The disease developed without any prior warning, with patients complaining of a loss of sensation and numbness in their hands and feet. They became unable to grasp small objects or fasten buttons. They could not run or walk without stumbling, their voices changed in pitch, and many patients complained of difficulties seeing, hearing, and swallowing. In general, these symptoms worsened and were followed by severe convulsions, coma, and eventually death. By October 1956, forty patients had been discovered, fourteen of whom had died: an alarming case fatality rate of 35%. Finding the cause
Researchers from Kumadai also began to focus on the cause of the strange disease. They found that the victims, often members of the same family, were clustered in fishing hamlets along the shore of Minamata Bay. The staple food of victims was invariably fish and shellfish from Minamata Bay. The cats in the local area, which tended to eat scraps from the family table, had died with symptoms similar to those now discovered in humans. This led the researchers to believe that the outbreak was caused by some kind of food poisoning, with contaminated fish and shellfish being the prime suspects. On 4 November, the research group announced its initial findings: "Minamata disease is rather considered to be poisoning by a heavy metal, presumably it enters the human body mainly through fish and shellfish." Identification of mercury
As soon as the investigation identified a heavy metal as the causal substance, the wastewater from the Chisso factory was immediately suspected as the origin.
Patients who were certified by a Ministry of Health and Welfare committee would be compensated: adult patients received ¥100,000 ($917) per year; children ¥30,000 ($275) per year, and families of dead patients would receive a one-off ¥320,000 ($2935) payment. Wastewater treatment
On 21 October 1959, Chisso was ordered by the Ministry of International Trade and Industry to switch back its wastewater drainage from the Minamata River to Hyakken Harbour and to speed up the installation of wastewater treatment systems at the factory. Chisso installed a Cyclator purification system on 19 December 1959, and opened it with a special ceremony. Chissos president Kiichi Yoshioka drank a glass of water supposedly treated through the Cyclator to demonstrate that it was safe. In fact, the wastewater from the factory, which the company knew still contained mercury and led to Minamata disease when fed to cats, was not being treated through the Cyclator at the time. Testimony at a later Niigata Minamata disease trial proved that Chisso knew the Cyclator to be completely ineffective: "The purification tank was installed as a social solution and did nothing to remove organic mercury. "The deception was successful and almost all parties involved in Minamata disease were duped into believing that the factorys wastewater had been made safe from December 1959 onward. This widespread assumption meant that doctors were not expecting new patients to appear, resulting in numerous problems in the years to follow as the pollution continued. In most peoples minds, the issue of Minamata disease had been resolved.