Modelling the impact of mass administration of ivermectin in the treatment of onchocerciasis (river blindness)

Onchocerciasis (river blindness) is a disease spread from black flies to humans. This disease is responsible for chronic morbidity in sub-Saharan Africa. The principal strategy to achieve onchocerciasis elimination is through mass drug administration with ivermectin, a drug that is effective in the short term but wanes quickly. Ivermectin kills the skin-dwelling microfilariae. It may also kill and/or sterilize adult worms. This treatment protocol occurs bi-annually. Consequently, a system of impulsive differential equations is introduced to model both fixed and non-fixed mass drug administration with ivermectin. We determine the threshold for the proportion of treated individuals that reduces the infection in the human population. In the absence of impulsive mass drug administration with ivermectin, we determine the threshold for eradication and carry out stability analysis. The sensitivity analysis results reveal that the disease is unlikely to be eradicated without extremely low transmission levels or strong vector control. If treatment is included, then treatment at fixed intervals can control but not eradicate the disease. Treatment at non-fixed intervals may produce bursts of infection. Thus, bi-annual mass drug administration with ivermectin that is tailored to eradicate onchocerciasis, can only lead to significant reduction of onchocerciasis. However, to achieve 2020/2025 onchocerciasis elimination goals set by World Health Organization, the human-vector contact should be sufficiently reduced and vector control programmes implemented to supplement an intensive and effective mass drug administration with ivermectin.