Lead poisoning is a serious condition which occurs when lead (a highly toxic metal) builds up in the horse's body. Horses are one of the more sensitive animal species to lead poisoning. It often causes peripheral nerve damage in horses, resulting in muscular weakness and roaring, which results from paralysis of the recurrent laryngeal nerve. It presents in horses as acute and chronic poisoning, with the later being most common. Clinical signs of lead poisoning vary depending on the form of intoxication.
Prior to knowledge of its toxicity (before 1977 in the United States), lead use to be used in a wide range of products including most paint, shotgun pellets, car batteries, pipes, and roofing materials.
Most chronic lead poisoning cases in horses occur due to contamination of the environment or from exposure to lead paint used in the construction of older buildings in the Unites States. Environmental contamination occurs from horses living in or near a lead-contaminated environment, such as within proximity to a battery factory, mines, smelters, or other industrial operations which use lead. This can also occur in the backyards of hoarders of junk outside or near car junk yards. Lead can also leach into the soil where it remains and further contaminates growing vegetation such as pasture grasses. When horses graze on contaminated vegetation they can accumulate enough lead to produce clinical signs of lead poisoning. Paint-associated poisonings usually occur from horses chewing on the wood of painted structures or from environmental contamination due to recent removal of the paint from the structure through the use of paint removal tools such as sand blasting and power washing.
Seven horses and one donkey died from lead poisoning in the later 1990s due to grazing on pasture land that was within close proximity of an old battery-recycling site located south of Madrid in Spain. All of the animals developed neurological signs (muscle twitching, joint stiffness, loss of vision, depression, ataxia), weight loss, paralysis of the laryngeal nerve and pneumonia prior to death. The animals were grazing on the land for a year before they developed clinical signs of chronic lead poisoning.