Mycobacteria and the Moggie
Ruth Gostelow
"Cats can be infected by a whole range of mycobacteria," Ruth Gostelow from the Royal Veterinary College, London, told delegates at the BVNA Congress in Telford on Saturday 11 October.
Mycobacteria are a very large group of bacteria, some of which are highly pathogenic. They have a very thick, fatty cell wall that means that they are resistant to environmental factors and can survive in soil and in immune cells in the host. They can also continue to multiply in tissue to form tubercles.
TB and leprosy are mycobacterial diseases that have infected humans for millennia. Mycobacteria were first isolated from cats over 50 years ago and up to 10 per cent of the UK feline population may show evidence of contact with this group of bacteria.
There has been a recent report of M bovis infection in seven cats in Berkshire with evidence that they had transmitted the disease to some of the human owners.
There are three groups of mycobacterial infection in cats – the TB Complex, Non-TB and Feline Leprosy Groups. The source of infection in the first group is either unpasteurised milk or infected rodent prey species, in the second group infection is usually manifested as skin lesions as a result of wound contamination from the environment, and group three infections follow contact with rodents and environmental sources.
How much of a risk do infected cats pose to people? Potentially, the most zoonotic risk is from cats infected with mycobacteria from the TB Complex group, including M bovis. This risk to humans is low, with few reported cases; but it is important to remember that TB is a notifiable disease.
The most likely cats to be infected are males, living in the South West, that go out and hunt and, therefore, have greatest exposure to vector species and environmental sources. Immune-compromised cats are also at risk.
Diagnosis is difficult and initially based on geographical location, lifestyle of the cat and imaging. If there are lesions, then smears and biopsies may be useful, together with microscopic granulomatous changes and specialist staining. Confirmation of the mycobacterial species ultimately requires complex diagnostic tests, and mycobacterial culture is a prudent measure in order to determine antibiotic sensitivity.
Treatment can be challenging and the zoonotic risk is of paramount concern, especially if owners are susceptible – HIV patients, for instance. Therapy may be for an indefinite period, possibly life-long, and costly. It may involve a multi-antibiotic approach and there are often side effects, so compliance and cost are major considerations.
Overall, the outlook for affected cats is highly variable, with the best prognosis in cats with localised skin lesions that can be removed surgically. It should be made very clear to owners that treatment will be costly, time-consuming and of uncertain outcome. In the event that euthanasia is considered, then the body should be cremated.
Veterinary practices are being urged to help owners safely dispose of any unused and expired pet medicines - not just antibiotics - during November's Animal Medicines Amnesty. 
