Mycobacterial infections in cats

© Carolyn O'Brien BVSc FACVSc, University of Melbourne

Mycobacteria cause a range of clinical symptoms in cats, ranging from localised skin conditions to disseminated, often fatal, infections. Mycobacteria are Gram-positive, aerobic, non-spore forming, non-motile members of the Actinomycetales order of bacteria. The high lipid content of the cell wall results in the retention of hot carbolfuschin stain after treatment with acid and alcohol, rendering the organisms acid-fast when stained with ZN of Fite's stain. Mycolic acid, the major lipid in the cell wall, plus cord factor and wax D are partly responsible for the capacity of the organism to survive within phagocytes, which gives rise to the typical granulomatous immune response by the host.

Conceptually, mycobacteria can be divided into three groups:

1. those species that are obligate pathogens (e.g. tuberculous bacteria)

2. those that cause leproid/tuberculous granulomata and cannot be cultured by routine laboratory methods (lepromatous mycobacteria)

3. those that have pathogenic potential but are generally considered to be opportunistic saprophytes (rapidly growing and slowly growing mycobacteria)

Feline tuberculosis

Aetiology

The mycobacteria that cause so called 'feline tuberculosis' are obligate parasites with the capacity to cause zoonotic infection. Several species are implicated, including Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium microti (which is normally associated with voles in the United Kingdom) and an unclassified variant, which seems to share properties of M. tuberculosis and M. bovis (M. microti-like), Cats can become infected by exposure to people infected with M. tuberculosis (anthropozoonosis) or via ingestion of unpasteurised milk or uncooked meat or offal from cattle infected with M. bovis. Wildlife reservoirs (e.g. the Australian brush tail possum, which is endemic in NZ) are a potential source of M. bovis infections in cats. In the UK, M. microti and M. microti-like infections are thought to be acquired by ingestion of infected prey species.

Mycobacterium avium infections have been described in a number of species, including dogs, cats, primates, swine, cattle, sheep, horses, and humans ( 1, 2, 3, 4, 5, 6, 7, 8, 9). Descriptions of disseminated infections of M. avium in cats are rare, because cats are naturally resistant to the organism ( 1, 2). An exception to this appears to be the Siamese breed, which is over-represented in reports of M. avium infection and infection with other intracellular organisms ( 3). Mycobacterium avium belongs to the M. avium-intracellulare complex (MAC), which is a group of slow-growing, atypical mycobacteria ( 3, 4). The complex consists of 28 serovars; 1 through 6 and 8 through 11 are considered to be M. avium ( 3). Most cases of Mycobacteriosis in dogs, cats, and horses are due to serotypes 1, 2, or 4 ( 4). Mycobacterium avium is a ubiquitous, saprophytic, acid-fast, aerobic, non-spore-forming bacillus that is widely distributed in the environment, especially in water and soil ( 3, 4). Pigs and birds are very susceptible to M. avium infections and may serve as reservoir hosts ( 4), although the organism may remain viable in the soil for up to 4 y ( 3, 4). Although M. avium is considered an opportunistic organism, it is the most likely of the MAC organisms to produce bacteremia and disseminated disease ( 3). Mycobacterium avium is important, because the granulomatous lesions it produces are indistinguishable from the tubercular lesions of M. tuberculosis and M. bovis ( 3). Mycobacteria produce a cell-mediated, delayed-type hypersensitivity response, characterized by granulomatous inflammation ( 4). The progression of disease depends on the ability of the macrophages to inhibit intracellular growth of the organisms ( 4). A competent cell-mediated immune response is more likely to result in elimination of the organism than is an antibody-mediated immune response ( 3, 5).

Clinical signs

Clinical signs are typically organ-specific, but nonspecific generalized symptoms, such as weight loss, anorexia, and fever, are common, as was seen in this case ( 5). Cutaneous lesions with M. avium infection are only rarely reported, but are typically displayed in cats infected with M. lepraemurium ( 6, 7). Mycobacteria often gain entry to the body through either the respiratory tract, the gastrointestinal tract, or the skin, where they are phagocytized by local tissue macrophages and then disseminated to adjacent tissues ( 8, 3, 6). Infrequently, intracellular organisms can be seen in neutrophils and monocytes on blood smears. 

