Progress towards elimination of onchocerciasis transmission in Mali: A "pre-stop MDA" survey in 18 transmission zones

BackgroundOnchocerciasis control activities in Mali began in 1975 with vector larviciding carried out by the Onchocerciasis Control Programme (OCP), followed by the distribution of ivermectin from 1998 until the closure of the OCP in 2002. At that time, epidemiological evaluations, using skin snip microscopy and O-150 pool screening PCR in black flies, indicated that the disease had been largely controlled as a public health problem. Ivermectin distribution was nevertheless continued after 2002 in 34 of the 75 health districts in Mali as these were known to still be meso- or hyper-endemic for onchocerciasis. In addition, the onchocerciasis sites known to be hypo-endemic for onchocerciasis benefited from the distribution of ivermectin treatment as part of the mass drug administration (MDA) program for lymphatic filariasis. Various entomological and epidemiological evaluations have now indicated that Mali may have achieved successful interruption of onchocerciasis transmission.MethodsA series of cross-sectional surveys to update vector breeding sites throughout the endemic areas, followed by a pre-stop ivermectin mass drug administration (Pre-stop MDA) survey, were undertaken in 2019-2020. Based on breeding site findings, historical epidemiological assessments, and vector collection site maps, 18 operational transmission zones (OTZ) were delineated within which a total of 104 first line villages were selected for evaluation. Dried blood spots (DBS) samples were collected from 10,400 children (5-9 years old) from these 104 first line villages and processed for the presence of OV16 antibody using a lab-based rapid diagnostic test.ResultsWithin the 544 Simulium damnosum s.l. breeding sites visited in all five endemic onchocerciasis endemic regions of Mali 18.01% (98/544) were seen to be active with the presence of at least one stage of S. damnosum. The overall prevalence of OV16 positive children was 0.45% (47/10,400). However, two hotspots were identified: 2.60% (13/500) seroprevalence in the OTZ number 5 in Kayes Region and 1.40% (7/500) in the OTZ number 1 of Sikasso Region.ConclusionThese data show that onchocerciasis prevalence in the five endemic regions has declined to levels that indicate that Stop-MDA surveys should be now carried out in most of the OTZ except for one in the Kayes Region. This latter site will need additional ivermectin treatment before reevaluation, and an OTZ in the Sikasso Region requires revaluation before possibly reinitiating MDA. The control of onchocerciasis, then an endemic disease in Mali, began through vector control by spraying insecticides in Simulium damnosum s.l. breeding sites in an effort known as the Onchocerciasis Control Program in West Africa (OCP). As the OCP's activities began to wind down, control activities moved to a combination of vector control together with the distribution of the anthelminthic, ivermectin administered to those populations at risk of infection in the endemic areas. This drug administration was carried out by mobile teams using the framework of community-directed ivermectin treatment (CDTI). It is also important to note that lymphatic filariasis is endemic in all health districts, and thus the onchocerciasis endemic districts, regardless of endemicity, all therefore also benefited from the effects of the additional ivermectin treatment.After various evaluations of endemicity it was found that Mali qualified to conduct pre-stop ivermectin treatment surveys according to World Health Organization (WHO) guidelines. Thus, between the end of 2019 and the beginning of 2020, a large-scale serological survey of children 5-9 years was conducted in all health districts of Mali suspected of being endemic for onchocerciasis. The results of this investigation showed the seroprevalence thresholds measured by the OV16 rapid diagnostic test in the laboratory were below the WHO defined 2% level for success in all operational transmission zones, except for an operational transmission zone in the Kayes region, namely the KA05 zone, where there was 2.60% seropositivity to OV16.