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Mercury exists in a variety of organic and inorganic forms.
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Toxic effects of the two forms are largely dissimilar.
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Organic mercurials are less toxic than inorganic compounds.
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The replacement of common mercurial compounds viz. antiseptics, diuretics and fungicides by other agents has decreased the likelihood of mercurial toxicosis; however, the possibility of exposure to environmental sources exists.
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Both forms of mercury, arising as effluents from industrial processes are converted into soluble methyl mercury in the lakes and rivers.
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This is then carried down to sea, where it is taken in by living organisms, notably fishes, the concentration factor being as high as 300 folds.
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This entails serious risk to creatures, including human beings, living largely on fish.
Sources of poisoning
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Sources of inorganic mercurial poisoning include elemental mercury and salts such as mercuric chloride (corrosive sublimate), mercurous chloride (calomel), yellow mercuric oxide, red mercuric iodide and mercuric nitrate.
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Dogs and cats are particularly susceptible to the toxic effects of mercury from mercurial ointments, both due to direct absorption and licking of parts to which the ointment has been applied.
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Poisoning in cattle has also been recorded after the use of ointments containing mercuric iodide, mercuric nitrate etc.
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Mercury vapours appear to be extremely toxic to sheep and cattle and are absorbed through respiratory tract.
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Main sources of organic mercurial toxicity include fungicides (ethyl mercuric chloride and hydroxide) and methyl mercuric dicyandiamide and methoxy ethyl mercuric silicate used as seed dressing agents in agriculture.
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Antiseptics (mercurochrome, thiomersal, phenylmercuric acetate and nitrate) and diuretics (mersalyl) have also been implicated in toxicity.
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Secondary poisoning may arise from the ingestion of flesh of animals which have been fed on mercurial fungicides.
Clinical signs
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Clinical signs of inorganic mercurials poisoning include vomiting, diarrhoea and colic, polydipsia, albuminuria and anuria (severe cases) is common.
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In cattle, death may occur rapidly after the ingestion of mercury with signs of colic and subnormal temperature as the only symptom.
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Inhalation of mercury vapours causes dyspnoea and coughing, nasal discharge, fever, loss of appetite and condition and sometimes bleeding of mucosa.
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Animals poisoned with organic mercury compounds exhibit symptoms which are quite different from those of inorganic compounds.
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Symptom of CNS stimulation and locomotor abnormalities are seen after a lengthy latent period of weeks.
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Signs of toxicity may include blindness, excitation, abnormal behaviour and chewing movements, incoordination and convulsions.
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Cats show hind leg rigidity, hypermetria, cerebellar ataxia and tremors. Neurological signs may be irreversible once they develop.
Post mortem lesions
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The lesions of mercury toxicosis include gastroenteritis, stomatitis, gingivitis and acute parenchymatous nephritis.
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There is oedema of lungs, hydrothorax, hydropericardium, haemorrhages in epicardium and endocardium following inhalation of mercury vapours.
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Microscopic changes include necrosis of convoluted tubules of kidneys.
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In animals affected due to organic mercurials changes are seen in the brain which include degeneration of neurons (sensory in particular) and perivascular cuffing in the cerebrocortical grey matter, cerebellar atrophy of granular layer and damage to Purkinje cells.
Diagnosis
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