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Mass drug administration (MDA) with ivermectin appeared to interrupt transmission of onchocerciasis, or river blindness, in several Nigerian states, and may have eliminated it altogether in other parts of the country, researchers said.

With a consistent MDA program, transmission of river blindness was "suspected" to be interrupted in four states in southern Nigeria, and transmission was interrupted in another state, reported Emmanuel Emukah, MD, PhD, of the Carter Center in Nigeria, during a presentation at the virtual American Society of Tropical Medicine & Hygiene (ASTMH) annual meeting.

A serosurvey of young children ages 5 to 9 determined "suspected" transmission interruption, and actual interrupted transmission was determined by an entomological survey of black flies to ensure lack of the Onchocerca volvulus parasite, which causes the illness.

If an OV16 rapid diagnostic test or ELISA diagnostic assay has a point prevalence of <1% in children, the state is eligible for evaluation by the WHO for a "Stop MDA" decision, which occurs if OV16 ELISA test positivity in 3,000 children is <0.1% and PCR of at least 6,000 flies is <1 infective fly per 2,000.

The process has two steps: the serosurvey and the entomological survey. With ongoing transmission in the four states of Abia, Anambra, Enugu, and Imo, the first step was taken: a serosurvey in October 2020 among children ages 5 to 9. Dried blood spots were collected via finger prick and then tested via OV16 ELISA.

Delta State, which already had "suspected" interrupted transmission, had an entomological survey from May to November 2020, for which black flies were collected at breeding sites via human landing capture or glue traps. Previously, Delta State had its own serosurvey in May/June 2019.

Emukah noted that all five states had an OV16 seroprevalence of <1%. Looking at the 2019 and 2020 data, there were nine positive specimens out of 16,098 total tested, for a 0.06% positivity rating.

Delta State, which was further along in the process, had two positive samples in 2019, but these then tested negative via skin snip PCR. They also passed the entomology criteria, with zero positive pools of black flies out of 73, with a total of 6,591 flies sampled.

Emukah said that based on these criteria, Delta State should move on to the next classification: "transmission interrupted," and the four other states should move from "transmission ongoing" to "transmission suspected interrupted." Based on these results, MDA for river blindness has stopped in Delta State, he added.

A second presentation at ASTMH by Abel Eigege, MD, also of the Carter Center, detailed how two other Nigerian states, Plateau and Nasarawa, became the first in the country to qualify for "Stop MDA" in 2017, after 8 to 26 years of treatment with ivermectin.

Post-treatment surveillance was conducted from June to October in both 2019 and 2020, where zero black flies tested positive out of 93 pools with 7,925 flies in Nasarawa and 89 pools with 7,368 flies in Plateau. WHO elimination criteria is defined as a transmission infectivity threshold of less than 0.05%.

In a statement, Frank Richards, MD, scientific advisor and former director of the Carter Center, noted that MDA with ivermectin has eliminated river blindness from four countries in the Americas, and led to significant progress in Nigeria.

"Ivermectin is of enormous proven value for a number of neglected tropical diseases that are a significant burden for millions of people. Before ivermectin arrived, we really had nothing to safely treat river blindness," he said. "Look at where we are now!"