April 6th, 2023
An Update On Mycobacteria In Cats
‘If you’re not seeing cases of mycobacteriosis in cats then they are seeing you’ was the statement delivered by Professor Danielle Gunn-Moore in last week’s Platinum Member webinar. This was one of the key take home messages Danielle wanted vets to take on board about mycobacteriosis in cats and explained this disease is more common than most of us think. The reason why some of these cases are missed is because some affected cats can present with subtle clinical signs which can easily be mistaken for other conditions or, sometimes, as ‘nothing to worry about’.
Danielle showed an array of pictures demonstrating the variety of presentations from the obvious large skin nodule to a tiny raised area just a millimetre or two in size which could be easily missed or even, if spotted, assumed to be completely innocent. Data received by Danielle from 21 UK wide laboratories also showed that 1% of eighteen thousand biopsies taken from anywhere on or within the cat had changes highly suggestive of mycobacterial infection, once again demonstrating that this disease is more prevalent than we think.
Difficulty in diagnosing mycobacteria in cats may also play a role in vets missing cases. Most affected cats usually present with cutaneous lesions which, as stated before, can present in a variety of ways. Other clinical signs include mandibular lymphadenopathy, respiratory signs, weight loss, gastrointestinal signs, arthritis and ocular signs. Some of these clinical findings may lead the vet to consider neoplasia as a potential diagnosis. For example, osteomyelitis found in cats infected with mycobacteria when bitten by a rodent over the joint could be mistaken for an osteosarcoma on radiography. Lymphadenopathy and the presence of nodules can also lead the clinician to be suspicious of neoplasia. Due to the similarity in clinical signs, Danielle’s advice is that, whenever cancer is being considered as a potential diagnosis in the cat, mycobacteriosis should also be placed on your list of differentials, especially if clinical signs correlate.
Biopsy of nodules can also provide evidence of a mycobacterial infection but Danielle explains that once specimens are placed in formalin, further tests cannot be performed to confirm this diagnosis. For this reason, she always advises that, as well as placing some tissue in formalin, some should also be placed within a plain pot which can be frozen for later use. This tissue can then be used to perform a PCR and to culture for mycobacteria. These tests, however, also have limitations, with only 50% of cases actually showing any growth on culture and with PCR only being useful if there is a lot of bacteria on the histopathology sample due to its lack of sensitivity. Danielle explained there is now a new blood test which has been developed called the Interferon Gamma Release Assay (IGRA) which can also be used and is proving useful in cases which are difficult to diagnose.
In cases where clinicians are suspicious or have confirmed mycobacterosis, Danielle always advises performing a chest x-ray to check for pulmonary TB. This is particularly important as few cats with this condition actually show any respiratory signs despite there often being evident lung changes on x-ray with cats presenting with a diffuse interstitial lung pattern. Lymphadenopathy, mineralisation and pleural/pericardial fluid may also be present.
Of course once a case of mycobacteriosis in the cat is confirmed, the zoonotic potential for that organism needs to be considered. Danielle advises the risk of zoonosis is low but is present as confirmed by an outbreak of M. bovis in nine cats in Newbury which left two humans with latent infection and two humans with fulminant infections. For this reason, Danielle advises if an affected cat has a simple small nodule with no other clinical signs then treatment is an option. However, if the owners are immunocompromised, i.e. the very young, the pregnant, or the very old, then the cat should be euthanased. Also, if disease is extensive, if there is the presence of dripping pus, or if there is the presence of any cavitation or caseation, cats should also be euthanased.
Treatment for the tuberculosis form of mycobacteriosis (which includes M. bovis and M. microti), involves the use of three drugs simultaneously, rifampicin, pradofloxacin and azithromycin. Treatment needs to be continued for at least two months after the cessation of clinical signs which is usually for a period of three-six months. In Danielle’s experience usually 70% of cases go into long-term remission when using triple therapy over a long period of time. Non-tuberculosis mycobacteriosis which includes Feline Leprosy Syndrome, usually presents as cutaneous nodules and panniculitis in the cat due to the lypophillic nature of the bacteria. Once again treatment is usually with double or triple therapy using different drugs to the treatment of tuberculosis and is often prolonged with treatment usually being administered for more than a year.
Mycobacteria in cats is not a disease I thought I encountered frequently but after digesting this webinar, it may well be that this disease has been presenting itself to me time and time again, I have just not been recognising or considering it. The variety of presentations demonstrated by the pictures shown by Danielle make it evident that this is a condition which can present in a number of sometimes innocuous ways. This webinar delivers the information needed to try and make sure we diagnose all the cases we encounter in general practice and as one in 100 biopsies taken from the cat are likely to show changes consistent with mycobacterosis, this makes last week’s webinar essential viewing to ensure we appropriately diagnosis and manage cases of feline mycobacteriosis.