There are approximately 15 essential trace elements, namely, arsenic, chromium, cobalt, copper, fluoride, iodine, iron, manganese, molybdenum, nickel, selenium, silicone, tin, vanadium, and zinc (1). Ten of these have been associated with clinical problems in animals, when in excess or deficiency. These are arsenic, cobalt, copper, fluoride, iron, manganese, molybdenum, nickel, selenium, and zinc. Supplementation of animal diets has resulted in numerous incidents of poisoning (2). Organic arsenicals are often used as feed additives for disease control and to improve weight gain and feed efficiency in swine and poultry (2). Poisoning with these arsenicals has often been due to misformulations or insufficient water intake. Because .of major differences among species with regard to copper requirements and the influence of low molybdenum intake, copper toxicosis has been common in sheep and calves (3-5). The high species sensitivity of young pigs to iron supplementation has been a common cause of iron toxicosis in swine (6). Supplementation of selenium in selenium-deficient areas has resulted in selenium toxicosis through miscalculation in feed mixing and in the use of injectables (7-9). Although zinc has been added to animal feeds, its low toxicity has resulted in only occasional incidents of toxicosis (10,11); however, heavy use of this element in galvanized coatings has resulted in toxicosis in pets (12).
Cadmium, lead, mercury, and nickel are metals that have become common environmental contaminants (1). As a result of mining, industrial activities, and commercial applications, these metals may be released into the environment. Cadmium, lead, and nickel are often components of sewage sludge used as a soil amendment for agriculture or home gardening applications (12).
Cadmium and nickel tend to be taken up into the leafy portions of plants and may reach high concentrations in grains (13). Domestic animals that graze are exposed to heavy metals in soil and plants in ways that humans and other animals are not. Several of these heavy metals accumulate in animal tissues and often lead to chronic toxicosis or residues, particularly in organ meats (14). All forms of mercury entering the environment are converted to methyl mercury and to a lesser extent ethyl mercury by anaerobic bacteria. Methyl mercury is the principle residue accumulating in aquatic organisms and birds from polluted watersheds (15). Mercurial diuretics and dermatological ointments are no longer commonly used (16). Although the use of lead in gasoline and paint has been reduced, lead is still commonly used in plumbing, batteries, fishing sinkers, and shotgun pellets. These products provide a significant source of environmental contamination. Heavy metals, such as, cadmium, lead, and mercury are able to exert toxic effects at low doses in all species (15-17).
In the past, mixtures of inorganic arsenic, cadmium, copper, chromium, mercury, and zinc were used as fungicides, herbicides, and insecticides in agriculture. These metals accumulated in the soil and their use resulted in frequent animal poisonings (16-18). These pesticides are now primarily of historical importance and no longer a practical threat, since they have been replaced with the new organic pesticides (19). Today, wood and insulation products are frequently treated with arsenic, copper, or chromium as preservatives (20). After burning these products, the ash may present a danger to domestic animals. Prohibiting the use of thallium as a rodenticide and insecticide has dramatically reduced the occurrence of thallium toxicosis in North America (20). Copper sulfate, however, commonly used as an algicide in water and fungicide in foot baths, is a source of acute animal poisoning.
In Ontario, the significance of lead toxicosis has been reported previously (21-23); however, very litttle has been documented on the excess of other metals in domestic animals. The objectives of this study were to document the frequency of requests of suspected heavy metal toxicosis in Ontario between 1990 and 1995, to report the positive diagnosis of heavy metal toxicosis during this period, and to determine if any other metal analysis should be made available to Ontario veterinarians.