Merck Manual

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Pet Owner Version

Tularemia in Horses


Janet E. Foley

, DVM, PhD, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis

Reviewed/Revised May 2019 | Modified Oct 2022

Tularemia is a bacterial disease that affects people and many species of wild and domestic animals. It is caused by toxins in the blood produced by the bacterium Francisella tularensis. The bacteria can survive for weeks or months in a moist environment. Tularemia can infect people and can be transmitted by aerosol, direct contact, ingestion, or ticks and deer flies.

Among domestic animals, sheep are the most likely to show signs of the disease, but disease has also been reported in horses. The most common source of infection for people and horses is the bite of an infected tick. Signs of infection in the horse include the sudden onset of high fever, swollen lymph nodes, and lethargy. Prostration and death may occur in a few hours or days. Sporadic cases are best recognized based on signs of illness and observation of heavy tick infestation. Very mild cases without signs may be common.

Animals with signs of illness are treated with an antibiotic. Early treatment should prevent death; however, prolonged treatment may be necessary. Control is difficult and is limited to reducing tick infestation and to rapid diagnosis and treatment. Animals that recover develop a long-lasting immunity.

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