Mycobacterial infections in cats

Mycobacteria can cause disease in both man and animals. However, cats appear to have a greater risk of developing mycobacteria infections than other animals. Recovery depends on the type of mycobacteria involved, and the extent and severity of the infection. Many infected cats will respond favorably to treatment, although a prolonged course of treatment is necessary.

Mycobacteria are a large group of bacteria, some of which can cause disease in man or animals. They can be very difficult to grow in the lab so identifying the exact strain involved can be difficult. Because they tend to grow very slowly in infected cats, disease usually develops slowly over a few weeks.

A number of different species of mycobacteria can infect cats. These include Mycobacterium microti (from mice and voles), Mycobacterium bovis (from cattle via badgers, mice and voles), Mycobacterium avium (from birds), and a number of mycobacteria that usually live in soil. The prevalence of the different infections varies around the world; most reports of tuberculosis (TB; caused by M. microti and M. bovis) come from the UK, while cases of non-TB infections are seen in tropical and subtropical areas of the world, including GB and mainland Europe, US, Canada, Australia and New Zealand. Cats appear to have a greater risk of developing mycobacterial infections than other animals.

If your cat hunts and eats mice or voles it is at risk of catching TB – that said, while very many cats hunt small mammals, very few ever develop TB. Adult male cats have an increased risk, and M. bovis-infected cats are found in areas where cattle, badger, mice and other small rodents are infected with M. bovis, while M. microti is found where M. microti-infected rodents have been detected. Infections can also enter from the soil via any skin wounds, and cats that fight frequently could be at an increased risk.

The majority of cats present with skin disease (non-healing lumps and/or ulcers) – these lumps and ulcers may develop slowly, and tend not to heal even if the cat is given routine antibiotics. Disease can sometimes spread from the skin to the lungs which may cause the cat to breathe more quickly and sometimes cough. If disease is very extensive the cat may lose weight.

First, your veterinarian must suspect that your cat may be infected with TB. Non-healing wounds should be biopsied and samples sent for histopathology and culture (unfortunately, some of bacteria eg M. microti, can take up to three months to grow); a chest x-ray should be taken to see if the infection has spread to the lungs. A blood test (the interferon gamma release assay – IGRA) can be used to identify M. microti, M. bovis and M. avium infections. Molecular tests, eg PCR tests, can be run on biopsied tissue to identify some of the mycobacteria.

Cats with TB will need a long treatment consisting of two to three drugs, usually given for six months, sometimes more. This can be difficult given cat non-compliance (it is not easy to give some cats multiple drugs at least once a day), the toxicity of some of the drugs, and the financial costs involved. That said, many cats have been successfully treated when they have dedicated owners and a supportive veterinary team.

Possibly, perhaps 70% of cats infected with TB can recover, but only when their disease is not too advanced, and the cat receives typically six months treatment of two to three drugs, which need to be given daily.

This appears to occur only very rarely. While people can be infected by M. bovis, M. microti and M. avium, especially if they are immunosuppressed, only M. bovis is known to have crossed from cats to their owners. When this has occurred the cats involved typically had a very wet-looking wound that was dripping pus.

This appears to occur only rarely. However, where a cat is infected with M. bovis and has a very wet-looking wound that is dripping pus, then there is a risk of the infection spreading to other cats and, possibly dogs.