A 12 year old female neutered domestic short hair cat presented with lymph node enlargement in the submandibular area.  Histological examination revealed a granulomatous lymphadenitis with necrosis.

Figure 1: Large areas of necrosis are present in the lymph node (stars). Magnification x 7.5. H&E stain. Scale bar =2.5 mm.

Figure 2: Granulomatous inflammation within the lymph node. Magnification x 200. H&E stain. Scale bar =100 µm

Figure 3: Acid-fast (pink) bacilli within the lymph node (arrows), consistent with mycobacteria. Magnification x 400. Ziehl-Neelsen stain. Scale bar = 50 µm.

PCR analysis revealed a mycobacterium of the Mycobacterium avium complex (MAC). Mycobacteria in this group are non-tuberculous mycobacteria. Typically, these are saprophytic and infections may occur via contaminated wounds.

Clinical signs of mycobacterial infections in cats:

The most common presentation of mycobacterial infection in cats is skin lesions. Cats may present with single or multiple nodular masses, non-healing wounds and granulomatous panniculitis, as well as regional lymph node enlargement. In some cases, such as in the case presented here, lymph node enlargement may be the only or most obvious clinical sign.

Most commonly nodular skin lesions are found in areas predisposed to bites during hunting, such as the face and extremities. Granulomatous panniculitis is most commonly found in the inguinal area, flanks and tail base.

Systemic dissemination may occur in particular to lungs, liver and spleen. An alimentary form has been associated with ingestion of M. bovis contaminated milk or pet food.

Diagnosis:

• Ziehl-Neelsen (ZN) stain on cytology samples or histological sections may detect the presence of acid-fast rods. Mycobacteria may be present in low numbers and ZN stains may therefore be false negative in some cases.
• Mycobacterial culture – please note that not all mycobacteria can be cultured and more than 50% of ZN positive lesions in cats were found to be negative by culture in one study (Gunn-Moore et al. 2011).
• PCR analysis may be performed on non-fixed and formalin-fixed samples. Please note that formalin-fixation may inhibit PCR analysis.

If a mycobacterial infection is a potential differential diagnosis, and tissue samples are taken, preparation of an impression smear should be considered.  Ziehl-Neelsen staining of the impression smear will be performed, and if found positive for mycobacteria, the sample can be prioritised for specialist culture and PCR.

Tissue samples may be divided, with one third submitted in formalin for histopathological analysis and the remainder frozen in 2 separate containers for subsequent culture and/or PCR. This may not be an option for small samples.

• The interferon-gamma release assay uses peripheral blood mononuclear cells obtained from a blood sample of the patient. The cells are stimulated with synthetic peptides representing mycobacterial epitopes. In cats with a mycobacterial infection, this will lead to a release of interferon-gamma, which is measured in an ELISA assay.

Please contact us prior to sampling, if you wish to discuss sample handling, sample submission and test options.

Zoonotic risk:

Most mycobacteria have zoonotic potential. If the presence of mycobacteria is confirmed by ZN stain, stringent precautions should be taken, until further characterisation of the causative agent has been performed. Cats with ulcerated wounds or draining tracts pose an increased risk of exposure. If samples are submitted to diagnostic laboratories, any suspicion of a mycobacterial infection should be clearly stated on the submission form to prevent exposure of laboratory staff.

Mycobacteria of the M. tuberculosis complex (including M. tuberculosis, M. bovis and M. microti) pose the most significant risk to all in contact persons, not only those that are immunocompromised.

Non-tuberculous mycobacteria cause opportunistic infections in immunocompromised human hosts, including those with underlying disease, undergoing chemotherapy, pregnant women and the elderly. M. avium (subspecies avium) poses the greatest risk of those NTM found in cats.

The identification of M. bovis in cats (and any other non-human mammals) is notifiable to the APHA.

Reference:
Gunn-Moore DA, McFarland SE, Brewer JI, Crawshaw TR, Clifton-Hadley RS, Kovalik M, Shaw DJ. (2011). Mycobacterial disease in cats in Great Britain: I. Culture results, geographical distribution and clinical presentation of 339 cases. J Feline Med Surg. 13(12):934-44.