TB in Cats and the risk to people
With the news this week that two people have been infected with tuberculosis, almost certainly from their cats it is a worrying time for cat owners. Evidence at present suggests that the risk of transmission to people from their cat is considered very low and in the Milton Keynes area this risk is luckily probably even lower. Tuberculosis is caused by a group of organisms called Mycobacteria. These organisms are generally quite slow-growing and also quite resistant to antibiotics.
There are three main species of mycobacterium that we need to concern ourselves with regards to cats and they are Mycobacterium tuberculi, Mycobacterium bovis, and Mycobacterium microti. 90% of human TB is caused by M.tuberculi and some by M.bovis. Infection in people with M.microti is extremely rare. Cats on the other hand are quite resistant to M.tuberculi with M.microti and M.bovis causing most of their infections. TB in cats invariably presents with non-healing, discharging skin sores, rather than respiratory complaints. In the days before pasteurisation it was commonly caught from drinking infected cows milk but now it is invariably caught by being bitten on the front legs and head by small rodents which themselves have TB, so cats that hunt are at much higher risk.
We had an unusual case of TB in a cat Milton Keynes a few years ago. She initially presented with a discharging lump below her eye which on analysis was confirmed as TB but was unable to be classified. She had 6 months of triple antibiotic therapy and seemed cured. A year or so later she appeared with inflammation in the left eye which resolved with treatment. A few months later she came back with this lesion in her eye, a similar one in the back of her right eye and a gritty lesion in the lymph node in her back leg from which we were able to culture the T.microti. She also had extensive lung involvement on x-rays. After several more months of treatment the lesions resolved although she remains blind. A year or so later she relapsed with lesions on her spine but she is now on continuous treatment with antibiotics but remains a very sweet happy cat.
The problem with TB is that the organisms are extremely difficult to grow and over 50%, in most studies are unable to be cultured. For those of us in Milton Keynes however the good news is that in those cats where we do culture there TB the typing has a strong relation to geography. In a study done between 2004-8 nearly all the cases of M.bovis in cats came from the West of England and Wales, while nearly all the cases from the South East of England were M.microti. TB is difficult to treat in both people and animals and there is also the ethical dilemma of using what are the most effective treatments in people in animals which may lead to greater resistance. TB in cats is uncommon, but also easy to miss therefore if owners have a pet that is unwell they should consult a vet. However the one ray of sunshine is that cats in our area that do catch TB are likely to be carrying M.microti making the risk to their families very low.
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