Copper Poisoning in Horses

Like many microminerals, copper holds a rightful place in the lineup of nutrients required by horses. The importance of copper to cartilage and bone metabolism has been underscored in various studies that indicate a deficiency could bring about developmental orthopedic disease in immature horses. Further, copper is also involved in hemoglobin formation and in nerve conductivity and coordination, making it essential for performance horses.^*

While copper deficiency can wreak havoc on skeletal and neuromuscular systems, from a historical perspective, copper toxicity in horses has been rare. Recent work by Brazilian researchers has indicated an upswing in copper poisoning cases, however.⁺

The retrospective study described five cases of copper toxicity. According to the researchers, diagnosis was made based on clinical history and signs, laboratory test results, copper levels in feed and soil, and histopathological findings.

Copper toxicity may be acute or cumulative. Acute copper toxicity occurs when large amounts of copper are ingested in a short time. In cases of cumulative copper toxicity, copper collects in the liver until it exceeds the organ’s storage capacity, and then it is released into the bloodstream, leading to liver and kidney failure.

A general review of these five cases of cumulative copper toxicity includes:

Case 1. A two-year-old Quarter Horse filly died from copper toxicity due to soil and pasture contamination by poultry litter in months just prior to death. Because poultry diets are rich in minerals, including copper, the litter was a likely source of contamination. Moreover, poultry litter is often treated with copper-based fungicides, and these contaminants can leach into the environment.

Case 2. A two-year-old colt diagnosed with cumulative copper toxicity died. At least 10 other horses on the property had a history of clinical signs indicative of copper toxicity. An adjacent coffee plantation was subjected to periodic treatment with a copper hydroxide fungicide.

Cases 3 and 4. An eight-year-old Lusitano mare and a two-year-old Lusitano gelding. Soil copper concentrations of soil and forages were above the upper reference limits in most paddocks and forages, with higher limits in onetime vineyards, where both horses were kept. Estimated copper intake exceeded dietary requirements for immature and mature horses. Copper-based products are frequently used by grape growers.

Case 5. An eight-year-old Mangalarga Marchador gelding was diagnosed with copper toxicity and recovered, but then had subsequent problems with copper overload a year later. Grass crops were treated with poultry litter, so soil and pasture contamination was suspected.

Environmental contamination is often the cause of mineral toxicities in animals. The horses in this retrospective study were exposed to excessive copper through poultry litter and crop fungicides. For other horses, though, the answer may be found in the feed room. For instance, horses often consume more microminerals than necessary through oversupplementation. Nutrients fed in excess of requirements are typically excreted without causing harm, but some accumulate within tissues, ultimately causing health problems.

A well-balanced diet prevents overconsumption of any single nutrient. The best way to ensure proper nutrition is through consultation with an expert nutritionist who has specialized knowledge and experience with horses.

The most accurate ration evaluations include forage (pasture and hay, if both are consumed), concentrate, and supplement analyses. Once the nutritionist has this information, she can evaluate the ration’s adequacy and identify any shortages or overages of nutrients based on the horse in question (age, workload, health problems).

^*Jackson, S.G. 1998. Trace minerals for the performance horse: Known biochemical roles and estimates and requirements. In: J.D. Pagan, editor, Advances in Equine Nutrition, Nottingham University Press, Nottingham, U.K. p. 205-214.

⁺Belli, C.B., W.R. Fernandes, L.N. Torres, M.C.A. Sucupira, L.R. Marques de Sa, P.C. Malorka, H.M. Neuenschwander, A. Barros, and R.Y.A. Baccarin. 2021. Copper toxicity in horses: Does it exist? Journal of Equine Veterinary Science. In press.