective larvae. Lymphatic filariasis or other parasitic worm infections that may elicit antibodies that cross-react with Ov16 are not endemic in the study area.