Mycobacteriosis

Chapter 39 Mycobacteriosis



Andrew Hillier, Alan C. Mundell


Mycobacteria are gram-positive organisms that cause sporadic disease in human beings and in animals. All mycobacterial organisms contain a lipid-rich cell wall that inhibits host defense mechanisms and imparts a characteristic staining property in the laboratory. This staining property involves the retention of carbolfuchsin after acid and alcohol decolorization. Thus mycobacteria are classified as acid-fast staining organisms.


Recently, a new conceptual framework for classification of mycobacterial infections of dogs and cats was proposed. This classification takes into account the mammalian host, the infecting bacteria, and the environment. Consideration of mycobacterial infections within this framework provides the clinician with important information for diagnosis, treatment, and prevention. The categories of mycobacterial diseases as they relate to skin disease specifically are as follows:


1. Diseases caused by obligate mycobacterial organisms (e.g., feline leprosy syndromes)

2. Localized disease caused by saprophytic mycobacteria in immunocompetent hosts
a. Where the infection is self-limiting and the immune response eventually eliminates the organisms (e.g., canine leproid granuloma syndrome)

b. Where the infection remains localized but requires intervention for cure (e.g., panniculitis syndromes, caused by rapidly growing mycobacteria, and Mycobacterium avium complex infections)

3. Disseminated disease caused by mycobacteria in immunodeficient hosts


FELINE LEPROSY SYNDROMES


Feline leprosy is a mycobacterial disease of cats that was first recognized in Australia during the early 1960s. The condition has since been identified in New Zealand, Great Britain, France, the Netherlands, and the west coast of the United States and Canada. The disease is usually confined to cats living in coastal areas.


Feline leprosy refers to mycobacterial disease in which single or multiple skin granulomas are present. The granulomas contain large numbers of organisms that cannot be cultured using standard techniques. Until recently, all cases of feline leprosy were thought to be caused by M. lepraemurium and to occur in young cats. Recent studies have identified different syndromes of feline leprosy.



Feline Leprosy Caused by Mycobacterium Lepraemurium



Etiology



• Caused by M. lepraemurium, the rat leprosy organism. Although the rat leprosy organism had been suspected for many years, it was only in the 1980s that sophisticated culture techniques and biochemical analysis could identify M. lepraemurium as the etiologic agent.

• The mode of transmission is unknown, although cats are thought to become infected following bite injuries from infected rats.

• In experimental infections, the incubation period is 2 to 18 months.


Clinical Signs



• Young adult cats are affected, predominantly males living in either suburban or rural environments.

• Lesions are painless, raised, fleshy, freely movable nodules of a few millimeters up to 4 cm in diameter. They may develop rapidly, and some ulcerate when large.

• Nodules are usually multiple (but may be single), tend to initially concentrate in one body region, and may spread to adjacent areas and regional lymph nodes.

• Nodules may occur anywhere on the body, but the head and limbs are more frequently affected. Additionally, lesions may occur on the lips, oral mucosa, tongue, and nasal mucosa. The condition disseminates rarely to the spleen, bone marrow, liver, kidney, lung, or adjacent muscle. Cats with disseminated disease usually show signs of systemic illness, which are not seen in cats with localized infection.

• Lesions tend to recur following surgical excision.


Diagnosis



• The diagnosis is achieved primarily through histologic evaluation of biopsy samples in conjunction with compatible history, physical examination, cytology, culture, and laboratory animal inoculation findings.

• The differential diagnoses for feline leprosy include atypical mycobacteriosis, tuberculosis, foreign body dermatitis, deep mycotic infections, mycetomas, dermatophyte pseudomycetomas, chronic bacterial infections, eosinophilic granuloma complex, and neoplastic diseases.


History



• History must be compatible with feline leprosy. The cat will typically be young, allowed to roam, and living in a cool, moist, coastal climate.


Cytology



• Cytology of needle aspirates or lesional impression smears reveals mixed inflammatory cells with macrophages and histiocytes containing variable numbers (often many) of acid-fast bacilli (long slender rods). In Diff-Quik–stained preparations, negative-staining organisms are seen within macrophages. Ziehl-Neelsen stain and a modified Fite’s stain are the acid-fast stains most commonly used.


Culture




Key Point


Culture of the feline leprosy mycobacterium is difficult because the organism fails to grow on blood agar and standard mycobacterial media (Löwenstein-Jensen and Stonebrink).



• A 1% Ogawa egg yolk medium can be used to grow the organism when incubated under precise temperature and CO2 conditions. Most commercial laboratories do not perform Ogawa egg yolk medium cultures.


Biopsy



• Biopsy specimens of lesions are fixed in 10% buffered formalin and evaluated histologically, using both hematoxylin-eosin and acid-fast stains (Ziehl-Neelsen and modified Fite’s stains). (See Chapter 37 for skin biopsy technique.)

• Histologically, two forms are seen: tuberculoid and lepromatous forms. The tuberculoid form (most common in the United States and Canada) consists primarily of non-encapsulated epithelioid granulomas that are interspersed with neutrophils and surrounded by a zone of lymphocytes with low numbers of organisms. The lepromatous form is composed of sheets of large foamy macrophages that contain large numbers of acid-fast bacilli. The tuberculoid form is associated with a more competent host immune response.


Polymerase Chain Reaction



• Polymerase chain reaction (PCR) amplification of gene fragments from tissue samples provides a rapid and accurate diagnosis and identification of the causative organisms. Fresh (frozen) tissue or freeze-dried tissue is best, although PCR can also be performed on formalin-fixed, paraffin-embedded samples. (PCR for mycobacterial organisms is performed at the School of Veterinary Medicine, University of California, Davis, CA.)


Laboratory Animal Inoculation



• Laboratory animal inoculation is used primarily in research to propagate and investigate the feline leprosy organism. This procedure is of greatest value for differentiating feline leprosy from tuberculosis. Only tuberculosis-producing mycobacteria routinely kill guinea pigs within 6 to 8 weeks after inoculation.


Treatment


Treatment may not be necessary when lesions are small, unobtrusive, and likely to be self-limiting. However, this strategy is seldom effective and is recommended only for lesions that appear to be resolving spontaneously at the time of diagnosis.




Key Point


Surgical excision is the treatment of choice when lesions are limited in number and wide surgical margins can be obtained. However, disease recurrence is common and may require additional surgical intervention and/or medical management. Even then, adjunct antimicrobial therapy starting a few days prior to surgery is recommended.


Clofazimine is an iminophenazine dye with antimycobacterial properties that is suspended in olive oil and packaged in 50- and 100-mg capsules. Although the pharmacokinetics of clofazimine in the cat are unknown, a dosage of 10 mg/kg once daily PO (25–50 mg every 1–2 days), for 6 to 12 weeks past complete clinical resolution appears to be very effective. Wear latex gloves to prevent staining of hands. Clofazimine is not approved by the U.S. Food and Drug Administration (FDA) for animal use.


• Clofazimine may cause a reversible hepatopathy, and regular biochemical monitoring of cats is mandatory. Vomiting and inappetence are strong indications to reduce the dose and to perform a biochemical evaluation of the patient. Photosensitivity and pitting corneal edema have also been noted in cats.

Related posts:

  1. Feline Infectious Peritonitis (Feline Coronavirus)
  2. Borreliosis (Lyme Disease)
  3. Immune-Mediated Dermatoses
  4. Canine and Feline Demodicosis

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Mycobacteriosis

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