Introduction

Classification

Taxonomy

• Order: Actinomycetales
• Family: Mycobacteriaceae
• Genus: Mycobacterium; morphologically similar, aerobic, non-spore forming, non-motile bacteria.
• All mycobacteria are morphologically similar, aerobic, non-spore forming, non-motile bacteria.
• Mycobacterium are closely related to other intracellular pathogens such as Corynebacterium, Nocardia spp  Nocardia spp and Rhodococcus.
• The various mycobacterial species that have been identified in dogs can be grouped into 2 major categories:

1. The Mycobacterium tuberculosis - complex (MTBC)

• The MTBC consists of several closely related species of mycobacteria capable of causing tuberculosis (TB) in man and other animals.
• Of the MTBC pathogens, only Mycobacterium bovis and Mycobacterium tuberculosis Mycobacterium tuberculosis have been frequently detected in dogs with very rare cases of Mycobacterium microti.
• Mycobacterium tuberculosis infection, the leading cause of human TB, is most frequently contracted by dogs from their owners. As such any diagnosis of TB in a dog should instigate a search for the infecting human and the the appropriate public health authorities should be notified.

2. Non-tuberculous mycobacteria (NTM)

• Otherwise referred to as environmental mycobacteria, atypical mycobacteria or mycobacteria other than tuberculosis (MOTT).
• NTM agents are environmental mycobacteria found in the soil, water, aerosols, protozoa, deep litter and fresh tropical vegetation.
• More than 140 species have been identified within this group, but not all are capable of causing disease in animals or humans. The species of clinical significance to animals can be divided into:
  • 1. Canine leproid granuloma (CLG) Skin: canine leproid granuloma syndrome: canine granulomas is a condition previously only seen in Australia, New Zealand, Brazil and the USA but is now also seen in Europe. It is characterized by nodular skin lesions, typically seen on the head and dorsal pinnae, which spontaneously resolve over a period of weeks to months. The etiological agent has yet to be identified but the histological apparance is consistent with granulomatous disease and intracellular ZN-positive bacteria are commonly seen, suggesting a mycobacterial species as the causative agent. Short-coated hunting dogs have been found to be predisposed and active hunting work has been shown to be a risk factor for disease. If diagnosed, cases rarely require pharmacological intervention.
  • 2. M. avium-intracellulare complex (MAC) Mycobacterium avium: MAC infections are significant as the most frequently confirmed NTM infection in companion animals are also (theoretically) potential zoonoses. Miniature Schnauzers Schnauzer: miniature are reportedly predisposed to MAC infections.
  • 3. Slowly growing NTM: these organisms take longer than 7 days to culture in laboratory conditions and are divided into sub-types based on the pigmentation of colonies.
  • 4. Rapidly growing NTM: these organisms (eg M. fortuitum, M. abscessus and M. chelonae) grow in laboratory culture conditions in fewer than 7 days.
• All these groups of opportunistic pathogens most often infect dogs with underlying immune dysfunction (which may or may not be identifiable) and results in systemic disease.

Etymology

• Gr: myces - a fungus; bakterion - a small rod.

Active Forms

Clinical Effects

Epidemiology

Transmission

Tuberculous Mycobacteria (M. tuberculosis, M. bovis and M. microti)

• Dogs and cats acquire M. bovis infection when they consume or are bitten/scratched by infected prey, drink contaminated milk or scavenge contaminated carcasses.
• Aerosolized droplets are the primary transmission mechanism of M. tuberculosis.

Non tuberculous mycobacteria 

• Infection most frequently occurs via contamination of open wounds from contaminated soil but can also occur by ingestion of meat or contact with infected soil, water sources or fomites.

Pathological effects

Tuberculous Mycobacteria (M. tuberculosis, M. bovis and M. microti)

• Tubercule bacilli enter via the respiratory tract, skin penetration or the gastrointestinal system where they have a chronic incubation period.
• Local multiplication of the bacillus can develop at the initial site (primary complex), resulting in granuloma (nodule) formation with associated local lymphadenopathy.
• Cats and dogs respond erratically to intradermal tuberculin testing.
• Immunity largely cell-mediated (TH1 response), and so lesions are formed in part by the immune response.
• Antibodies, in the few cases where they are detectable, are markers of infection, not indicators of exposure.
• The bacilli survive in macrophages and possess a number of mechanisms to subvert the immune response and maintain survivability.

