A poisoned sea
Between 1932 and 1968, the Chisso Corporation's chemical factory in Minamata discharged an estimated 27 tons of mercury compounds into Minamata Bay. The factory produced acetaldehyde, a process that used mercury sulfate as a catalyst. The industrial wastewater, containing the neurotoxin methylmercury, was released directly into the bay. This toxin then entered the food chain.
Bacteria in the sediment may have also metabolized some inorganic mercury into the more toxic methylmercury. This lipid-soluble compound bioaccumulated in the fatty tissues of shellfish and fish. As larger fish consumed smaller ones, the concentration of methylmercury biomagnified, reaching levels in the bay's seafood up to 50 parts per million. For comparison, the U.S. FDA limit for commercially sold fish is 1 ppm. The local population, whose diet relied heavily on fish from the bay, ingested the poison with their daily meals.
The dancing cats of Minamata
The first signs of the poisoning appeared in the local animal population. Cats that ate fish scraps from the bay began to exhibit strange, uncontrolled movements, convulsions, and would sometimes run in circles. Locals called it "cat dancing disease" (猫踊り病, neko odori byō).
On May 1, 1956, the director of Chisso's factory hospital reported an "epidemic of an unknown disease of the central nervous system," marking the official discovery of Minamata disease. Patients experienced a range of severe neurological symptoms: numbness in their hands and feet, a lack of muscle coordination known as ataxia, muscle weakness, and damage to hearing, speech, and vision. In extreme cases, victims fell into comas, became paralyzed, or died within weeks.
The poison also crossed the placental barrier. Mothers who showed few or no symptoms gave birth to babies with severe congenital defects, a condition similar to cerebral palsy. The placenta, rather than protecting the fetus, concentrated the methylmercury from the mother's bloodstream.
Dr. Hajime Hosokawa, a doctor at the Chisso factory hospital, began his own experiments in July 1959. He fed factory wastewater to healthy cats. After 78 days, a subject known as "Cat 400" developed the same symptoms seen in the "dancing cats" and human patients. An autopsy confirmed organic mercury poisoning. The Chisso Corporation suppressed these findings and ordered Hosokawa to stop his research. The Japanese government did not officially acknowledge Chisso's industrial waste as the cause until 1968. By March 2001, 2,265 people had been officially certified as victims, and over 10,000 had received financial compensation from Chisso.