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Nocardiosis in Horses


Márcio Garcia Ribeiro

, DVM, PhD, Department of Animal Production and Preventive Veterinary Medicine, Faculty of Veterinary Medicine and Animal Science, São Paulo State University (UNESP), Botucatu, SP, Brazil

Reviewed/Revised May 2019 | Modified Oct 2022

Nocardiosis is a chronic, noncontagious disease caused by the bacteria of the genus Nocardia. These bacteria are found commonly in soil, decaying vegetation, compost, and other environmental sources. They enter the body through contamination of wounds or by inhalation. Species in this genus are found in temperate regions, as well as in tropical and subtropical areas. In horses, nocardiosis is an opportunistic infection and usually related to immune disorders, such as pituitary pars intermedia dysfunction or severe combined immunodeficiency in Arabian foals.

Poor appetite, fever, lethargy, and weight loss are common nonspecific signs associated with all infection sites. Skin infection and lymph node abscesses (localized collections of pus) are common signs in horses, with respiratory or disseminated disease occurring in animals with weakened immune systems. The first sign of infection is the appearance of a hardened nodule or pustule, which ruptures and produces pus. Individual lesions may be connected by channels, with frequent development of chronic, progressive disease. Nocardial inflammation of the mouth produces swelling and inflammation of the gums around the teeth and ulceration of the oral cavity, with severe bad breath. Abortion may occur in infected mares.

Your veterinarian will prescribe an antibiotic based on identification of the bacteria. Nocardial infections are resistant to some types of antibiotics. Treatment must often be continued for more than 3 months. It is important to continue treatment as directed to allow the best possibility for recovery. The outlook is guarded due to the long treatment time and the likelihood of relapse.

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