Mycotic Mastitis in Three Dogs Due to Blastomyces dermatitidis

Introduction

Blastomycosis is a systemic fungal infection due to Blastomyces dermatitidis and is most commonly diagnosed in humans and dogs.^1–5 It is a dimorphic fungus with both a yeast and mycelial form that is endemic in much of the central and southeastern United States, with increased incidence in the Ohio, Missouri, and Mississippi River valleys.^1–5 The mycelial form is present in the environment, but upon inhalation of the conidia, the temperature increase in the host converts the spore into the spherical yeast form.^1–5 As a result, primary mycotic pneumonia is the most common presentation, although secondary mycotic dermatitis is frequent.^1–5 Young, large breed, sporting dogs with access to a body of water are at higher risk for infection, but due to the high environmental load in endemic areas, Blastomyces pneumonia can be seen in many species without breed, sex, or other predilections.^1–5 Mycotic mastitis due to B dermatitidis is uncommon, and mastitis or mammary masses as the presenting clinical sign is rare. This retrospective case series described three dogs over a 5 yr period from which mammary samples and lymph nodes were submitted for histopathology.

Case Report

Mammary tissues from three dogs were submitted to the Department of Pathobiology, University of Tennessee, College of Veterinary Medicine (UTCVM) for histopathology.

A 2 yr old, spayed female mixed breed (dog 1) presented to the referring veterinarian in June 2003 for multiple mammary nodules of a few weeks’ duration. The right first mammary gland and the right axillary lymph node were excised and submitted to UTCVM. Dog 2, a 4 yr old, spayed female pug, presented to the referring veterinarian in April 2008 for coughing and difficulty breathing. On physical examination, there were multiple, firm, enlarged mammary masses. Thoracic radiographs revealed enlarged tracheobronchial lymph nodes, and lymphosarcoma was suspected. The mammary masses and the left popliteal lymph node were excised and submitted to UTCVM. Dog 3, a 5 yr old, intact female boxer was adopted by the new owners in late December 2008, approximately 6 wk postpartum. Mammary glands were swollen at the time of adoption and increased in size over the following month and a half with recent onset of ulceration and bleeding. Per owner, a veterinarian treated the dog with antibiotics (cefalexin and ciprofloxacin hydrochloride, unknown dosages) approximately 2 wk after adoption with minimal to no response. Over the course of the next 3–4 wk, the mammary lesions progressed to multiple large draining abscesses, and the dog became dyspneic. Again, per owner, another veterinarian treated the dog with a glucocorticoid injection (triamcinolone, unknown dosage) with subsequent worsening of respiratory difficulty. The dog was relinquished shortly thereafter to the humane society and was presented to another veterinarian in February 2009 (approximately 6 wk after adoption) depressed, dyspneic, and anorexic with a temperature of 40.3°C and multiple ruptured mammary glands. A complete blood count was done, an intravenous catheter placed, and intravenous fluids started. There was a leukocytosis (21.25 K/μL; reference interval, 5.50–16.90) due to neutrophilia (18.05 K/μL; reference interval, 2.00–12.00). Thoracic radiographs were consistent with mycotic pneumonia, and the dog was euthanized. Necropsy was not performed, and mammary samples were harvested postmortem and submitted to UTCVM.

Tissue samples were fixed in 10% neutral buffered formalin, processed routinely, and stained with hematoxylin and eosin. Histopathologic examination of the skin, dermis, subcutis, and mammary gland from each dog revealed a marked multifocal pyogranulomatous inflammatory response that surrounded and partially effaced the mammary glands and surrounding subcutis (Figure 1). Inflammation was composed of numerous epithelioid macrophages and neutrophils mixed with fewer lymphocytes and plasma cells and occasional multinucleate giant cells (Figure 2). Rarely within granulomas of dog 1 and scattered throughout the inflammatory foci of dogs 2 and 3, were spherical, Gomori's methenamine silver-positive yeasts, up to 30 μm in diameter, with a double-contour wall, basophilic nucleus, and occasional broad-based budding consistent with Blastomyces dermatitidis (Figure 3). The granulomas of dog 2 were surrounded by a fibrous stroma. There were also multiple large lakes of proteinaceous fluid admixed with neutrophils and macrophages that were surrounded by a wall of epithelioid macrophages. Multiple intralesional vessels of dog 3 were occluded by fibrin thrombi with variable recanalization. Fibrin and hemorrhage were within the surrounding tissue. The epidermis of dog 3 also had regionally extensive ulceration with a thick layer of fibrin and necrotic material. The subcapsular and medullary sinuses of the lymph nodes from dogs 1 and 2 were multifocally expanded by hemorrhage, and the pyogranulomatous inflammation composed of variable numbers of macrophages had fewer eosinophils and neutrophils. Both the lymph node and the mammary glands of dog 2 had multiple areas of necrosis. The owners of both dogs 1 and 2 moved shortly after diagnosis, and the dogs were lost to follow up.

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Discussion

Dermatitis secondary to mycotic pneumonia or generalized blastomycosis has been reported in multiple species, including humans, dogs, cats, and horses.^1–5 Cutaneous inoculation of humans via accidental lacerations during necropsy, needle puncture, dog bite, or outdoor trauma have rarely been reported.^6–10 There is a single previous case report of naturally occurring Blastomyces mastitis in veterinary medicine, in a horse with widespread involvement of the skin of the ventrum and legs.¹¹ In a blastomycosis retrospective case series in dogs, four dogs (3%) had a mammary gland mass at physical examination, and three of those four also had multisystemic involvement.¹²

Unfortunately, none of the cases in this report were available for follow-up, and information was limited, particularly for dog 3. As B dermatitidis is endemic in this region, inhalational blastomycosis with systemic spread was most likely for dog 1. Although a radiographic miliary pattern is common with blastomycosis, lymphadenopathy without a radiographic fungal pattern, as in dog 2, can also be seen with blastomycosis.⁴ As such, dog 2 was likely to have had subclinical mycotic pneumonia with subsequent spread to the mammary glands. Similarly, for dog 3, given the pulmonary involvement, inhalational blastomycosis with spread to the mammary glands was likely, but environmental inoculation during lactation and nursing could not be ruled out.

In retrospect, a fine needle aspirate and cytology would likely have given a diagnosis of blastomycosis. This is particularly important with dogs that present in respiratory distress, as in dog 2, when the risks of anesthesia and surgery were increased. Although B dermatitidis is endemic in this region, other fungi, including Histoplasma capsulatum and Coccidioides immitis are differential diagnoses. The yeast form of B dermatitidis is approximately 30 μm in diameter with a thick, double contoured wall and granular basophilic nuclear material.^1–5 Blastomyces can be differentiated from other yeasts by size, nuclear morphology, and broad-based budding.^1–5

These cases were unusual in that the original presentation was for mastitis and/or mammary masses, and mycotic pneumonia was diagnosed secondarily. Blastomycosis should be considered in the differential diagnosis of mammary lesions in dogs from endemic areas.