Intracellular success: cytologic findings in an ulcerated submandibular mass from a cat

Abstract

A 1-year-old neutered male domestic shorthair cat had an ulcerated, proliferative lesion in the submandibular area that did not respond to antibiotic therapy. Impression smears from the mass revealed septic pyogranulomatous inflammation, with large numbers of pleomorphic bacteria observed intracellularly within macrophages as well as neutrophils. Bacterial culture was consistent with a diagnosis of Rhodococcus equi, a facultative intracellular coccobacillus capable of replicating within macrophages. The cat's lesion resolved after treatment with rifampin and clarithromycin. R equi should be considered as a differential diagnosis when coccobacilli are recognized within macrophages in cytologic samples.

Pathology in Practice

“…The negative acidfast stain results further ruled out mycobacterial infection and supported R equi as the agent, although R equi can be partially acid fast at some growth stages. 4,5 There are numerous case reports 3,[5][6][7][8][9][10][11] of R equi infections in cats; the cutaneous lesions in those cats and the cat of this report were similar, as was the cytologic appearance of smears of fine-needle aspirate specimens. Once the cytologic evaluation supports a diagnosis of R equi infection, bacterial culture and antimicrobial susceptibility testing should be strongly advised because of the variability of antimicrobial resistance of this organism in cats.…”

“…Once the cytologic evaluation supports a diagnosis of R equi infection, bacterial culture and antimicrobial susceptibility testing should be strongly advised because of the variability of antimicrobial resistance of this organism in cats. 9 There has been success in the treatment of infected cats solely with amoxicillin-clavulanic acid, 6 despite R equi being an intracellular organism. However, because of the risk of death as a result of infection with this organism, it is advisable to treat cats with a combination of antimicrobials, 12 at least until the outcomes of additional animals treated with single antimicrobials are reported.…”

“…Cutaneous infection with R equi is an emerging disease in cats, with cases reported from Canada, the United States, Australia, Brazil, New Zealand, and the United Kingdom. 3,[5][6][7][8][9][10][11] This organism is an aerobic, gram-positive, pleomorphic coccobaccillus. 4 It has simple nutritional requirements and can be found in soils where herbivore manure is present.…”

“…4 It has simple nutritional requirements and can be found in soils where herbivore manure is present. 4 It is most commonly associated with bronchial and gastrointestinal tract lesions in young foals, 4,6 and the number of reports 4,7,9 of infections in immunosuppressed humans with HIV is increasing. Immune status should be considered if R equi infection is suspected; however, similar to the present case, all cats (including 1 that died as a result of the infection 3 ) described in previous reports 3,[7][8][9] were negative for FeLV antigen and anti-FIV antibodies.…”

“…4 It is most commonly associated with bronchial and gastrointestinal tract lesions in young foals, 4,6 and the number of reports 4,7,9 of infections in immunosuppressed humans with HIV is increasing. Immune status should be considered if R equi infection is suspected; however, similar to the present case, all cats (including 1 that died as a result of the infection 3 ) described in previous reports 3,[7][8][9] were negative for FeLV antigen and anti-FIV antibodies. Although living conditions were not reported routinely, some of the reported cases involved cats that were in contact with areas where horses resided.…”