Onchocerciasis transmission indicators following long-term ivermectin treatment in Imo river Basin, Nigeria

Abstract

Simulium damnosum sl, the vector of Onchocerciasis in Nigeria breeds in fast flowing and well oxygenate rivers and streams with nutrients. To monitor the impact of repeated community directed treatment with Ivermectin on the interruption of transmission and/or elimination of Onchocerciasis, the transmission indices of female flies caught between May 2019 and March 2020 were calculated. Captured black flies at Lolo and Ibii Riverswere morphologically identified, dissected and processed for parity, infection/ infectivity as well as transmission status using standard protocol. Of the 889 adult females caught, rainy season months (May-September) had greater proportion than dry season months (October-March). Nulliparous black flies density (78.97%) were significantly higher than parous (21.08%) black flies (P< 0.05). 0.67% of the parous flies were infected with different stages of the parasite but none had infective larvae (L3 in the head). The calculated monthly biting rate (MBR) ranged from 259 (in February) to 899 (in September) with a mean biting rate of 662.925 bites/ person/ month. Monthly parous biting rate (MPBR) ranged from 23 (in February) to 247.5 (in November) with a mean parous biting rate of 140.225 bites/person/month. The transmission potential (TP), number of infective bites/ person/ month was zero. These findings indicated entomological evidence of reduced vector biting activities and low risk of river blindness infection in the study area. The public health implications were discussed especially with regards to the on-going community directed treatment with ivermectin (CDTI) and elimination strategy.