What Minamata disease is about
Occurrence and background of Minamata disease
Cause and Damage of Minamata Disease
Minamata disease is an encephalopathy and peripheral neuropathy caused by daily intake of ﬁsh and shellﬁsh highly contaminated by methylmercury. Through gills and gastrointestinal tracts, ﬁshery products such as ﬁsh, shrimp, crabs, and shellﬁsh, take in methylmercury discharged from chemical plants into rivers and seas. Flesh-eating ﬁsh that eat those contaminated ﬁsh also accumulate the toxicsubstance (food web). Thus accumulated methylmercury in sea food intoxicates people who have eaten a lot of such sea food daily.
Minamata disease broke out in the Yatsushiro Sea coastal area, particularly around Minamata Bay in Kumamoto Prefecture. At ﬁrst the disease was regarded as a kind of disease of nervous system of unidentiﬁed causes. Later it occurred in the Agano river basin in Niigata Prefecture.
In the Minamata bay area, an outbreak of Minamata disease was ﬁrst reported in May 1956. At the end of that year the number of identiﬁed patients reached 52, of which 17 had already died. In 1957, this unknown disease was named Minamata disease. In the Agano river basin, the ﬁrst report on Minamata disease patients was ﬁled in May 1965; by July 1965, a total of 26 patients were recognized, of which 5 had already died.
Today, whether or not a patient is aﬄicted by Minamata disease is certiﬁed by either governors of prefectures concerned or the national government in accordance with the Law Concerning Compensation and Prevention of Pollution-related Health Damage.
Social Background of Minamata Disease
It was Chisso Corp. (Minamata factory) and Showa Denko Co., Ltd. (Kanose factory) that caused methylmercury pollution. They were both chemical engineering companies that were then playing the role as a driving force to bolster Japan, which was in the midst of its rapid economic growth following the period of recovery from World War 2, and to spur its development.
In particular, Chisso, a company with high development skills, worked on upgrading its production capacity successively by applying original techniques. Keeping pace with Chisso's growth, Minamata City also developed rapidly. As taxes paid by its factory and employees eventually accounted for over 50% of the City's tax revenue, among other consequences, Chisso increasingly exerted a major inﬂuence on the regional economy and public administration, which resulted in Minamata transforming into a so-called factory town.
Thus shored up by the local community, Chisso was able to boost its production more and more, owing also to the fact that it took advantage of inexpensive labor, ample supplies of water, electricity generated in house, as well as raw materials readily available in the locality, such as limestone and coal from Amakusa, and benefited from favorable treatment it was given in terms of waste material and wastewater disposal. On the other hand, concerns about labor conditions and the natural environment were given a lower priority.
Medical explanations of Minamata disease
Pathological Changes of Methylmercury Poisoning in the Nervous System
Methylmercury causes severe damage to particular areas of the nervous system (areas highlighted i in pink). This triggers disorders corresponding to the role each affected area has. They appear as a variety of symptoms that characterize Minamata disease, as illustrated below.
Disorders in the nervous system caused by methylmercury
- ❶ Ataxia
Gait disturbance, loss of balance
- ❶ Dysarthria
- ❷ Constriction of visual ﬁelds
Sight disturbance of peripheral areas in the visual ﬁelds
- ❸ Disturbance of sensation
- ❹ Disturbance of movement
Muscle weakness, muscle cramp
- ❺ Hearing disturbance
Hardness of hearing
- ❻ Disturbance of sensation
Disturbance of sense of pain, touch or temperature
Lesions of Damaged by Methylmercury
Examples: Cross-sectional images of the right hemisphere of the cerebrum
A horizontal section of right hemisphere of cerebrum showing lesions (unstained and strongly atrophic) in the calcarine cortex of visual cortex (↑↑), the temporal transverse gyrus containing auditory center (↑).
Examples: Cross-sectional images of the cerebellum
Severe atrophy of the hemisphere of the cerebellum. Total loss of granule cells in the cerebellum except vermis (↑)
Pathway of Methylmercury from Factory to Human
A model of acetaldehyde plant (new ﬁfth term) in which highest byproduction of methylmercury could be
occurring: When acetaldehyde was made from acetylene and water, methylmercury was by-produced in the reaction chamber.
Methylmercury contaminated into the vacuum evaporator, the ﬁrst still, and even to the second still, and then, discharged with the drain from each equipment.
Methylmercury released from factory into water accumulated in aquatic organisms Via their gills or intestine directly, or though their food chain. Human had taken it by eating polluted ﬁsh. This process is called bioaccumulation.