Onchocerciasis and lymphatic filariasis (LF) are neglected tropical diseases (NTD) that affect millions of people in tropical settings, especially in remote and inaccessible areas. Onchocerciasis causes itching, skin lesions, blindness and epilepsy; whereas lymphatic filariasis can result in lymphedema and hydrocele. These two diseases represent an important socio-economic burden due to permanent disabilities, including elephantiasis and blindness, that drive communities into extreme poverty.
After many years of successful mass drug administration (MDA), Mali, like several other countries, is eager to confirm the interruption of transmission of onchocerciasis and LF, stop the distribution of MDA and enter into an elimination and surveillance phase.
This thesis assessed preparedness for onchocerciasis and LF MDA interruption and lymphedema morbidity management in Mali. More specifically, we evaluated a new rapid diagnostic test (SD Bioline Onchocerciasis/LF IgG4 Rapid test) for detection of O. volvulus and W. bancrofti infection in children 6-7 years of age according to the LF TAS guidelines in two evaluation Units. This test appeared to be a useful tool for monitoring onchocerciasis and LF exposure in co-endemic areas. We observed that all Ov16 antibody-positive subjects were residents of two previously known onchocerciasis meso- and hyperendemic districts. Onchocerca volvulus antibody prevalence levels were 0.19% and 0.18 % in the two districts. These values are above the 0.1% threshold proposed by WHO for stopping ivermectin MDA. However, given the specificity of the Ov16 Elisa test, this threshold is now considered to low. The results of our study suggest that in many onchocerciasis hypo-endemic areas in Mali, ivermectin MDA can be stopped.
In a second field study, we determined the all age serological profile of onchocerciasis and LF in the previously onchocerciasis hyperendemic setting of Bakoye and Falémé. The Ov16 and Wb123 seroprevalence in ≤14-year olds was less than 0.1% and 2% in the two areas respectively, consistent with elimination of onchocerciasis transmission and elimination of LF as a public health problem since stopping treatment in 2016.
We also studied the distribution and clinical presentations of lymphedema in previously LF endemic health districts in the post ivermectin and albendazole MDA period. We found that active lymphedema case identification (through village meetings, calls, visits, etc.) was more efficient than passive case reporting (through community health workers), the method used by most LF elimination programs, including the one in Mali.
We studied risk factors for W. bancrofti infection in a setting where LF is co-endemic with M. perstans. After controlling for age, gender, and village of residence, the likelihood of being W. bancrofti infected was higher in individuals with detectable M. perstans microfilariae, despite the fact that these infections are transmitted by different vectors. This finding is important for the LF elimination program, because of the possibility of false positive LF serological testing caused by cross-reacting antibodies in areas where M. perstans is present.
In conclusion, we did not observe evidence of ongoing LF transmission after ivermectin and albendazole MDA was stopped. With respect to onchocerciasis, our study suggests that stopping of ivermectin should be considered in the near future. Regardless, Mali needs to continue periodic surveillance for both onchocerciasis and LF including using entomological indicators to detect and respond to early signs of resurgence/re-introduction. Moreover, the country needs a stronger LF morbidity management program, that includes identification of active lymphedema cases.