The signs and symptoms of one of the most deadly accidental mass poisonings the world has ever seen first  manifested in the cats of Minamata, Japan. In the 1950s, residents of Minamata noticed a strange and increasingly present phenomenon – cats in the city were falling ill with a bizarre sickness that caused them to convulse, make odd noises, twitch uncontrollably, and ultimately die. Citizens were baffled by what appeared to be a plague devastating the city’s cats.

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The culprit turned out to be a Japanese chemical company called the Chisso Corporation. Unbeknownst to local citizens, the Chisso chemical plant had been dumping harmful byproduct into Minamata Bay for quite some time.

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Among other chemicals, Chisso was discarding mercury, and, unfortunately for the local environment and residents, the bay’s anaerobic bacteria reacted with the dumped mercury to transform it into methylmercury, the most noxious and dangerous variant of the metal.

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Mercury, in its elemental form, is not easily absorbed by the gut and instead is mostly flushed from our systems (eating mercury is still bad, to be clear), living creatures can soak up much larger quantities of any methylmercury that travels through our digestive tracts.

Plants and fish in the bay continued absorbing ever-rising levels of methylmercury, and over time the contaminant made its way up the food chain. The increasingly poisoned fish were consumed by the city’s cats, leading to the phenomenon of the “Dancing Sickness,”  the strange feline plague that left the citizens of Minamata so baffled.

And, as the residents of Minamata, Japan continued to eat more and more contaminated seafood, the poisonous substance began to ravage humans, too.

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Over 900 people died, and birth defects in the area surged. Thousands of Minamata residents have since been identified as suffering the effects of mercury poisoning – in fact, the neurological syndrome that severe mercury poisoning causes is now known as Minamata disease. Though the Chisso Corporation ceased dumping mercury in 1968, for many people it was simply too late.

If you’d like a more detailed explanation of what occurred in Minamata, take a look at this video:

h/t: Mental Floss