Mycobacteria are a specific group of bacteria with special characteristics, and 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 involving the subcutaneous tissues (tissues just below the skin itself)
In humans, the organism Mycobacterium tuberculosis causes the majority of cases of tuberculosis. This species of Mycobacterium rarely infects other mammals and cats are naturally resistant, so infection is rare. Other mycobacteria that can cause tuberculosis in humans include M bovis, and the M avium complex (MAC) - a group of related mycobacteria that are widespread in animals and in the envrionment.
In cats, tuberculosis has historically been caused by infection with Mycobacterium bovis. M bovis infects cattle and a wide range of animals including deer, badgers, ferrets, dogs and cats. Cats may be infected through consuming unpasteurised infected cows milk, or from contact with infected other animals or their immediate environment.
Other tuberculosis-causing mycobacteria can also infect cats, including M microti - this is a mycobacterial species that mainly infects rodents (e.g., voles) and is an important cause of tuberculosis in cats that hunt actively. Most cat infections have been reported in the UK, but the disease is seen in other countries as well. Infection with M avium complex (MAC) also occurs occasionally in cats and can result in development of dessimentated granulomas (inflammatory masses).
Clinical signs of tuberculosis in cats
If disease is acquired from ingesting infected food (e.g., drinking unpasteurised milk infected with M bovis), then intestinal infection usually results with inflammatory granulomas in the intestine leading to diarrhoea, vomiting, inappetence and weight loss. However, infection via the skin probably occurs more commonly (with organisms acquired from bite wounds during hunting or perhaps from the environment), and initial signs may be non-healing sores and/or nodules (small firm swellings) in the skin, often associated with swollen lymph nodes. The lymph node swelling may be marked, and sometimes this is the only (or most obvious) sign. As the disease progresses, infection may spread elsewhere, including to the cat's lungs, and so may cause progressive coughing or difficulty breathing. Disease usually progresses slowly, and cats often develop lethargy and weight loss with disseminated infections.
Infection tends to be more common in adult male cats that are active hunters. As infection may often be acquired from fight wounds during hunting, initial skin lesions are often seen around the head and face, or on the extremities. Swollen lymph nodes under the chin often develop after infection to the skin around the head area. In some cases, local infection may spread to tissues under the skin, including muscle and bone that may lead to signs of lameness.
Risk to humans
All mycobacteria that cause tuberculosis pose a potential threat to humans (and other animals too), although the risk of spread of infection from cats to humans or other animals appears very low. Nevertheless, as M tuberculosis and M bovis are important causes of tuberculosis in humans, if cats are infected with these bacteria, treatment is not recommended.
Diagnosis of tuberculosis in cats
Infection may be suspected on the basis of clinical signs, but these are not diagnostic for mycobacterial infections. Investigations will often include X-rays to identify spread of infection to lungs, bone, abdominal organs etc. Routine blood tests may sometimes show anaemia and/or hypercalcaemia (high blood calcium concentrations, that can occur secondary to the inflammatory granulomas).
In some animals, a skin test can be used to diagnose tuberculosis but this is unreliable in cats and so not used. To confirm mycobacterial infection, aspirates and/or biopsy samples of affected tissue should be obtained. These sample can be:
- Stained (using a stain called Ziehl Neelsen - ZN) - which specifically stains mycobacteria to identify their presence - careful examination is require though as sometime the number of organisms present is small
- Submitted for routine histology to look for typical inflammation, and also for the presence of organisms using ZN stains
- Submitted to a laboratory for culture of the organism to determine the exact species involved. Since these are very slow-growing bacteria, culture typically takes several months and unfortunately, many samples that are seen to have ZN positive organisms may fail to culture anything
- Submitted to a laboratory for PCR analysis (genetic testing for the presence of mycobacteria and the species involved). However, these tests are only available for some of the organisms causing mycobacterial infections in cats.
If the species of mycobacteria is identified, the zoonotic risk, potential source of infection, and treatment options can be evaluated.
Treatment of tuberculosis in cats
It is not generally recommended that cats with M tuberculosis or M bovis are treated, as although the risk to humans is low, these are important causes of tuberculosis in humans. Also, in some countries, these infections have to be notified to the appropriate health authorities.
Other tuberculous mycobacteria may also carry a very low risk of transmission to humans so careful consideration of treatment is needed, especially if there is contact with very young, very old, or immunocompromised people. If treatment is undertaken, this needs to be long-term (for many months) and it is important that cats are monitored during treatment as side effects can occur. In some cases the drugs may suppress the disease, at best, and indefinite treatment may be required. Surgical removal of small skin lesions may sometimes help, but does not remove the need for follow-up drug therapy.
Because antibacterial drug resistance is a problem with mycobacteria, treatment usually involves the use of a combination of two or three antibiotics. Those most commonly used include:
- A fluoroquinolone, such as marbofloxacin, pradofloxacin, or orbifloxacin
- A macrolide, such as clarithromycin or azithromycin
The prognosis for infected cats depends on the type of mycobacteria, and the extent and severity of the infection. While many cases, especially those caused by M. microti, may respond favourably to treatment, the prognosis should always be viewed as guarded.