In Sri Lanka’s North Central province, a land of crumbling Buddhist temples and gently swaying palms, farmers have cultivated rice for millennia. Until the 1960s, they relied on oxen, not tractors, to plow their fields. But the introduction of mechanized and chemical methods has rendered such time honored techniques extinct. An island nation that once boasted nearly 3,000 ancient rice varieties now produces a handful of modern strains in fields routinely drenched with herbicides and synthetic fertilizers.

As a result, Sri Lankan rice yields have increased by 60 percent since 1979. Unfortunately, a burgeoning public health disaster has arisen alongside this “progress.” Today, a mysterious kidney disease afflicts an estimated 400,000 people in the province, representing about a third of its population.

First-world renal disorders typically accompany obesity, diabetes, or high blood pressure, all rare risk factors in rural Southeast Asia. Baffled epidemiologists refer to the illness ravaging Sri Lanka as “chronic kidney disease of unknown etiology,” or CKDu, which results in a slow, torturous death. Sufferers are unable to pass urine, so their limbs swell with toxic fluid, causing constant pain.

And Sri Lanka is far from the only place affected: CKDu has been identified in the agriculture-dependent tropical areas of at least 10 other countries. New York City photojournalist Ed Kashi, who captured the images on these pages, began chronicling the epidemic four years ago, in Nicaragua and El Salvador, then India, before traveling to Sri Lanka last summer. “I met guys in their twenties who could barely get out of bed for me to take a portrait,” he reports. “There was one Sri Lankan family in which both the father and the son were sick. Neither could work, and the women were forced to abandon their dreams of getting an education in order to become the breadwinners.”

Many of the Sri Lankan farmers Kashi spoke to suspected that the illness stems from contaminated drinking water. Arsenic, cadmium, and other heavy metals found in synthetic fertilizers and pesticides frequently surface in the country’s groundwater. Channa Jayasumana, Ph.D., a medical toxicologist at Sri Lanka’s Rajarata University, believes there’s no single smoking gun, but rather a confluence of factors at play, among them: malnutrition, chronic dehydration, a prevalence of infectious diseases like leptospirosis, and the interaction between groundwater supplies and agricultural chemicals. Specifically, his research focuses on whether those chemicals and heavy metals, along with the water’s naturally occurring calcium and magnesium, are forming new, more acutely toxic compounds. Jayasumana notes that glyphosate, the active ingredient in Monsanto’s Roundup, was originally patented in part as a chelating agent—a substance that readily binds with metals—and may act as a vector, carrying heavy metals to the kidneys.

Though Monsanto vigorously denies Jayasumana’s claims, he counters that the corporation tests its products in temperate Western countries. “Our research shows that pesticides behave differently depending on climate, soil type, and groundwater conditions. It is the duty of Monsanto to show that these chemicals are not toxic in other parts of the world,” says the doctor.

Meanwhile, the Sri Lankan government isn’t taking any chances. President Maithripala Sirisena instituted a national ban on glyphosate upon taking office in 2015. Pesticides containing propanil, carbaryl, chlorpyrifos, or carbofuran had already been outlawed in 2014. Yet farmers continue to purchase the prohibited products at inflated black-market prices.

Companies like Hayleys Agriculture Holdings Ltd. are now funding the installation of reverse-osmosis water filters in the North Central Province’s villages. “The irony,” explains Kashi, “is that this company is a distributor of the pesticides and fertilizers that many Sri Lankans believe caused the epidemic.” Which is why he’ll continue to document the global crisis. “It is unacceptable to me,” the photographer says, “that farmers who produce the rice we consume on a daily basis could be dying on the job.”