Chronic kidney disease linked to heat stress could become a major health epidemic for millions of workers around the world as global temperatures increase over the coming decades, doctors have warned.

More research into the links between heat and CKDu – chronic kidney disease of uncertain cause – is urgently needed to assess the potential scale of the problem, they have said.

Unlike the conventional form of chronic kidney disease (CKD), which is a progressive loss of kidney function largely seen among elderly people and those afflicted with other conditions such as diabetes and hypertension, epidemics of CKDu have already emerged primarily in hot, rural regions of countries such as El Salvador and Nicaragua, where abnormally high numbers of agricultural workers have begun dying from irreversible kidney failure.

CKDu has also started to be recorded as affecting large numbers of people doing heavy manual labour in hot temperatures in other parts of Central America as well as North America, South America, the Middle East, Africa and India.

Kidneys are responsible for fluid balance in the body, which makes them particularly sensitive to extreme temperatures. There is an emerging consensus that CKDu should be recognised as a heat stress-related injury, where workers are developing subtle damage to their kidneys each day while they are in the field. This in turn can develop into severe kidney disease or complete renal failure over time.

This repetitive low-grade assault on the kidneys does not necessarily come with symptoms, so workers may not even know they are getting sick over time until things get so bad that they end up with end-stage kidney disease, said Dr Cecilia Sorensen, director of the global consortium on climate and health education at Columbia University.

“I think we just have no idea what the scope of the problem is because we’re not doing surveillance for it,” she said. “There are some regions that are clearly hotspots but in terms of its prevalence and how serious a problem it is, I don’t even think we’ve begun to wrap our brains around it.”

The documented epidemics, however, have similar characteristics. Those affected tend to be people who work in hot conditions outdoors and come from disproportionately vulnerable backgrounds – socially and economically – with limited access to medical care or insurance, or live in areas with modest healthcare infrastructure.

Sorensen said that, according to current data, it appears that the severity of the kidney damage gets worse the more vulnerable and desperate the worker is. She says that those who have no control over their working conditions or are incentivised to work for longer hours with no breaks, such as workers paid for how many berries they pick or how much sugar cane they cut, are likely to be those worst affected.

“They’re getting sick from the work that they’re doing, but they have no other options, and there’s very little regulatory oversight in the work environment that prevents this from happening. It’s a huge blind spot and a human rights issue,” she said.

Dr Ramón García Trabanino, a clinical nephrologist and medical director at El Salvador’s Centre of Hemodialysis, first noticed an unusual number of CKD patients saturating his hospital as a medical student more than two decades ago.

“They were young men,” he said, “and they were dying because we didn’t have the budget or the capacity to give them dialysis treatment. We did the best we could, but they kept dying and more kept coming.”

Since then he has started researching similar epidemics in Mexico, Nicaragua, Costa Rica and Panama.

“If you take a look at the maximum temperature maps in the region in Central America, you will notice that they match the regions where we are describing the disease, the hotspots,” he said. “El Salvador and Nicaragua – every year we have a fight for the first place for the country with the highest mortality due to CKD. Our mortality rates are about 10 times higher than what we should expect. The number of new patients is overwhelming.”

Although the consensus view is that CKDu is related to heat exposure and dehydration, some scientists believe exposure to agrochemicals and infectious agents, as well as genetic makeup and risk factors related to poverty, malnutrition, and other social determinants of health, are also likely to play a role.

Prof Richard Johnson, of the University of Colorado’s school of medicine, said: “What is less clear is the fact that recurrent heat stress is not just a problem in the sugar cane fields of Nicaragua. Even in our own societies, the possibility that heat stress and dehydration can be playing a role in kidney disease is not as appreciated.”

Prof Tord Kjellstrom, of the Australian National University’s national centre for epidemiology and population health, said that heat stress is not getting the attention it needs in debates around how to mitigate the worst effects of the climate emergency.

“As the number and intensity of hot days increases, more and more working people will face even greater challenges to avoid heat stress, particularly the two-thirds of the global population who live in tropical and sub-tropical areas. Heat exhaustion threatens the livelihoods of millions and undermines efforts to reduce poverty,” said Kjellstrom, who is also a former member of the Intergovernmental Panel on Climate Change.

“Global heating is a serious threat both to workers’ lives and the livelihoods of millions of people. Emerging policies on climate must take this into account if we are to have any chance of getting to grips with what is ahead.”