Disseminated mycobacteriosis in a Siberian cat (Mycobacterium avium subsp. hominissuis)
Emily Rätsep, Joseph Cyrus Parambeth
Animal Health Laboratory, University of Guelph, ON (Rätsep), Alta Vista Animal Hospital, Ottawa, ON (Parambeth)
AHL Newsletter, 2021;25(2):32.
A 3-year-old male neutered, indoor-only Siberian cat was presented to Alta Vista Animal Hospital on an emergency basis for dyspnea on August 28, 2020. Abdominal free fluid and enlarged lymph nodes were noted on examination. As the cat was maintained on a commercial raw food diet (rabbit meat), and on cyclosporine (Atopica) for possible atopy, a potential bacterial infection was suspected. Cytological evaluation of the aspirated enlarged lymph nodes showed neutrophilic inflammation. Empirical treatment with amoxicillin and clavulanic acid and pradofloxacin was initiated. Further testing was negative for FeLV, FIV and FIP, and no bacteria were isolated from routine aerobic and anaerobic cultures. PCR was negative for mycobacterium on the abdominal fluid and lymph node aspirates
The cat then represented to internal medicine on September 2nd, wherein aspirates of the lymph nodes demonstrated neutrophils and foamy macrophages with negatively-staining, intracytoplasmic rod-shaped bacteria, consistent with Mycobacteria spp. (Fig. 1). Treatment was initiated with clarithromycin, doxycycline and rifampicin.
On October 10th, the cat represented to internal medicine with ataxia, and a wide-eyed stare. Euthanasia was elected due to the progression of clinical signs (hyporexia, weight loss, icterus), and the body was submitted for postmortem examination.
On postmortem, multiple enlarged and coalescing abdominal lymph nodes were observed, and there were multiple adhesions throughout the abdomen. Histologically, there was severe granulomatous inflammation in multiple tissues, including: intestines, cecocolic and mesenteric lymph nodes, liver, bone marrow and spleen. Macrophages in all sites contained intracytoplasmic, acid-fast, rod-shaped bacilli consistent with Mycobacterium spp. (Fig. 2). Granulomatous inflammation in the section from the ileocecocolic junction was severe, effacing mucosal and muscular layers. No lesions were observed to explain the history of ataxia, though weakness secondary to systemic illness could not be ruled out.
Culture and PCR speciation performed by the Canadian Food Inspection Agency confirmed mycobacteriosis and speciation as Mycobacterium avium, subsp. hominissuis.
The species M. avium subsp. hominissuis can persist in the environment, making determination of the infection source difficult. Enteric localization with subsequent systemic dissemination suggests ingestion as a likely route of infection. Intestinal mycobacteriosis (M. bovis) resulting from a contaminated raw food diet has been reported in the United Kingdom (1). Disseminated mycobacteriosis with a largely enteric distribution was also observed in U.K. ‘wild-type infections’ (M. avium species identified in outdoor-only hunting cats), although cutaneous lesions were also observed in this population (2). Similar outbreaks have not been reported in North America.
The zoonotic transmission potential for this bacterial species in healthy individuals remains low, but a risk to immunocompromised populations remains. While one case does not represent an outbreak, there is value in raising awareness of systemic mycobacteriosis as a potential differential diagnosis for granulomatous enteritis and lymphadenomegaly in small animals. As intestinal mycobacteriosis can occur via ingestion, there is potential risk of transmission by raw food diets, consumption of small wildlife by domestic cats or other environmental contamination. In this case, it is suspected that previous cyclosporine use may have contributed to the establishment of infection. While the raw food diet was not confirmed to be the source of mycobacteria in this case (mycobacterial culture has been attempted with results pending), it remains a strong potential source of infection in the absence of any likely environmental exposure. AHL
Figure 1. Cytology of foamy macrophages with negatively-staining, intracytoplasmic rod-shaped bacteria (consistent with mycobacteria spp.). Wright’s stain (1000x).
Figure 2. Transmural granulomatous inflammation in the colon associated with Mycobacterium avium subspecies hominissuis. 2A. Histologic section of colon illustrating numerous infiltrating macrophages effacing mucosal and muscularis layers. H&E stain. 2B. Multinucleated giant cell with acid-fast intracytoplasmic bacteria (arrow). Ziehl Neelsen stain.
References
1. O’Halloran C, et al. Tuberculosis in UK cats associated with a commercial raw food diet. J Feline Med Surg 2019;21(8):667-681.
2. O’Halloran C. Personal communication. November 2, 2020.