A mysterious epidemic is sweeping Central America - it's the second biggest cause of death among men in El Salvador, and in Nicaragua it's a bigger killer of men than HIV and diabetes combined. It's unexplained but the latest theory is that the victims are literally working themselves to death.

In the western lowlands of Nicaragua, in a region of vast sugar cane fields, sits the tiny community of La Isla.

The small houses are a patchwork of concrete and wood. Pieces of cloth serve as doors.

Maudiel Martinez emerges from his house to greet me. He's pale, and his cheekbones protrude from his face. He hunches over like an old man - but he is only 19 years old.

"The way this sickness is - you see me now, but in a month I could be gone. It can take you down all of a sudden," he says.

Maudiel's kidneys are failing. They do not perform the essential function of filtering waste from his body - he's being poisoned from the inside.

When he got ill two years ago, he was already familiar with this disease and how it might end. "I thought about my father and grandfather," he says. Both died of the same condition. Three of his brothers have it too.

All of them worked in the sugar cane fields.

Kidney disease has killed so many men here that locals now call their community not simply La Isla - which means "The Island" - but La Isla de las Viudas - "The Island of the Widows." (You can see a slideshow from Nicaragua at PRI's The World).

It is wasting away our populations Maria Isabel Rodriguez, El Salvador health minister

The epidemic extends far beyond Nicaragua. It's prevalent along the Pacific coast of Central America - across six countries.

"It is important that the chronic kidney disease (CKD) afflicting thousands of rural workers in Central America be recognised as what it is - a major epidemic with a tremendous population impact," says Victor Penchaszadeh, a clinical epidemiologist at Columbia University in the US, and consultant to the Pan-American Health Organization on chronic diseases in Latin America.

El Salvador's health minister recently called on the international community for help. She said the epidemic is "wasting away our populations".

Heat stress

At a health clinic in El Salvador, in the farming region of Bajo Lempa, Dr Carlos Orantes recently found that a quarter of the men in his area suffered from it.

What is chronic kidney disease? Image caption Jesus Sosa Mancia is a CKD patient in Bajo Lempa, El Salvador Chronic kidney disease, or CKD, is permanent or long-lasting kidney damage. CKD is often without any symptoms in the early stages, so many people don't know they have it until the later stages, when symptoms include anaemia (with weakness/breathlessness), bone disease, nausea and vomiting. Final-stage CKD patients may die without dialysis or a kidney transplant. In the developed world, the primary causes of CKD are diabetes and high blood pressure, which are becoming more common as a result of increasing obesity, lack of exercise, and high salt intake. In the developing world, the main causes are chronic infections like HIV, viral hepatitis, malaria, and tuberculosis. Dr Charles Tomson, President, UK Renal Association

What's more, he says, most of the men who are ill show no signs of high blood pressure or diabetes - the most common causes of CKD elsewhere in the world.

"Most of the men we studied have CKD from unknown causes," he says.

What the men in his area have in common is they all work in farming. So Dr Orantes thinks a major cause of their kidney damage is the toxic chemicals - pesticides and herbicides - that are routinely used here in agriculture.

"These chemicals are banned in the United States, Europe and Canada, and they're used here, without any protection, and in large amounts that are very concerning," he says.

But he's not ready to rule out other possible causes. For instance, the overuse of painkillers can damage the kidneys, and so can drinking too much alcohol. Both are major problems here, he says.

In Nicaragua, the disease has become a political issue.

In 2006, the World Bank gave a loan to Nicaragua's largest sugar company to build an ethanol plant. Plantation workers filed a complaint, saying the company's working conditions and use of chemicals were fuelling the epidemic. They said the loan violated the bank's own standards for worker safety and environmental practices.

In response, the bank agreed to fund a study to try to identify the cause of the epidemic.

"The evidence points us most strongly to a hypothesis that heat stress might be a cause of this disease," says Daniel Brooks of Boston University, who is leading the research.

His team has found it's not just sugar cane workers who are falling ill. Miners and port workers also suffer high rates of kidney disease, yet they're not exposed to farm chemicals. What these men have in common, he says, is they all work long hours in extreme heat.

Image caption Nicaraguan sugar cane workers leaving the fields

"Day after day of hard manual labour in hot conditions - without sufficient replacement of fluids - could lead to effects on the kidney that are not obvious at first but over time accumulate to the point that it enters into a diseased state," says Mr Brooks.

"This has never been so far shown to cause chronic kidney disease, so we would be talking about a new mechanism that has not so far been described in the scientific literature."

But Mr Brooks says a new preliminary study bolsters this hypothesis. His team tested blood and urine from sugar cane workers who perform different jobs. The scientists found more evidence of kidney damage in the workers who have more strenuous jobs outside.

Professor Aurora Aragon of Nicaragua's National University in Leon says this explanation makes sense. She's long suspected that part of the problem is the way sugar cane workers are paid - receiving more money the more sugar cane they cut.

"This way of working forces people to do more than they are able to do, and this is not good for their health," she says.

No alternative

"Working in the field made us feel dizzy and nauseous," says Jose Donald Cortez who cut sugar cane for 18 years. "We often had fevers."

Cortez now has kidney disease and heads an organisation of sugar cane workers in Nicaragua who are ill. He's convinced that something on the sugar plantations is causing the sickness.

Whatever it is, he says, those who are ill need treatment with dialysis - which can keep them alive when their kidneys fail. But few can get it because dialysis is extremely expensive and rarely available.

Image caption A hydration drink being given to workers in the fields of Ingenio San Antonio, Nicaragua

"If you ask the ministry of health they say they don't have the money. If you ask the sugar company if they are responsible, they say 'No'."

For their part, the sugar cane companies say they're not convinced that farm chemicals or working conditions on their plantations are to blame for the epidemic. Still, they say, they are trying to protect their workers' health.

One conglomerate that owns several sugar plantations in Central America - the Pellas Group - says it's started giving workers an hour-long lunch break and now employs staff to make sure the men drink water. The company also routinely tests its workers' kidney function.

Company spokesman Ariel Granera says if a worker is found to have kidney disease, he is let go - out of concern, says Mr Granera, for his well-being.

But the sick workers who have been dismissed say what they receive from the companies and from social security isn't enough to live on - and when they lose their jobs, they lose the right to be treated at company clinics.

In La Isla, and many other villages like it, the men often take jobs with contractors who do not check for kidney disease.

Everyone fears that working in the sugar cane fields is a big risk, but there are no other jobs around.

"There is no alternative," says one woman, who recently lost her father. "No other way to support a family."

Listen to more of Kate Sheehy's story, and see a slideshow, at PRI's The World, a co-production of the BBC World Service, PRI, and WGBH in Boston. PRI's The World looked into this story with the International Consortium of Investigative Journalists (ICIJ), a project of The Center for Public Integrity.