A 17-year-old cat presented with a history of weight loss. On ultrasound there was a moderately enlarged, hypoechoic and rounded caudal mesenteric lymph node. The only other significant finding was mild degenerative changes to the kidneys. Fine needle aspirate samples for cytology were collected from the mesenteric lymph node.

What is Your Interpretation of the Cytology? 

Below are images of lymph node cytology. What is your interpretation?

Figure 1. Lymph node cytology from a cat. 100x objective.

Figure 1. Lymph node cytology. 100x objective.

Figure 2. Lymph node cytology. 100x objective.

Figure 2. Lymph node cytology. 100x objective.

Figure 3. Lymph node cytology. 100x objective.

Figure 3. Lymph node cytology. 100x objective.

Here is the pathologist’s cytology description:

The slides are highly cellular. There is a heterogeneous population of lymphocytes with small lymphocytes predominating. There are mildly increased numbers of intermediate and large lymphocytes. Plasma cell numbers are mildly increased. There are also low numbers of nondegenerate neutrophils and frequent macrophages. Within the macrophages and background are abundant non-staining bacterial rods consistent with Mycobacterium spp. The background also contains abundant material consistent with ultrasound gel. No overtly neoplastic cells are seen.

Findings are consistent with reactive lymphoid hyperplasia with histiocytic inflammation and non-staining bacterial rods consistent with Mycobacterium spp.

Additional Diagnostic Testing

Additional diagnostic tests were completed that included an acid-fast stain which showed acid fast bacilli present (figure 4).

Figure 4. Acid fast stain: showing Acid fast bacilli present. 100x objective.

Figure 4. Acid fast stain: showing Acid fast bacilli present. 100x objective.

Finally, Mycobacterium avium was cultured from the sample.

What’s Your Diagnosis?

Mycobacteriosis (atypical and presumably disseminated) due to Mycobacterium avium.

Conclusion: Mycobacterium Avium in Cat

Mycobacteria are gram positive, aerobic, acid fast, non-spore-forming bacilli that are widely distributed in water and soil as well as carcasses and feces. Infection with these bacteria causes granulomatous or pyogranulomatous disease. However, disease is variable depending on route of exposure, the infecting mycobacterium species, and host factors.

In general, mycobacteria are categorized into groups based on cultural growth pattern as either slow, rapid, or difficult. Furthermore, there are three forms of infection depending on the causative organism: classical tuberculosis, opportunistic or atypical infections, and leprosy. Opportunistic or atypical mycobacterial infections, as in this case, are typically due to slow-growing, non-tuberculous mycobacteria species in the mycobacterium avium-intracellulare complex (MAC). This group has 28 serovars of mycobacteria, of which ten are Mycobacterium avium, and most cases, including disseminated disease, are due Mycobacterium avium.

Mycobacterium avium generally has low virulence and cats (except for Siamese cats) are naturally resistant to infection with infrequent reports of disease. It is typically regarded as an opportunistic organism and the location of the lesion(s) reflects the route of exposure. Typically, exposure is via direct contact or ingestion and less commonly via inhalation. Infection of intra-abdominal lymph nodes (as in this case) or the gastrointestinal tract is most often caused by ingestion. Clinical signs vary from organ specific to non-specific generalized signs, but typically more severe disease is thought to occur in immunosuppressed patients, although there is no association with retroviral disease yet.

Diagnosis of mycobacteriosis is relatively straight forward on cytology as these bacteria have a characteristic appearance and Mycobacterium avium tends to be present in high numbers. But, if there is any question to the identity, acid fast staining on cytology samples can be performed. Additionally, acid fast staining on a biopsy sample could be performed to diagnose mycobacteriosis. To confirm infection with mycobacterium and for further information regarding species/group, culture (can take several weeks) and/or PCR is recommended. This is particularly important if treatment will be pursued. The mycobacterium group/species cannot be differentiated by morphology or clinical picture alone, and species/group helps determine prognosis and response to treatment as well as potentially guide a treatment regimen. Additionally, while the risk of transmission is very low, from the species/group we can determine zoonotic potential.


Ultimately, prognosis is guarded to poor and depends on immunocompetence, distribution of lesions, group/species, and susceptibility to antimicrobials. Many have a poor response and slow growing, non-tuberculous Mycobacteria spp like Mycobacterium avium (as in this case) are the most difficult to treat. Typically, combination antibiotic therapy is recommended for a minimum of three months to over a year or even the rest of the life of the patient, and it may also require surgical debulking, if possible. There is ultimately rare success with treatment of both local and less often disseminated atypical/opportunistic mycobacteriosis in cats.


  • Latimer, K. S., Crowell, W. A., Duncan, J. R., Currin, K. P. and Jameson, P. H. (1997), Disseminated Mycobacterium avium Complex Infection in a Cat: Presumptive Diagnosis by Blood Smear Examination. Veterinary Clinical Pathology, 26: 85-89. doi:1111/j.1939-165X.1997.tb00716.x
  • Blauvelt, M., Weiss, D., McVey, A., Bender, J. and Aird, E. (2002), Space‐occupying Lesion Within the Calvarium of a Cat. Veterinary Clinical Pathology, 31: 19-21. doi:1111/j.1939-165X.2002.tb00272.x
  • Barry M, Taylor J, Woods JP. Disseminated Mycobacterium avium infection in a cat. Can Vet J. 2002;43(5):369-71.
  • Greig B. https://www.marshfieldlabs.org/proxy/RefPointWinter11.1.pdf Reference point: Mycobacterial infections in dogs and cats
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