Blastomycosis in a cat presenting as suspected mammary carcinoma
Dominique Comeau,
Animal Health Laboratory, University of Guelph, Guelph, ON
AHL Newsletter 2024;28(3):26.
A 12-year-old female spayed cat was presented to her veterinarian for swelling and ulcerated lesions on her abdomen following a small laceration which occurred while in the garden. There were inflamed and excoriated growths noted on both sides of the caudal mammary gland that reduced in size with cleaning and treatment, but did not fully regress. Due to this lack of response, the top differential was inflamed mammary adenocarcinoma, and bilateral caudal mastectomy was performed. The tissue was submitted to the Animal Health Laboratory for histologic assessment.
On examination, the mammary tissue and associated lymph nodes were extensively effaced by coalescing nodules composed of abundant neutrophils, macrophages, and eosinophils surrounding large areas of necrosis that were in turn surrounded by lymphocytes, fibrous tissue, and plasma cells (Fig 1). There were frequent multinucleated giant cells in these areas. Within the foci of necrosis and inflammation there were numerous spherical, approximately 7 um to 15 um diameter organisms with a thick, double-contoured capsule which very rarely had evidence of broad-based budding (Fig 2). This histologic appearance is classic for the fungal infection blastomycosis.
Blastomycosis is a common fungal infection in dogs, and is rarely described in cats. The fungus is thermally dimorphic, growing as a mold in the cooler soil environment, and growing as a yeast in human or animal tissue. It is reported worldwide, and cases have been reported across Ontario. Endemic locations generally have moist, acidic soil which is the presumed reservoir for this agent. The noted laceration which occurred on the abdomen of the above-described cat while in the owner’s garden is the most likely source of inoculation for the fungus. Gardening is a described risk factor for human blastomycosis cases due to disruption of the soil and possible inhalation of fungal spores or inoculation into the skin. Clinical presentations vary from respiratory or cutaneous infections to systemic disease. In the above-described case, only cutaneous lesions were noted, and no respiratory or systemic disease was described in the reported history. In previous studies of cats with blastomycosis, animals without respiratory signs often had radiographic evidence of respiratory disease. Therefore, systemic treatment is often started regardless of presentation.
Compared to blastomycosis, mammary neoplasia is very common in cats; it is the third most common tumor type in female cats after lymphoma and skin tumors. Mammary tumors are reported to represent 17 % of feline tumors, and the prevalence of malignancy is very high, with up to 90 % of these tumors reported to be malignant. Tissue necrosis and inflammation is common in these masses, which was mimicked by the fungal infection found in this case. While rare, blastomycosis should be considered as a differential diagnosis for masses in the mammary chain of cats. AHL
Figure 1: Low magnification histologic view of the mammary tissue showing extensive effacement by pyogranulomatous inflammation. The skin surface is at the upper left. H&E stain.
Figure 2: High magnification histologic view of the mammary tissue showing fungal organisms (inside black outlines) in a background of severe pyogranulomatous inflammation. One of the fungi is undergoing budding (black rectangle). H&E stain.
References
1. Zygmunt A, Kus J, La Delfa A. Blastomycosis in Ontario: Public Health and Clinical Considerations. Public Health Ontario. Webinar presentation 2022-08-22. Slides accessed 2024-08-08 from www.publichealthontario.ca.
2. Gilor C, et al. Clinical aspects of natural infection with Blastomyces dermatitidis in cats: 8 cases (1991-2005). JAVMA. 2006; 299(1):96–99.
3.Morris J. Mammary tumours in the cat: Size matters, so early intervention saves lives. J Fel Med and Surg. 2013;15(5):391–400.