The machete blades turned red with heat in the fire that the rubber workers built on a Liberia plantation, Thomas Unnasch remembers from a visit in the 1980s.

This was how the men tried to quell the intense itchiness that comes with river blindness, a rare tropical disease.

"You can imagine how bad the itching must be, that running a red-hot machete up and down your back would be a relief, but it was," said Unnasch, whose laboratory works on diagnostic tests for the disease.

About 18 million people have river blindness worldwide, according to the World Health Organization, but more than 99% of cases of this disease are found in Africa. It goes by the technical name "onchocerciasis," and it spreads through small black flies that breed in fast-flowing, highly oxygenated waters. When an infected fly bites a person, it drops worm larvae in the skin, which can then grow and reproduce in the body.

Unlike malaria, river blindness is not fatal, but it causes a "miserable life," said Moses Katabarwa, senior epidemiologist for the Atlanta-based Carter Center's River Blindness Program, which has been leading an effort to eliminate the disease in the Americas and several African countries.

Some strains cause blindness, while others come with more severe skin disease. With time, generally all strains of the disease can lead to rough "lizard" skin, depigmented "leopard skin" and hanging groins. Another big problem among patients is itching, which happens when the worms die inside a person.

In southwest Uganda, the locals call the disease "Obukamba," referring to the symptoms of distorted skin appearance and itchiness, Katabarwa said. In western Uganda, he said, "the fly is called 'Embwa fly' or dog fly, for it bites like a dog!"

There is no vaccine for river blindness, but there is a drug, called ivermectin that paralyzes and kills the offspring of adult worms, according to the Mayo Clinic. It may also slow the reproduction of adult female worms, so there are fewer of them in the skin, blood and eyes. The pharmaceutical company Merck has been donating the treatment, under the brand name Mectizan, since 1985.

Elimination efforts

Great strides have been made against this disease. In the Americas, it was eliminated in Colombia in 2007 and in Ecuador in 2009.

The biggest area of concern is Africa. In 1995 the African Programme for Ocnhocerciasis Control was founded by a group of nongovernmental organizations, governments, and United Nations agencies, with the World Health Organization overseeing it.

The APOC partnership, which includes the Carter Center, has given out 500 million ivermectin treatments between 1995 and 2010, according to a new study in the journal PLOS Neglected Tropical Diseases.

The organizations and governments involved in APOC have collectively avoided 8.2 million years of life that would have been lost to unhealthiness as a result of this disease from 1995 to 2010, the study said.

The study projected that APOC's impact will be even greater from 2011 to 2015, avoiding 9.2 million years of life that would have otherwise been lived in agony from this disease.

In Uganda, Katabarwa's home country, the country's goal is to eliminate the disease by 2020. The Ministry of Health and the Carter Center, partners of APOC, are working toward making that happen.

Instead of the standard ivermectin treatment of once a year, Katabarwa and colleagues have been working on distribution every six months. More frequent treatments eliminate more baby worms and reduce the reproductive capacity of the adult female worms inside patients' bodies, Katabarwa said.

Where possible, they are also treating fast-flowing waters with a biodegradable insecticide called Abate to wipe out the larvae of the infectious black fly population. And they are training local populations to give out the medicine and keep track of who has been treated.

'You never sleep'

It can't be overstated that the worms itch, a lot. Sometimes, said Unnasch, people infected with the worms are driven to suicide.

Katabarwa also has stories of people inflicting harm upon themselves because of the itchiness. There are cases where people boil water to pour on themselves, he said.

"You scratch yourself until you damage your skin," Katabarwa explains. This leads to other health problems: The scratch marks bleed, making you more prone to bacterial infections. Children with the worms can't concentrate because they are scratching themselves all day and night.

"You never sleep," Katabarwa said. "Some people use a clay pot and break it to scratch themselves because the nails are not enough."

Relief from itchiness comes with the ivermectin -- even for someone who has lived with the disease for more than 50 years, he said. By taking the drug frequently, the skin can partly heal.

Unfortunately, for patients who lose all sight, blindness is irreversible because the optic nerve is damaged. Skin that has hardened, sometimes to the point where needles can't penetrate it, will not reverse itself, either.

The side effects from the medicine can also be nasty: Ivermectin may cause fever, itching, skin rash, joint or muscle pain, rapid heartbeat, and painful, tender glands. Less common side effects include headache and swelling of a person's hands, feet, legs, face or arms. A person can also suffer a serious allergic reaction from it, so health workers bring antihistamines.