Clusters of cases were reported by local nongovernmental organizations in the Kamwenge District of Western Uganda, although the region wasn't known to be affected or at risk for the condition.
The disease is typically caused by infection with parasitic worms, transmitted through certain mosquito species. But an investigation by the country's Ministry of Health has revealed a new root cause: volcanic soil.
This method of contracting the disease is known as podoconiosis, or non-filarial elephantiasis.
The region's soil comes from volcanic rocks formed more than 2.5 million years ago, and irritant minerals in remaining volcanic soil can cause itching, pain and eventual swelling and scarring after prolonged exposure.
Officials believe the current epidemic has been happening silently for more than 30 years.
Until the 1960s, the land had been covered in forest and generally occupied by hunter-gatherers. Farmers moved in and cleared the area, triggering the onset of exposure to these soils.
"When we looked at the data, we found a stable illness, not an increase," said Christine Kihembo, a senior field epidemiologist with the Ugandan Ministry of Health and the lead author of the study
on the outbreak, published Tuesday.
"This had been happening silently," she said.
Kihembo believes the epidemic was aided by stigma and isolation of people with severe symptoms, as well as a lack of awareness among local health workers.
"Affected people were not reporting to health facilities," she said.
Officials in the ministry are now working with local health workers to improve their knowledge and ability to manage the disease -- which has no treatment -- as well as inform the communities at risk to prevent it from developing.
"This disease is totally preventable with protective shoes and foot hygiene," Kihembo said.
But the people living with this form of elephantiasis are generally among the country's poor, meaning access to footwear is not always possible.
In August 2015, the World Health Organization's country office in Uganda received a report from a local organization of a "perceived increase in the number of elephantiasis cases," Kihembo's team wrote in their paper. A similar report was filed in 2014, leading the new reports to trigger an investigation.
"We highly suspected it was podoconiosis when these reports came in," Kihembo said, adding that people living in the region were predominantly poor farmers.
The team identified 52 people living with elephantiasis who, more intriguingly, had probably developed it between 1980 to 2015.
Further investigation found that 93% of those affected never wore shoes at work -- typically farming -- and 80% did not wear shoes at home. Another factor investigated was whether people washed their feet immediately after work or waited until the end of the day.
Seventy percent of people affected by the disease, as opposed to 40% from a control group, were found to wash their feet at the end of the day.
All signs pointed to contaminants in the soil, as people worked the land with bare feet and samples revealed earth rich in volcanic clays containing certain irritant minerals that enter the skin. The minerals cause itching and burning sensations followed by skin markings, rigid toes and swelling.
"Little crystals get into your lymphatic system. ... After many years, they block it so the lymph can't draw back and your foot swells up," said David Mabey, professor of communicable diseases at the London School of Hygiene & Tropical Medicine, who was not involved in the research.
The district has the characteristic traits of a region where this form of elephantiasis is endemic: typically, high altitude and high annual rainfall leeching calcium from soil, leaving it acidic. It's volcanic soils, added Mabey.
"The condition takes a long time to develop. It doesn't happen overnight," Kihembo said, adding that people may suffer for years before symptoms of elephantiasis become obvious.
"Once elephantiasis has set in, you cannot reverse the disease," she said.
The importance of shoes
Podoconiosis was discovered to be endemic in eastern parts of Uganda, near Mount Elgin, in 2001, leading health officials to predict that this may have been the underlying cause of the new epidemic in the west of the country.
Kihembo says the disease affects 4 million people globally, mainly in Central and Eastern Africa as well as Southeast Asia. One-quarter of those are in Ethiopia, according to the World Health Organization
In these countries, "there are volcanic mountains, and people don't wear shoes," Mabey said.
Previous studies have showed a genetic element to the disease, with family clustering reported in Rwanda, Ethiopia and Burundi. One study found siblings of affected people to have five times the risk of developing it themselves.
The new study did not find a significant association based on family history.
The study "is robust and has used appropriate techniques to identify podoconiosis and describe the extent of the problem in Kamwenge," said Gail Davey, professor of global health epidemiology at Brighton and Sussex Medical School, who was not involved in this investigation.
"Podoconiosis is still widely under-recognized in tropical Africa," she said. This finding, along with the investigation in 2001, "will help mobilize resources to map the distribution of podoconiosis throughout the country and then direct prevention and care programs to affected districts."
"The way to prevent this is to encourage people to wear shoes. ... There are a lot of parasites you can get from not wearing shoes," Mabey said. "Once your foot is swollen, there's not much that can be done."