Diagnosis

Collection of representative tissue and/or fluid samples for cytology or histopathology and microbiology are key to diagnosis. Occasionally, organisms can be visualised in circulating leukocytes, bone marrow aspirates or urine. As with other mycobacteria, these organisms do not stain with Romanowsky stains (e.g. DiffQuik), but are visible with stains that test for acid- fastness. Intradermal tuberculin testing in cats is apparently unreliable, as is serologic testing. Ideally, an attempt to culture the organism should be attempted in every case in which tuberculous mycobacteria infection is suspected.

A presumptive diagnosis of M. avium infection can usually be made following acid-fast staining of affected tissue, although occasionally staining will be negative in affected animals ( 6). Unlike other mycobacteria, M. avium tends to be present in high numbers within cells ( 6, 11). Definitive diagnosis requires culture of the organism, which can take several weeks in the case of M. avium ( 3, 10). Due to the progressive nature of the disease, treatment should be initiated upon finding acid-fast bacilli and not withheld pending culture results. Recently, polymerase chain reaction has shown promise for the rapid detection of mycobacteria in both humans and cats, although presently its use is limited to the identification of M. tuberculosis ( 3, 12).

In humans, infections with MAC organisms are often associated with immunodeficiency or immunosuppression ( 3, 9). Although one would expect FeLV- or FIV-positive cats to be predisposed to mycobacterial infections, a significant association has not been found ( 3, 9). In several reports of feline Mycobacteriosis, the cats were found to be FeLV negative at the time of diagnosis ( 1, 2, 6, 10).

Figure 1. Fine-needle aspirate of mesenteric lymph node. Single population of large macrophages with intracytoplasmic, long, slender, negative-staining, rod-shaped organisms, consistent with mycobacteria; Wright's stain.	Figure 2. Tissue section of bone marrow. A confluent sheet of stained, fine, rod-shaped organisms is evident interspersed among the hematopoietic cells; Ziehl-Neelsen.

There are rare reports of cats with M. avium infection being successfully treated using combination antibiotic therapy including rifampin, enrofloxacin, and clarithromycin ( 3, 6). Treatment times can range from 6 wk to 6–8 mo ( 3). Although an initial response to treatment can be achieved, resulting in remission of clinical signs, relapses are common and have been reported up to 2 y following treatment ( 3). Consideration must also be given to the possible zoonotic potential of affected cats prior to attempting treatment.

Human cases of M. avium infection are becoming increasingly common ( 9). It has generally been assumed that only immunosuppressed individuals or those with existing pulmonary disease are at risk of contracting M. avium infection through direct acquisition of the organism from contaminated environments ( 3, 9). However, there are increasing reports of M. avium infection in humans who do not have predisposing risk factors ( 9). Although there are no confirmed reports of transmission of M. avium between animals and humans, reasonable precautions should be taken by anyone handling affected animals ( 3). In particular, special attention should be paid to the appropriate handling and disposal of used needles to prevent accidental inoculation of the organism.

Mycobacterium avium infections are only rarely reported in domestic animals and are usually opportunistic infections. Unfortunately treatment failures and relapses are common in dogs and cats infected with M. avium. With the increasing prevalence of human infections with MAC organisms, appropriate handling of known or suspected cases to prevent accidental zoonotic transmission is prudent.

Feline leprosy

Introduction

Mycobacterium leprae, the aetiological agent of leprosy in humans, gives rise to a chronic granulomatous disease that affects primarily the skin and peripheral nerves, and secondarily some internal organs such as the testis and the eye; viscera are seldom involved. Depending on host resistance, leprosy may present as a benign disease (tuberculoid leprosy) or as a malignant disease (lepromatous leprosy), with a spectrum of intermediate stages appearing between the two. Immunity against leprosy depends on the cell-mediated immunity of the host, and this is severely compromised in the malignant (lepromatous) form of leprosy. Although culture of M. leprae has never been achieved in artificial media, the bacterium may be grown in several experimental animals, including the armadillo, non-human primates, and to a certain extent, rodents. Naturally acquired leprosy has been reported in wild nine-banded armadillos (Dasypus novemcinctus) and in three species of non-human primates (chimpanzees [Pan troglodytes], sooty mangabey monkeys [Cercocebus atys] and cynomolgus macaques [Macaca fascicularis]), thus qualifying leprosy as a zoonosis.

