Onchocerciasis (river blindness) is an eye and skin disease caused by a worm (filaria) called Onchocerca volvulus. The disease is transmitted to humans through the bite of a blackfly (simulium species). The flies breed in streams and rivers, which accounts for the common name of the disease. Within the human body, the adult female worm produces thousands of larval worms which migrate in the skin and the eye. The drug ivermectin remains the first-line treatment of choice.
Researchers from the Washington University School of Medicine in St. Louis have undertaken an in-depth analysis of the genetic make-up of the disease, with the aim of pinpointing a weakness.
The main way to eliminate the disease is through inset control programs, and the World Health Organization aims to achieve global eradication by 2025. An obstacle to this is with some parasites showing resistance to the ivermectin drug.
To understand the basis of resistance requires an understanding of what is taking place at the genetic level. However, the matter is complicated by there being two separate strains of this parasite, which vary by geographical area: a savanna strain and a forest strain. The savanna strains are more virulent.
What is of interest is whether the parasites evolving to survive the treatment, or whether these worms are actually new strains. To answer this library samples of worms have been compared with new isolates, using advanced gene sequencing technology.
The results suggest the genomes of the worms have been shaped by massive drug-administration programs; therefore adaptation by the worms appears to have taken place rather than new strains emerging. There is also evidence of gene transfer, via the flies, from the more virulent savanna strain to the forest strains, with the potential to make the forest strains more dangerous.
The outcome affirms that as part of the strategy to eliminate the disease worldwide, an alternative drug to ivermectin may be needed to complete the process.
The research is published in the journal Nature Microbiology, in a paper called “Genomic diversity in Onchocerca volvulus and its Wolbachia endosymbiont.”
In related news, Guatemala recently became the fourth country in the Americas to eliminate river blindness disease. The announcement represents tough action by national and international bodies.
