After you see a case of elephantiasis, you can never forget it.
People's legs, feet and toes swell up so much that they can't walk. Or move easily. The skin thickens and breaks open, creating ulcers and infections.
"It causes so much pain. So much pain," says epidemiologist Christine Kihembo, at Makerere University School of Public Health in Kampala, Uganda.
The trigger for the disease is typically a tiny worm. About the width of human hair, the worm lodges inside lymph nodes. Instead of the bodily fluid draining and moving around the body properly, it gets trapped in the extremities but also sometimes around the genitals.
So when Kihembo started hearing reports of farmers with swollen extremities and thickened skin in western Uganda in 2015, she and her colleagues thought they knew right away what the problem was: an outbreak of elephantiasis.
They traveled out to the community, which lives high in the foothills of western Uganda. "This is a very remote community in tropical forest," Kihembo says. "They receive about 4 feet of rain each year and live at an average elevation of 4,000 feet."
When Kihembo and her colleagues reached the area, the team identified about 50 farmers with the telltale signs of elephantiasis. And they started looking for the worm.
They took blood samples from people. Hunted down mosquitoes that spread the parasite. They even checked soil around the farmers' homes.
But time and again they came up empty-handed. There were no signs of the worm anywhere, Kihembo and her team report Monday in the American Journal of Tropical Medicine and Hygiene.
So Kihembo started doing disease epidemiology the old-school way: talking to people.
They interviewed more than 40 people with the disease and found that nearly all of them had something in common: They farmed in volcanic rock — without wearing shoes.
Back in the 1970s, Ernest Price was a British surgeon living in eastern Ethiopia. He noticed people had a risk of developing swollen feet, thickened skin and ulceration when they farmed barefoot in red clay soil.
He called the disease podoconiosis, from the Greek "podos" and "konos," which mean "foot" and "dust." And since then, health workers have found cases of podoconiosis on three continents, from the mountains of Mexico to the volcanic ridges of Sri Lanka.
The disease is widespread along the equator in Africa, with 10 countries reporting cases. Ethiopia alone has at least a million cases and Cameroon at least half a million, the World Health Organization says.
Scientists aren't exactly sure how red clay soil triggers the podoconiosis. One hypothesis is that silica — or some other mineral in the soil — creates tiny slices in the skin and makes its way into the lymphatic system. Then the immune system attacks the mineral, creating a massive inflammatory response that eventually damages lymph vessels.
Instead of worms trapping fluid, the body's own immune system causes the swelling.
The disease takes years to develop, Kihembo says.
"At first, farmers say they have pain, itching and swelling in their feet, in the evening, after working on the farm," she says. "But in the morning it's gone."
This cycle happens for years until one morning, the swelling and pain don't go away in the morning. Then the damage is hard to reverse.
"These people probably have been suffering silently without help for more than 30 years," Kihembo says.
And there's a surprisingly simple way to stop progression of the disease and prevent the problem in the future: Wear shoes while working in the fields.
"These people are not in a position to afford shoes," Kihembo says. "As much as we wanted, we didn't give them shoes because we didn't have the means to.
"But really what the community needs is economic capacity so they can buy shoes," she adds. "We pray and hope that we have the support to continue this work so we can help this community."
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