Merck Manual

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Pet Owner Version

Malignant Edema in Horses


Carlton L. Gyles

, DVM, PhD, Department of Pathobiology, Ontario Veterinary College, University of Guelph

Reviewed/Revised May 2019 | Modified Oct 2022

Malignant edema is a disease in which there is a severe, usually fatal spread of bacterial toxins through the bloodstream of horses and other large mammals. It is usually caused by Clostridium septicum bacteria, often accompanied by other clostridial species. The bacteria are found in soil and intestinal contents of animals (including humans) throughout the world. Infection ordinarily occurs through the contamination and infection of wounds such as those caused by accident, castration, tail docking, unsanitary injections, or during birth.

Signs, such as loss of appetite, intoxication, and high fever, as well as swelling around the site of the infection, develop within a few hours to a few days after a predisposing injury. The swellings extend rapidly because the toxins cause inflammation, severe fluid accumulation, and tissue death. Gas may accumulate also. The muscle in such areas is dark brown to black. Malignant edema associated with lacerations of the vulva during labor is characterized by swelling of the vulva, spread of bacterial toxins through the bloodstream, and death in 24 to 48 hours. Treatment with high doses of penicillin or broad-spectrum antibiotics early in the disease may be attempted, as well as surgical incision of the skin and underlying tissue to allow drainage. If not treated promptly, death can occur rapidly after infection.

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