Introduction

• Cause: infection with Mycobacterium spp (mycobacteria other than tubercuolosis MOTT). Many different mycobacterial species have been associated with disease in reptiles, including Mycobacterium chelonae, M. fortuitum, M. avium, M. intracellulare, M. marinum, M. phlei, M. smegmatis, M. ulcerans, M. confluentis, M. haemophilum, M. hiberniae, M. neoaurum, M. terrae, M. phamnopheos, M. agri, M. szulgai, M. kansasiii and M. nonchromogenicum. M. tuberculosis complex has not been reported in a reptile to the author’s knowledge.
• Signs: mycobacteria may colonize many tissues, and so clinical signs will be dependent on the tissue and system affected. Lesions are usually granulomatous. Weight loss is commonly seen, as are respiratory signs or cutaneous nodules or swellings.
• Diagnosis: detection of acid-fast organisms with appropriate staining, eg Ziehl-Neelsen staining of affected tissues. Broad range PCRs may be performed followed by gene sequencing. Culture can be diagnostic but can take an extended period of time.
• Treatment: given the zoonotic potential of mycobacterial infection, treatment of affected reptiles is usually not recommended, and euthanasia is advised. Some treatment protocols have been described but should not be undertaken lightly.
• Prognosis: variable, although most confirmed cases will be euthanized due to zoonotic risk.

Pathogenesis

Etiology

• Mycobacterium is a genus of Arcanobacteria. They do not take up gram stain and are often seen as unstained ‘ghost cells’. They stain acid fast.
• Mycobacteria are ubiquitous in the environment, often as biofilms in aquatic environments, and infection usually requires chronic immunosuppression of the host. Infections are often opportunistic, often occurring via skin wounds. Multiple different species of mycobacteria have been implicated in reptile infections, but the taxonomic validity of some species is not unique, leading to differing reports.

Predisposing factors

General

• Immunosuppression, eg concurrent illness, stress.

Specific

• Reptiles exposed to aquatic environments may be more prone, due to the ubiquity of mycobacteria in aquatic biofilms.
• Captive wild animals may be at greater risk due to likely chronic stress.

Pathophysiology

• Granulomatous infection associated with mycobacteria may be acute or chronic.
• Heterophilic granulomas are caused by extracellular microorganisms, whereas histiocytic granulomas are caused by intracellular infections.
• Acute granulomatous inflammation will develop into the chronic form, which is characterized by populations of epithelial cells, lymphocytes, plasmacytes and sometimes giant cells with four nuclei. These all surround a central portion of the granuloma which is often necrotizing.
• A fibrous capsule will develop around this which sometimes becomes calcified.
• Clinical signs (eg cutaneous, respiratory or multifocal) will then develop depending on the site and extent of the granulomas.

Timecourse

• Lengthy: months to years in some cases.

Epidemiology

• Minimal available studies so prevalence and transmission in reptiles are currently unclear.

Diagnosis

Treatment

Prevention

Outcomes