Onchocerciasis is a vector-borne parasitic disease caused by the filarial nematode Onchocerca volvulus and transmitted by Simulium species. According to WHO, an estimated 217.5 million people live in areas at risk of onchocerciasis, the vast majority in sub-Saharan Africa. This estimate does not include low transmission areas that still need to be mapped . The main complications of this infection are severe eye disease that lead to blindness and skin disease characterized by papular or hypo-pigmented lesions and intense itching. It is the microfilariae (mf) which drive this pathology.
The main strategy for combatting onchocerciasis is mass ivermectin (Mectizan®) distribution. Following the discovery of ivermectin and its subsequent donation by Merck and Co, African control programmes adopted mass ivermectin treatment as their main control strategy, using community-directed treatment (CDTi) to distribute . Ivermectin (Mectizan®) is a potent microfilaricide that has limited effect on the viability and reproductive capabilities of adult onchocercal worms. Thus, repeated treatment is needed in order to suppress the manifestations of the infection over time and limit transmission. In 2009 the first evidence of elimination of the parasite from African foci using ivermectin alone emerged from Mali and Senegal . This led to a wider evidence gathering effort by the African Programme for Onchocerciasis Control (APOC), and as evidence supporting the feasibility of onchocerciasis elimination in Africa using ivermectin alone mounted, countries switched the focus of their programmes from control to elimination. In 2020, evidence of onchocerciasis elimination through annual mass drug administration was documented in Plateau and Nasarawa states in Nigeria .
In Nigeria, Onchocerciasis is endemic in 31 of the 36 States of the country including the Federal Capital Territory of Abuja. Repeated annual treatment with ivermectin commenced in 1990 and scaled up by 1997 in most of the endemic communities after undertaking rapid assessment of communities to be treated using Rapid Epidemiological Mapping of Onchocerciasis (REMO) .
The treatment of onchocerciasis commenced in Kaduna State with the clinical trial of ivermectin in Lere LGA in 1989. This was carried out in collaboration with Guinness Ophthalmic Unit, University of London, World Health Organisation and Sightsavers. Treatment was later expanded to Birnin Gwari and Kauru LGAs of Kaduna State in 1990 . The success of the clinical trial, which ended in 1991, set the stage for expansion of treatment to other LGAs in the state. Over the years, treatment for onchocerciasis increased from 6,149 people treated in 1989 to over 1.9 million people in 2012.
Wide-spread treatment with ivermectin has been ongoing at large scale in endemic LGAs in Kaduna State since 1997. Therefore, monitoring of both human and entomological indices of transmission is essential to determine the impact of ongoing treatment, and collect adequate entomological data to inform the design and implementation of future stop-MDA surveys. The World Health Organisation (WHO) guidelines on stopping onchocerciasis MDA stipulate that fly-catching and serological surveys must be conducted at the transmission zones before stop- MDA is implemented . Previous epidemiological studies by Tekle et al in 2008 but published in 2012 reported zero cases of onchocerciasis in the endemic communities in Kaduna State . In the present study, we present the results of entomological assessment of effects of 17 years mass distribution of ivermectin in two onchocerciasis foci in Kaduna State, Nigeria.