Mycobacterial diseases in cats – leprosy and opportunistic infections

Mycobacteria are a specific group of bacteria with special characteristics, and are capable of causing disease in both animals and man.

Several species of Mycobacteria can cause disease in animals, being either primary pathogens, or becoming pathogenic (disease causing) under certain circumstances. Mycobacterial infections generally cause one of three different types of disease:

• Tuberculosis: the term used to describe disease where there is the formation of granulomas (inflammatory nodules) in the body;
• Leprosy: the term used to describe disease where infection results in the formation of granulomas in the skin (seen as skin lumps or nodules);
• Opportunistic infections: these are infections that usually involve the subcutaneous tissues (tissues just below the skin itself).

Leprosy in cats

Leprosy is the name used to describe skin infections with a number of mycobacterial species that cause single or multiple inflammatory nodules (granulomas) to develop in the skin or just under the skin (subcutaneous tissues).

What causes leprosy in cats?

In up to 50% of cases, feline leprosy may be due to infection with Mycobaterium lepraemurium. However, studies have shown that a number of other mycobacterial species may be involved in causing this syndrome in cats (including M malmoense, M avium complex, M mucogenicum and some, as yet, unidentified species).

Disease has been reported from many different countries, but the organisms involved and their relative importance, may vary between different countries. The source of infection for cats is uncertain, but might include rodent bites, insect bites or contamination of wounds from the environment.

Clinical signs of feline leprosy

Cats affected with feline leprosy have single or multiple skin nodules often on the head or limbs, and occasionally the body. The nodules often lose their overlying hair, and may become ulcerated. If there are multiple nodes they usually affect one area of the skin. Local lymph nodes may become enlarged later in the disease, but other signs are of illness are rare.

Disease may occur in cats of varying ages – M lepraemurium may be more common in younger cats, with some other species being more common in older cats. With some (but not all) of the mycobacteria, underlying immunosuppression (eg, FeLV or FIV infection) may predispose to infection.

Risk to humans

Feline leprosy is not really regarded as a zoonotic disease (ie, infection is not spread from cats to humans). Leprosy in humans is caused by infection with a different organism, Mycobacterium leprae, and M lepraemurium is not infectious for humans. However, some of the other mycobacterial species that can sometimes cause leprosy in cats could potentially be spread to humans through bites or scratches, although humans are more likely to be infected from the environment.

Diagnosis of leprosy in cats

Obtaining a sample of infected tissue by fine needle aspirate or biopsy is needed to make a diagnosis. Cytology and histology may show large numbers of typical ‘acid fast’ organisms (staining with a special stain known as ‘Ziehl Neelsen’) within macrophages (a type of inflammatory cell) seen within infected tissues.

Culture of samples is often unrewarding, as many of the species of mycobacteria involved are very difficult to culture. Nevertheless, this should be performed where possible. Molecular techniques (eg, PCR) are available to accurately identify some of the bacteria involved.

Treatment of leprosy in cats

Surgical removal of small nodules may be helpful, but is often not curative, so even if nodules are removed, a minimum of two months follow-up antibacterial therapy is usually recommended.

As with most mycobacterial infections using a combination of more than one antibacterial is recommended – common drugs used include:

• Rifampicin;
• Clarithromycin;
• Clofazimine.

In most cases, the long-term outlook for cats diagnosed with leprosy is good.

Opportunistic mycobacterial disease

Opportunistic mycobacterial infections are infections that occur with a diverse group of mycobacteria that are present in the environment, being ubiquitous in soil, decaying vegetation, water, etc. These mycobacteria are often collectively known as ‘rapidly growing mycobacteria’ or RGM as they are generally easy to grow (culture) in the laboratory, although slower growing mycobacteria may also sometimes be involved. Infection is usually thought to occur from contamination of a wound.

The following organisms have been implicated in causing this syndrome; M. chelonae-abscessus, M fortuitum/peregrinum group, M smegmatis and M phlei, and others. These bacteria are opportunistic, causing disease through contamination of skin wounds, and can be particularly pathogenic if inoculated into adipose (fatty) tissue, particularly that beneath the skin. Adult outdoor cats with a hunting or fighting lifestyle are more likely to be affected.

Clinical signs of opportunistic mycobacterial disease in cats

The most common clinical syndrome caused by opportunistic mycobacterial infections is panniculitis (inflammation of the fat under the skin). This usually affects the inguinal tissues (the groin area). Infection causes inflammation which may initially appear as ovules or plaques within or beneath the skin. Infection will then spread to surrounding tissues, where it usually causes multiple small skin ulcers which can exude fluid – this may resemble a cat bite abscess in some cases. As the infection progresses, more tissue may become infected and further inflammation occurs. Eventually the whole of the ventral abdomen (‘belly’) of the cat may be affected, and the infected tissues can be extremely painful. Spread to other tissues in the body (internally) is rare.

Risk to humans

Since opportunistic mycobacteria are present in the environment at all times, the risk of transmission from a cat to a human is low. However, it is recommended that normal hygiene measures are followed.

Diagnosis of opportunistic mycobacterial infection

Opportunistic mycobacteria may be seen on impression smears, fine needle aspirates or biopsies from infected tissues. Pyogranulomatous inflammation is seen but organisms may not be easy to identify as they can be quite scarce.

Culture of the organisms from a biopsy (or aspirate) is the test of choice, and the organisms are usually relatively easy to grow . This allows diagnosis, confirmation of the species involved, and also sensitivity testing (determining which antibiotics are best to use). Molecular diagnostic tests (eg, PCR) are also available in some situations.

Treatment of opportunistic mycobacterial infections in cats

Treatment should be with one or more antibacterial drugs, for example:

• A fluoroquinolone, such as pradofloxacin, merbofloxacin, or ciprofloxacin;
• Doxycycline;
• Clarithromycin;
• Clofazimine.

Ideally, once antibacterial sensitivity testing has been performed, drugs known to be effective against the species involved may be used. Therapy should be continued for several months and this may result in complete resolution in some cases.

With severe and widespread disease, usually after an initial course of antibiotics, surgery is needed to remove as much of the infected tissue as possible, followed up with several months (3-12 months typically) further drug treatment.

Prognosis for opportunistic mycobacterial infections in cats

The prognosis can be good if disease is identified early and treated aggressively. Some cases may be harder to manage than others, and sometimes permanent or intermittent antibiotic treatment may be needed to keep infection under control.