Non tuberculous mycobacteria 

• Organism enters an open wound from the environment.
• Engulfed by phagocytic cells (dendritic cells and macrophages).
• There is an unknown but presumed chronic incubation period.
• Granulomas form in an attempt to contain the organism.
• Spread to adjacent tissues or throughout the body via lymphatic or hematogenous dissemination.
• Primary clinical presentation of disseminated M.avium include enlarged lymph nodes, tonsillary inflammation, inappetence and/or anorexia.
• Submandibular, cervical and mesenteric nodes may also be affected
• Other reported symptoms have included fever, vomiting, bloody feces, breathing difficulty due to compression of lungs by enlarged nodes, and lameness.
• Infection disseminates throughout other tissues, including spleen, liver, and bone marrow.
• Progression of the disease depends on the ability of macrophages to inhibit intracellular growth of the organisms
• M. avium granulomatous lesions are indistinguishable from tubercular lesions of M. tuberculosis and M.bovis.

Control

Control via chemotherapies

Tuberculous Mycobacteria (M. tuberculosis, M. bovis and M. microti)

• Treatment can be challenging and prognosis is dependent on the species of mycobacteria present.
• Treatment of M.tuberculosis infections (almost exclusively seen in canines) should not be attempted due to the public health risks attendant with this infection.
• Surgical excision of lesions can be beneficial but is rarely curative and medical follow up is usually needed.
• Mycobacteria are resistant to most antimicrobials because of the high lipid content and complexity of their cell walls, together with their ability to reside within macrophages.
• A wide range of drugs have been used successfully: for MTBC infections the best outcomes are seen with a combination of rifampicin Rifampicin, azithromycin Azithromycin/clarithromycin Clarithromycin and pradofloxacin/marbofloxacin for a minimum of 3 months and for 2 months beyond the resolution of clinical signs.
• Long-term therapy is required to effect a cure and eliminate the organism (3-6 months or more).

Non tuberculous Mycobacteria

• Treatment can be challenging and prognosis is dependent on the species of mycobacteria present.
• Surgical exicion of lesions can be beneficial but is rarely curative and medical follow up is usually needed.
• Mycobacteria are resistant to most antimicrobials because of the high lipid content and complexity of their cell walls, together with their ability to reside within macrophages.
• A wide range of drugs have been used successfully: for NTM infections include combinations of rifampicin with clofazime Clofazimine, doxycycline Doxycycline, clarithromycin and ethambutol.
• Long-term therapy is required to effect a cure and eliminate the organism (9-24 months).
• Treatment for NTM infections is usually prolonged and resolution of signs takes, on average, a year.

Vaccination

• No vaccine available.

Diagnosis

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Gunn-Moore D A, Gaunt C, Shaw D J (2013) Incidence of mycobacterial infections in cats in Great Britain: estimate from feline samples submitted to diagnostic laboratories. Transbound Emerg Dis 60 (4), 338-344 PubMed.
• Gunn-Moore D A, McFarland S E, Brewer J I et al (2011) Mycobacterial disease in cats in Great Britain: I. Culture results, geographical distribution and clincial presentation of 339 cases. J Fel Med Surg 13 (12), 934-944 PubMed. 
• Rivière D, Pingret J L, Etievant M et al (2011) Disseminated Mycobacterium avium subspecies infection in a cat. J Feline Med Surg 13 (2), 125-128 PubMed. 
• Baral R M, Metcalfe S S, Krockenberger M B et al (2006) Disseminated Mycobacterium avium infection in young cats: over representation of Abyssinian cats. J Fel Med Surg 8 (1), 23-44 PubMed.
• Barry M, Taylor J & Woods P J (2002) Disseminated Mycobacterium avium infection in a cat. Can Vet J 43 (5), 369-371 PubMed.
• Arvand M, Mielke M E, Weinke T et al (1998) Primary isolation of Mycobacterium tuberculosis on blood agar during the diagnostic process for cat scratch disease. Infection 26 (4), 254 PubMed.
• Gunn-Moore D A & Shaw S (1997) Mycobacterial disease in the cat. In Practice 19 (9), 493-499, 501 VetMedResource.
• Hughes M S, Ball N W, Beck L A et al (1997) Determination of the etiology of presumptive feline leprosy by 16S rRNA gene analysis. J Clin Microbiol 35 (10), 2464-2471 PubMed. 
• Aranaz A, Liébana E, Pickering X et al (1996) Use of polymerase chain reaction​ in the diagnosis of tuberculosis in dogs and cats. Vet Rec 138 (12), 276-280 PubMed.

Other sources of information

• Greene C E (2006) Mycobacterial infections. In: Infectious diseases of the Dog and Cat. 2nd Edn. Ed. Greene C E. W B Saunders Co. pp 462-488.