A Dead Cat’s Brain Revives Discussion of 1960s Mercury Poisoning Disaster in Japan

The exact molecule behind the Minamata mercury disaster, caused by a chemical plant’s wastewater, remains a point of disagreement

A woman seems hesitant about buying fish at a shop in Tokyo in 1973
Archival image, July 9, 1973: (Original caption) A woman seems hesitant about buying fish at a shop in Tokyo recently (June 25) after the Japanese Health and Welfare Ministry's June 24th warning that no one should eat more than 567 grams (about one pound four ounces) of fish a week to avoid the possibility of dangerous mercury poisoning. Bettmann / Contributor

When the Chisso chemical factory in Minamata, Japan, dumped its wastewater into the bay in the 1950s and ‘60s, it poisoned thousands of people with mercury. Hundreds died in the immediate effects of the toxic environment, and thousands of others were left with neurological conditions, including chronic numbness and migraines or birth defects. Initial studies in the 1960s linked the toxicity to a molecule made of mercury and carbon called methylmercury.

But a new study published this January in the journal Environmental Science and Technology has reawakened debate about the exact chemical culprit behind the tragedy. Using a new, precise chemical-identifying method, the group found evidence of an understudied mercury molecule called alpha-mercuri-acetaldehyde. The researchers say this could change the story of methylmercury poisoning, but others in the field are critical, suggesting the results are overstated.

The research focuses on the brain tissue of a cat named 717. In 1959, a doctor associated with the Chisso chemical plant fed food mixed with wastewater to cats. The cats began to convulse, then became paralyzed and died. The doctor was ordered to stop his experiments, but he kept some proof of his studies. Brain tissue from 717’s autopsy was rediscovered in storage at Kumamoto University in 2001.

Initial re-analysis of 717’s brain turned up a mix of carbon-free, inorganic mercury molecules and methylmercury. But Graham George, co-leader of the experiment, tells Joshua Sokol at Science magazine that the new analysis reveals a major form of mercury that those previous studies, using less sensitive techniques, missed. “Were there other more prevalent forms of mercury present that were not detected? Yes, we think so,” George tells Science.

For decades, the prevailing theory has been that Chisso dumped wastewater containing either inorganic mercury or methylmercury into the bay. Bacteria in the bay converted the inorganic toxin into more noxious form of methylmercury. Either way, methylmercury could be absorbed by things living in the water, including plants and shellfish. Fish that eat plants would then also become infused with the toxic substance. And when people in and around Minamata ate the fish, they were poisoned as well.

"If you were poisoned with mercury, you'd go to the hospital and they would give you a substance which kind of binds up the mercury and then you would eliminate it out of your body," co-first author Ashley James, a toxicologist at the University of Saskatchewan, tells Alicia Bridges at CBC News. Taking a second look at the exact molecules involved in mercury poisoning could help researchers develop better treatments, she says.

Very little is currently known about the molecule they identified in 717’s brain. But the researchers suggest that alpha-mercuri-acedaldehyde was the main toxin involved in Minamata mercury poisoning and that it was present in the wastewater dumped by the Chisso chemical factory, per a University of Saskatchewan statement.

Other experts in the field tell Science that the team’s conclusions may be broader than the data suggests. “They did an admirable piece of chemistry, but it should not be interpreted beyond what it really shows,” Harvard University environmental toxicologist Philippe Grandjean, who was not involved in the study, tells Science. He says that the study identifies an unusual chemical in the cat brain, but no more.

Environmental scientist Charles Driscoll of Syracuse University, who was not involved in the study, points out to Science that residents of Minamata were ingesting mercury in their seafood, while cat 717 was fed wastewater directly. And, he says, the molecule could have been a result of the brain’s preservation or the cat’s metabolism.

“Quite a few things in [the study] give me pause,” Driscoll tells Science. “I was, frankly, surprised it would get published.”

In Japan, Chisso stopped dumping mercury-containing wastewater into the bay in the late 1960s, and it took decades of cleanup until the region’s shores were declared mercury-free. Today, thousands of people from Minamata and the surrounding towns still await recognition by the Japanese government for what happened to them.