"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.
"If your immune system is very efficient, it will try to attack the dead worms" that the medicine kills, Katabarwa explained. "The itchiness that will be triggered is incredible. It happens within 10 minutes."
But most of the reaction subsides after two days. During that time, other biting insects such as mosquitoes, lice and bedbugs also die if they attempt to bite the treated patient.
Because of the observed benefits of treating river blindness, infected people agree to take the medication despite the side effects, Katabarwa said.
"When people see you with that skin, they don't want to be near you," Katabarwa said.
How disease creates outcasts
One patient who made a big impression on Katabarwa was Semanza from the Rukungiri district of Uganda. In 1992, Semanza's skin looked like it was covered in dried mud, and flies swarmed around him. No one from his village wanted to be near him and he lived in a hut behind his family's home, separated from everyone else.
"He was really forgotten," Katabarwa said. Semanza's skin was so thick, because of river blindness, that it could break a needle.
Katabarwa told the local health workers to give Semanza a dose of ivermectin every three months, which is more often than the standard distribution.
Years later, when Katabarwa visited again, a man approached him and hugged him, smiling. "Who are you?" Katabarwa asked. It was Semanza. Two years later, they reunited again, and Semanza revealed that he had married and wanted many children.
Katabarwa also remembers educating a particular community in northwest Uganda in 1993, which had all the right conditions for black flies that transmit river blindness to breed.
A group of women in the community told Katabarwa that because of river blindness, people from surrounding areas did not want to marry anyone from their village. As a result, marriages had to happen within the community, including among relatives. Incest was strictly forbidden in their culture, but because of river blindness, it happened quietly, Katabarwa said.
When Katabarwa returned five years later, the skin of infected individuals had greatly improved, and marriages with outsiders were happening again.
"These are stories of sadness, but after a few years there is hope," Katabarwa said.
Katabarwa has taken the approach of making use of Ugandan villages' kinship structures to assist with health care.