River Blindness May Have Stopped Spreading in Most of the Americas

By Rob Goodier

November 30, 2020

NEW YORK (Reuters Health) - The spread of river blindness, or onchocerciasis, may have been halted in nearly two-thirds of the 34,000 people who remain at risk in the Americas, new evidence suggests.

The disease has been chased down to remote areas in the jungles of Venezuela and Brazil, where health workers have likely halted transmission in 61% of at-risk communities and 64% of the population, according to findings presented November 18 at the virtual annual conference of the American Society of Tropical Medicine and Hygiene.

"Interrupting transmission in over 60% of the communities in that area, which we believe has happened, means the countries have taken a big chunk out of the remaining at-risk population against real odds. So, if our suspicions are correct, we're now seeing transmission in less than 3% of the original population at risk in the Americas," lead author Lindsay Rakers of The Carter Center, a not-for-profit organization in Atlanta, Georgia, told Reuters Health by email.

River blindness is caused by the parasitic roundworm Onchocerca volvulus, which spreads through the bite of infected black flies. Since 1993, a public-private initiative called the Onchocerciasis Elimination Program for the Americas has battled the disease to elimination in Mexico, Guatemala, Ecuador and Colombia, four of the six countries where it was once endemic. Its last bastion in the Western hemisphere is now in indigenous Yanomami communities in the Amazon rainforest, straddling the border between Venezuela and Brazil.

Verification of the halt of transmission requires serology tests of children and polymerase chain reaction (PCR)-based assay tests of black flies in the region. The World Health Organization has set thresholds for declaring interruption at less than 0.1% positive serology in children and less than 0.05% infectious flies.

Rakers and her colleagues base their estimate on two pieces of evidence. One is the history of treatment in the regions that remain at risk for the disease. Past experience in other communities in the region has found transmission is interrupted when at least 85% of a community undergoes 20 or more consecutive rounds of treatment with the anthelmintic ivermectin.

Reports from 2019 indicate that 384 of the 629 communities (61%) that remain at risk for the disease had met that criterion, corresponding to 64% of the at-risk population, Rakers said.

The other piece of evidence comes from PCR tests of black flies. Of more than 84,000 flies collected in at-risk communities in Venezuela between 2006 and 2019 and some 76,000 collected in Brazilian communities from 2017 to 2018, 0.1% or fewer were found to be infected, meeting the WHO's threshold for interrupted transmission.

The data suggest there may be just 50 to 250 new cases per year, Rakers said. At this rate, she added, the WHO may be able to verify elimination in the Americas by the year 2027.

But getting there may be difficult.

"Indeed, the last mile, or last inch, is always hard, since you get down to finding the needle in the haystack to do it," said Dr. Daniel Bausch, a professor at the London School of Hygiene and Tropical Medicine, who was not involved in the research.

"In the end the barriers are often political and social more than scientific," Dr. Bausch told Reuters Health by email.

But Rakers noted that rather than present barriers, the governments in Venezuela and Brazil have carried out most of the work in the Yanomami communities to date.

"I would really like to emphasize that the success I'm reporting is due to the work of the Brazilian and Venezuelan Ministry of Health teams who are on the ground getting medicine into people's mouths against incredible challenges of access that one encounters in the Amazon Rainforest," she said.

Besides national governments other members of the Onchocerciasis Elimination Program for the Americas include the WHO, the Pan-American Health Organization, the U.S. Centers for Disease Control and Prevention, the Carter Center, Merck, Lions Clubs International Foundation and others.

SOURCE: https://bit.ly/39djIDS ASTMH 2020 annual meeting, November 18, 2020.