Previously, it was assumed that the causative agent was exclusively Mycobacterium lepraemurium, the bacteria that causes systemic disease in rats. Genetically, M. lepraemurium is most closely related to M. avium and M. avium sub paratuberculosis. Molecular methods of detection have identified the involvement of at least two other, as yet unnamed, species of mycobacteria. Cases of feline leprosy tends to occur in temperate, coastal regions and has been reported in Australia, NZ, western Canada, Netherlands, France, Greece, the UK, and the USA. A condition known as 'feline multisystemic granulomatous mycobacteriosis' has been documented in cats from western Canada and USA (Idaho and Oregon) caused by Mycobacterium visibilis (or M. visibile). This condition is characterised by diffuse cutaneous infection and widespread systemic dissemination.

Typically, cats with feline leprosy develop slowly growing, occasionally ulcerated nodules on the distal limbs. Clinical course is aggressive and locally recurrent, however cats may develop widespread lesions over several weeks. Feline leprosy comprises two different clinical syndromes, one tending to occur in young cats and caused typically by M lepraemurium and M. fortuitum/peregrinum group. Feline leprosy refers to a condition in which cats develop granulomas of the subcutis and skin in association with intracellular acid-fast bacilli that do not grow well on routine laboratory media. The clinical course of the disease is usually aggressive, with a tendency towards local spread, recurrence following surgery and development of widespread lesions over several weeks. Most affected cats tend to reside in both suburban and rural environments. 

Diagnosis

The diagnosis of feline leprosy is made via histopathological or cytological documentation of pyogranulomatous inflammation with negatively staining (Romanowsky stained) or acid fast bacilli (AFB) within macrophages.

Pathologically, feline leprosy can be divided into two forms; 

1. Lepromatous leprosy (LL) - characterised by pyogranulomatous dermatitis and panniculitis, immunologically ineffective multinucleate giant cells, absence of necrosis and abundant intrahistiocytic acid-fast bacilli (AFB). LL is thought to occur in individuals with a suboptimal cell-mediated immunological response to the infective organisms.

2. Tuberculous leprosy (TL) -  characterised by pyogranulomatous dermatitis and panniculitis with multifocal to coalescing areas of necrosis and moderate to scant intrahistiocytic AFB. In cases with TL, there is a strong cell-mediated response. Whether an individual has LL or TL may depend on host factors such as individual major histocompatibility complex haplotype or acquired defects of cell mediated immunity (e.g. retroviral infection, renal disease, etc), or may be dependant on the organism itself. Interestingly, cats with the novel mycobacterial species appear to have a  predominance of LL, whereas young cats with M. lepraemurium commonly have TL (although a number of cats with LL caused by M. lepraemurium have also been documented.

Although lepromatous organisms are slowly growing and fastidious, samples should be submitted for culture as occasionally a slowly growing saprophytic or tuberculous species may be the causative agent. Molecular methods such as PCR are becoming increasingly popular as a diagnostic tool in feline leprosy cases as they offer the advantage of rapid identification of organisms to the species level. Depending on the expertise of the laboratory, PCR can be performed on fresh (frozen) or paraffin embedded tissue.

Treatment

Definitive treatment guidelines for each of the causative agents are yet to be established.

Treatment with clarithromycin plus rifampicin and/or clofazamine for at least 2 months after clinical resolution seems to give the best chance of resolution. The concurrent use of multiple anti-mycobacterial agents is recommended because of the high incidence of resistance. 

Wide surgical excision, especially in the case of early, localised infections, may be helpful. New fluoroquinolones, such as moxifloxacin or pradofloxacin, may prove useful in the treatment of these infections.

Infections caused by opportunistic mycobacteria

Aetiology

Also known as 'atypical mycobacteria', the causative agents of rapidly growing mycobacterial (RGM) infections are ubiquitous saprophytes that are able to grown on synthetic culture media within 7 days at 24⁰C to 45⁰C. RGM include members of the M. smegmatis group (M. smegmatis sensu stricto, M. goodi, M. wolinski), M. fortuitum group (M. fortuitum, M. peregrinum and the third biovariant complex), M. chelonae/abscessus group, M. phlei, M. falvenscens and M. thermoresistible. Initially thought to be more common in tropical and subtropical areas (e.g. South eastern USA), many cases have now been documented in temperate regions, including Australia, Canada, Finland and Germany. In Australia, most infections in cats are caused by M. smegmatis group (followed by M. fortuitum group), whereas in the USA, infections with M. fortuitum group followed by M. chelonae/abscessus group appear more common, at least in NSW (Australia) and California, respectively.

