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