Mercury is available as elemental mercury, inorganic mercury and organic mercury.
Sources
Feed treated with mercurial fungicides, indiscriminate use of mercury containing drugs like ointments and diuretics, contaminated water, thermometers, mirror etc. serve as sources for mercury.
Mercury is also used in dental amalgams.
Mercury circulates in the environment because of its volatile nature and the earth continuously degases mercury.
Mercury is also released while burning coal.
Both acute and chronic mercury poisoning are rare because of the limited exposure to mercury.
More often, poisoning is due to consumption of obsolete mercurials.
Fish and other marine living organisms take up organic mercury from water and this mercury gets accumulated in these living things. This is known as bioaccumulation of mercury.
Though mercury is present only in small amounts in seawater, it is absorbed by the algae. It is efficiently absorbed, but only very slowly excreted by organisms.
Bioaccumulation and biomagnification result in build up in the adipose tissue of successive trophic levels: zooplankton, small nekton, larger fish etc.
Anything which eats these fish also consumes the greater level of mercury the fish have accumulated.
The consumers of such marine organisms are likely to have mercury poisoning.
Minamata disease sometimes referred to as Chisso-Minamata disease is a neurological syndrome caused by severe mercury poisoning.
Symptoms include ataxia, numbness in the hands and feet, general muscle weakness, narrowing of the field of vision and damage to hearing and speech.
In extreme cases, insanity, paralysis and death follow. This was first discovered in Minamata city in Japan in 1956. It was caused by the release of methyl mercury in the industrial waste water from a chemical factory. This highly toxic chemical bioaccumulated in shell fish and fish in Minamata Bay and the Shiranui sea.
When these were eaten, toxicity resulted in human, cat and dog. Toxic cases were reported for more than 30 years.
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