Washington, Nov 23 (IANS) Aiming to understanding why the roundworm that causes river blindness has started developing resistance to the drug used as the first-line treatment for the disease, a new study sheds light on the genetic makeup of the parasite.

The drug ivermectin remains the first-line treatment to target the parasite. The drug's discoverers were honoured with the Nobel Prize in Physiology or Medicine just last year.

But after several decades of exposure to ivermectin, these parasites are beginning to show resistance.

"We want to understand the origin of this resistance," said senior study author Makedonka Mitreva, Associate Professor of medicine at Washington University School of Medicine in St. Louis.

The parasite that causes river blindness infects about 37 million people in parts of Africa and Latin America, causing blindness and other major eye and skin diseases in about five million of them. 

The parasite is a roundworm called Onchocerca volvulus and is carried by black flies that thrive near rivers and streams. 

"Are these parasites evolving to survive the treatment, or are the surviving worms actually new strains that have been introduced due to migration of the black flies or of the parasite itself?" Mitreva asked.

To find out, Mitreva and her colleagues studied samples of the roundworms that have been stored since the early 1990s, before large-scale ivermectin treatment programmes began. 

They also are sequencing present-day strains for comparison. The current study, published in the journal Nature Microbiology, reports data only from the historical samples.

"We worked hard to find samples of the parasite that had not been exposed to the drug because we needed to establish an ancestral baseline," Mitreva said. 

The new study included 27 roundworm samples taken in the early 1990s from four distinct regions -- West African forest, West African savanna, Uganda and Ecuador.

"In West Africa, there are two separate strains of this parasite based on geographic areas -- the savanna strain and the forest strain," Mitreva said.

"The severity of the disease caused by each strain is very different. The savanna strains are much more virulent and have been associated with higher rates of blindness. The forest strains cause a mild skin disease. That's why we wanted to see how they differ genetically," she added.

According to Mitreva, the Uganda strains, collected in central Africa, could not be classified as either the forest or savanna strain. Similarly, the strains from Ecuador, in South America, were also very different from the African strains.

In general, Mitreva said, large-scale control efforts -- including mass treatment with ivermectin and the use of insecticides to reduce black fly populations -- have been very effective over the past 30 years. 

But since pockets of drug resistance are showing up, she said studies such as this one are important if the World Health Organisation's goal of eliminating the parasite by 2025 is to be met