Medical detectives in western Uganda recently discovered that the wrong culprit had been blamed for an outbreak of crippling elephantiasis — legs so swollen that they resemble those of an elephant.
As it turned out, one rare, neglected tropical disease had been mistaken for another.
In most affected countries, elephantiasis is caused by worms spread to humans by mosquito bites. The worms nest in lymph nodes, growing into big balls that stretch the glands and prevent lymph fluid from being pumped out of the legs.
The limbs swell, develop ulcerating sores and eventually rot. About 40 million people are disfigured or disabled by the disease, called lymphatic filariasis, the World Health Organization estimates.
In 2015, the Ugandan health ministry was told that an outbreak of elephantiasis was emerging in Kamwenge, in western Uganda. A team from the ministry, the W.H.O. regional office and Makerere University went to investigate.
The scientists found 52 suspected victims, and the rumor in the area was that the disease was caused by stepping in elephant dung. The investigators’ working assumption was that the worms that cause lymphatic filariasis had recently reached local mosquitoes.
But blood tests for worms came up negative, according to a report published Monday in The American Journal of Tropical Medicine and Hygiene. There were other clues that the hypothesis was wrong.
Some victims reported that their legs began swelling as far back as 1980. Most had swelling in both feet, while filariasis tends to affect one leg. And most were farmers who lived above 4,000 feet, where mosquitoes are less common.
Also, only one victim had a swollen scrotum. That symptom — scrotums so huge that, in extreme cases, they must be carried in wheelbarrows — is even more common than swollen legs among victims of lymphatic filariasis. It is also more stigmatizing and causes more lost work.
The researchers realized that they were facing something even rarer than worm disease: The victims had podoconiosis, a disease caused by walking barefoot in volcanic soils.
Such soils contains tiny, sharp, alkaline mineral crystals that work their way under the skin, causing fierce itching, and then are attacked by white blood cells, triggering inflammation that can develop over time into weeping sores and fibrous tissue.
Podoconiosis affects about four million people in Africa and Asia, but health care workers in western Uganda had never heard of it.
According to the study’s lead author, Dr. Christine Kihembo, a health ministry epidemiologist, the Kamwenge area had been forested and occupied until the 1960s by hunter-gatherers.
They were displaced by farmers migrating in from the south, who tilled the soil for the first time, exposing minerals deposited in the reddish clay during eruptions 2.5 million years ago.
Many farmers were too poor to afford shoes; 63 percent of those interviewed said they worked barefoot and 67 percent did not wash their feet right after leaving their fields.
“Many of the affected probably had been suffering silently without help for more than 30 years,” Dr. Kihembo said. She suggested that the government or donors give out rubber boots and teach people about the dangerous dirt.
Dr. Frank O. Richards Jr., a parasitologist at the Carter Center in Atlanta with a lot of experience working in Africa, said he was surprised that the cause of the outbreak had initially seemed mysterious.
“If all the patients you see have bilateral disease, that is, both feet are equally messed up, the clinical pearl you’re thinking about first is podoconiosis,” he said.
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.