Historically, mercury poisoning was a common occurrence in both animal and human populations. Replacement of mercury products used for medicinal, agricultural, or industrial purposes has resulted in a decline of sudden and longterm poisoning. Predator species near the top of the food chain, such as fish, seals, polar bears, and some bird species, accumulate significant quantities of mercury from dietary sources. Commercial fish products, such as tuna, have resulted in longterm poisoning in people and cats. Fossil fuels represent a significant environmental source of mercury.
Mercury exists in a variety of chemical forms. Elemental mercury (thermometers, light bulbs) may vaporize and cause breathing and nervous system signs. Inorganic mercury salts found in some batteries and latex paints are poorly absorbed when ingested and low in toxicity. Large amounts are corrosive, however, and can cause vomiting and diarrhea, colic, and kidney damage. Inorganic mercury is converted to the organic forms of mercury by microorganisms in the sediment of rivers, lakes, and seas. Marine life accumulates the most toxic form, methylmercury, and fish for consumption must be monitored for contamination..
The organic mercurials are absorbed through all routes and accumulate in the brain, kidneys, and muscle. Animals do not show signs until several weeks after being poisoned by organic mercury. Signs can include blindness, excitement, abnormal behavior and chewing, lack of coordination, and convulsions. Cats show hindleg rigidity, lack of coordination, and tremors. Neurologic signs may be irreversible.
Diagnosis may be made on the basis of laboratory analysis (blood, kidney, brain, and feed) in association with appropriate blood tests for organ function, clinical signs, and history. The clinical presentation may be similar to other conditions that produce gastrointestinal distress, kidney disease, and nervous system dysfunction (tremors, incoordination, or convulsions). The kidney and nervous system damage is often irreversible, and treatments may not be effective.
For recent exposures, oral administration of activated charcoal and sodium thiosulfate to bind the mercury will limit absorption. Antioxidants such as vitamin E and selenium may limit some types of damage. Dimercaprol and penicillamine are sometimes used in treatment.