A rare case of polyarthritis in a cat

Mathilda, a 7 year old domestic short hair cat, was referred to me with a one month history of high temperature and lethargy. There had been some response to antibiotics but the high temperature kept recurring once treatment was ceased. Blood samples had shown some non-specific signs of inflammation, but no dramatic abnormalities.

Mathilda was quiet but affectionate on examination. Her heart and lungs sounded normal and no abnormalities were felt on abdominal palpation. Her temperature was high, but the most striking aspect of the examination was her stiff, stilted way of walking, suggesting joint or muscle pain.

Persistent high temperature can be caused by an infection, a tumour, or a sterile inflammatory process which is often immune-mediated. In cats, a particular worry is often feline infectious peritonitis, however Mathilda did not show any signs typical of this. High temperature is such a vague sign, that often investigations have to be broad, although if there are any localising findings from history or examination these would form a particular focus for tests. In Mathilda’s case muscle enzyme levels were normal.

Fluid samples from several joints showed marked inflammation, suggestive of immune mediated polyarthritis (IMPA). IMPA is uncommon in cats. It can be a primary condition, but as it is not nearly as frequently seen in cats as it is in dogs, a greater emphasis was put on checking for an underlying condition. It would be possible for a tumour, a focus of infection, or certain infectious organisms to stimulate the immune system to attack the joints. Also, additional steps were taken to check for low-grade septicaemia and associated septic polyarthritis, as the inflammation in septic polyarthritis could mimic the inflammation in IMPA, and cats can present in this way from chronic low grade bartonellosis for example.

Imaging showed several mildly enlarged lymph nodes in the abdomen. Needle samples were collected with ultrasound guidance, and a skilled pathologist analysed these and the joint fluid samples at an external laboratory. Unexpectedly, minute bacteria called mycoplasma were suspected in both sites. A molecular test (PCR) confirmed the infection. This is an unusual finding, particularly in the UK, and I have only ever seen this in one other cat. Specific antibiotics are required to treat mycoplasma, and with a long course of these, Mathilda was soon back to her lively happy self.