MSD Manual

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Pet Owner Version

Besnoitiosis in Horses


Charles Mackenzie

, BVSc, PhD, ACVM, AO, Department of Pathobiology and Diagnostic Investigation, Michigan State University

Reviewed/Revised May 2019 | Modified Oct 2022

Although mainly a disease of cattle, besnoitiosis can cause infection in horses and other herbivores. This disease is caused by a tiny, single-celled organism called a protozoan, known as Besnoitia bennetti. It is usually found in horses living in the tropics and has not been reported in horses in the USA. It has been reported in donkeys in Africa, southern France, Mexico and in a number of locations in the USA.

The route of disease transmission remains unclear. The parasites are thought to have both definitive and intermediate hosts that are not clearly defined for most Besnoitia species. Cats are thought to be the definitive host with various animals, such as small rodents or other mammals acting as the intermediate host.

Once an animal becomes infected, the organisms form cysts in and under the skin and in mucous membranes of the eyes and other tissues. Other signs may include fever, fluid buildup in the tissues, loss of appetite, intolerance to light, inflammation of the mucous membranes of the nose, chronic hardening and thickening of the skin, and hair loss. The skin becomes hard, thick, and wrinkled and develops cracks that allow bacterial infections to develop. Severely affected animals become emaciated. The signs in horses tend to be less severe than in other animals.

Affected animals should be isolated and treated for specific signs. Recovery is slow in severe cases, and affected animals remain carriers of the disease for life. Reducing the number of biting insects and ticks may reduce transmission.

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