Introduction
Mycobacterium spp. have a wide host affinity and pathogenic potential. They are able to survive intracellularly and produce granulomatous inflammations. Mycobacterial infections can be divided into three forms: tuberculous, lepromatous and opportunistic.
The tuberculous bacteria, M. tuberculosis and M. bovis produce nodular granulomas in their hosts.
The M. avium-intercellulare complex (MAC) includes many saprophytic organisms that are opportunistic and produce granulomas in their hosts.
Dogs and cats are susceptible to M. tuberculosis and M. bovis but are more resistant to infections by MAC. There appears to be a higher degree of M. bovis infection in cats.
The incidence of true tuberculosis has decreased and rarely affects dogs and cats, but cases are seen in parts of the world where there is a high degree of exposure and where the disease is endemic.
Dogs and cats can become disseminators when the organism localises in the intestinal or respiratory tracts.
The lepromatous organism includes Mycobacterium lepraemurium which causes Feline Leprosy and which occurs through bites or from contact with infected rats.
See also the general page on tuberculosis.
Clinical signs
The predominant signs in small animals are respiratory and digestive, but there can be some skin lesions.
Infections are often asymptomatic and insidious.
In dogs, common signs include: fever, weight loss, anorexia and harsh non-productive coughing.
Dogs and cats may develop hypersalivation, retching, dysphagia and tonsillar enlargement due to oropharyngeal lesions.
Cats may have weight loss, anaemia, vomiting and diarrhoea.
Cutaneous lesions may include single or multiple ulcers, abscesses, plaques and nodules, commonly on the head, neck and limbs.
Diagnosis
Intradermal skin testing in dogs and cats is inconsistent and unreliable.
Radiographs may show: nodular soft tissue opacities in the lungs, most commonly in the caudal lung lobes.
The diagnosis is may by cytological and cultural examination, and histopathology of biopsy material.
On cytology: organisms vary in number, but numerous organisms may be visible in the smear. With Wright's stain, the organisms are rod-shaped, refractile and non-staining. An acid-fast stain is needed to highlight the organism.
Culture or PCR is then necessary to differentiate between mycobacterial species.
The pathogen is slow-growing and requires special media and several weeks to establish visible colonies.
Post-mortem examination will reveal: firm multifocal nodules, with necrotic centres in the caudal lung lobes, liver, kidney, pleura and peritoneum. The lesions are often exudative, discharging a yellow fluid into the thorax and abdomen.
Treatment
Dogs and cats with true tuberculosis infections can be a source of infection for humans with grave consequences for public health, and in most cases euthanasia is performed.
Because infections with the MAC organisms are opportunistic, usually with an environmental source, treatment may be attempted. Two or three antibacterials should be used intercurrently for 6-9 months. Drugs such as rifampicin, clarithrocymic, clofazimine, doxycyline and enrofloxacin have been used. Treatment should be continued until all clinical signs of the disease have resolved. The response is variable and treatment is usually more successful in cats.
References
Hoskins, J. (2001) Veterinary pediatrics Elsevier Health Sciences
Muller, G. (2001) Small animal dermatology Elsevier Health Sciences
Ettinger, S. (2001) Pocket companion to textbook of veterinary internal medicine Elsevier Health Sciences
Links
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