Clinical features

Localised infection of the skin and subcutis (mycobacterial panniculitis) is the most common clinical presentation. This localised infection tends to occur in immunocompetent individuals, where the organisms presumably gain entry into host tissues via a breach in the integument (usually via inoculation during a cat fight or other penetrative injury).

Typically, early lesions are found in the inguinal area (although infections can start in the axilla), then may spread to adjacent areas such as the perineum and the lateral abdominal and thoracic body walls. Because of the favourable environment offered by the subcutaneous fat deposits in this area, obese individuals may be predisposed. Adipose tissue aides in the survival and replication of organisms because it provides triglycerides for growth and/or physical protection from phagocytes.

Lesions are characterised by the presence of draining tracts, with characteristic purple-blue skin indentations ('pepper-pot' appearance) and patchy alopecia. Palpation of the affected skin and subcutaneous tissue reveals variable fluctuant and firm, 'ropey' areas, with the skin being adherent to the underlying tissue. The exudate from fistulae is typically watery, although secondary bacterial infections may cause this to become purulent. Typically, lesions are not overly painful and affected cats are not usually systemically unwell, but some may be depressed, pyrexic and have weight loss.

Pneumonia caused by RGM have been reported in cats, however this is rare. Predisposing factors were not identified in most cases, except one where the infection was thought to be secondary to aspiration of liquid paraffin. Disseminated systemic RGM infections are also rare, and are usually thought to arise as a result of impaired cell mediated immunity.

Diagnosis

The cytology and histopathology of lesions reveals pyogranulomatous inflammation with low numbers of AFB. The diagnosis can be readily obtained by culture of pus aspirated through intact skin disinfected with alcohol (in an attempt to avoid culturing surface bacteria and fungi) with or without the assistance of ultrasound guidance. Clinicians need to notify the laboratory of clinical suspicion of mycobacteria so that appropriate culture methods can be used. Culture of deep tissue samples can also be attempted. PCR to identify organisms can be achieved from either fresh or paraffin embedded samples.

Treatment

Empiric therapy with doxycycline and/or fluoroquinolones is reasonable, until susceptibility data is known. M. smegmatis is usually susceptible to doxycycline and fluoroquinolones but tends to be resistant to clarithromycin. M. fortuitum tends to show higher levels of resistance. M. chelonae tends to be resistant to all commonly  used drugs except clarithromycin, gatifloxacin and linezolid. A cure may be achieved with long-term administration of drugs but occasionally recalcitrant lesions require en bloc resection, with appropriate reconstructive techniques (e.g. skin fold advancement flaps). The total duration of therapy is usually 3-12 months (ideally, 1-2 months past clinical resolution of signs).

Slowly growing opportunistic mycobacterial infections

Aetiology

Slowly growing, non-tuberculous mycobacteria are ubiquitous in soil and water. Species include M. avium-intracellulare complex (MAC), M. genavense, M. terrae complex, M. simiae and M. xenopi. Usually, disseminated infection is observed in individuals with disturbances of cell mediated immunity. Such conditions may be inherent, such as the breed related susceptibility seen in Siamese and Abyssinians or acquired (illness, immunosuppressive drugs, etc). Occasionally, these organisms cause localised disease in apparently immunocompetent individuals, presumably due to introduction of organisms via a breach in the integument.

Clinical signs

Generally, clinical signs of disease are referable to the areas in which the infection and subsequent granulomatous inflammation occurs. As such, affected cats may develop signs referable to disseminated disease (generally involving the respiratory and/or intestinal tracts), with or without cutaneous, often ulcerated, granulomata and lymphadenitis (either peripheral or intracavitary). Cats may become anorexic and pyrexic. Chronic weight loss may be a feature, particularly if the intestinal tract is involved.

Diagnosis

As with other mycobacterial infections, documentation of AFB in cytology or histopathology specimens, plus identification of the organism via culture and/or molecular methods are the keys to a successful diagnosis.

Treatment

Treatment of these infections is often difficult and protracted, but successfully treated cases have been reported. Clarithromycin forms the cornerstone of therapy, but this agent should be combined with clofazimine, rifampicin and/or enrofloxacin to reduce the chance of antimicrobial resistance. Surgical excision of granulomatous tissue, if feasible, may be beneficial.