Tuberculosis
Peggy S. Marsh
Tuberculosis refers to any infectious disease caused by Mycobacterium spp. and associated with the formation of focal granulomatous lesions. In humans many forms of the disease exist, with the primary site of infection most commonly being the lungs.
Mycobacterium tuberculosis is the typical causative agent in humans, whereas Mycobacterium avium or Mycobacterium bovis is usually implicated in animal cases.1 Tuberculosis is extremely rare in horses, especially in the United States. Most cases in horses have reported isolation of M. avium2-5; however, there are reports of infection with M. bovis6~9 and M. tuberculosis? In equine cases it appears that the organism is usually ingested, although primary respiratory infection may occur. Spread is hematogenous, and the organism frequently settles in lymph nodes and the spleen.The most frequent presenting complaint is chronic weight loss with ensuing weakness and lethargy. Terminally ill horses with the pulmonary form are febrile and dyspneic and have a cough. Radiographic or sonographic evaluation of the thorax may be helpful in defining the pattern and distribution of lesions. Other clinical signs may be noted depending on the organ systems affected; these signs include intestinal tract abnormalities, hepatitis, and osteomyelitis of the cervical vertebrae.1-9
Diagnosis, usually made after the horse's death, is based on the presence of tubercles and isolation of the organism. The intradermal skin test is not reliable in the horse because a majority of clinically normal horses may have positive test results.9 Cytologic or histologic evaluation of a wash or aspirate of a lesion may reveal acid-fast bacilli. Biopsy of the lesion(s) is necessary for a definitive diagnosis. When tuberculosis is suspected, culture of an appropriate sample, such as a tracheal aspirate with primary pulmonary involvement, is advisable.
However, this is a very slow-growing organism, and isolation may take multiple weeks to months.6 PCR technology has proven to be a useful tool, often in conjunction with culture.6,7,9Due to public health concerns, treatment is not usually attempted. If treatment is undertaken, the horse should be isolated, with precautions taken to prevent the spread of disease and in consultation with the public health officials and/or the horse handler's physician. Tuberculosis is a reportable disease in other species. In one case a horse was treated with a prolonged course of antimicrobial medications (enrofloxacin and potassium iodide) while it took 4 months to identify M. bovis from biopsy samples from a lung mass. The horse went on to live at least another 8 years with no further problems related to the M. bovis infection.6 Because treatment is usually not a viable option in horses, prognosis is very poor.
Bacillus Calmette-Guerin vaccine has been used in humans with potential exposure, but the benefit of its use in horses to provide increased resistance after exposure to a tuberculosis infection is unknown.