History

A 14-year-old 568-kg Quarter Horse mare from New York was presented to the Cornell University Equine Hospital for unilateral right mammary gland enlargement at the end of fall 2018. During the previous fall, the mare and her filly were rescued from a kill pen. During quarantine, she was treated for enterocolitis, typhlitis, equine influenza, and secondary bacterial pneumonia. She was also diagnosed to be at approximately 60 days of gestation. After the mare stopped nursing her filly, a unilateral enlarged mammary gland was noted. A diagnosis of unilateral mastitis was made on the basis of bacteriological culture of Pseudomonas aeruginosa and Staphylococcus spp from mammary secretions taken from the affected gland. The mare was treated with trimethoprim/sulfamethoxazole (30 mg/kg) for a month on the basis of an antibiotic susceptibility panel with apparent clinical improvement, but the unilateral mammary gland enlargement did not resolve.

The mare presented approximately 200 days following the acute mastitis episode. The only abnormality detected on physical examination was enlargement and thickening of the right mammary gland (Figure 1). No signs of inflammation such as redness, heat, or pain were appreciated. A small amount of serous fluid could be expressed, and it was submitted for culture and sensitivity, which yielded no bacterial growth. Transcutaneous ultrasound examination of the right mammary gland demonstrated thickened mammary tissue with a generalized mild heterogeneous hyperechogenicity compared to the left side. Transrectal exam revealed an advanced pregnancy consistent with the history of 270 days of gestation. A CBC, chemistry panel, and serum amyloid A test were consistent with a mild chronic inflammatory process.

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Caudocranial image of the mammary glands of a 14-year-old 568-kg Quarter Horse mare evaluated for unilateral mammary gland enlargement. There is evident enlargement of the right mammary gland when compared to the left.

Citation: Journal of the American Veterinary Medical Association 261, 12; 10.2460/javma.23.08.0459

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Diagnosis

Punch biopsy was chosen due to the thicker skin of the mammary gland, and 2 representative biopsy samples were submitted for histopathology assessment by a board-certified veterinary pathologist (GED). Both biopsies consisted of abundant dense fibrous connective tissue and spindle cells with indistinct borders, variable amounts of fibrillar cytoplasm, small fusiform nuclei, and a heterochromatic chromatin pattern (Figure 2). Mitotic figures or other evidence of malignancy were not seen. The epidermis was hyperplastic, and a mild, mixed mononuclear inflammatory cell infiltrate suggestive of chronic local inflammation was noted within the superficial dermis. The final diagnosis was benign mammary fibroma.

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Photomicrograph of tissue sections obtained from a punch biopsy sample of the right mammary gland of the mare in Figure 1. Extensive dense fibrous connective tissue is evident. H&E stain; bar = 200 µm.

Citation: Journal of the American Veterinary Medical Association 261, 12; 10.2460/javma.23.08.0459

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Treatment and Outcome

Due to the benign nature of the mammary mass and the pregnancy status of the mare, surgical removal was not advised and the mare was discharged, and the owner was instructed to monitor lactation from the affected gland once the mare foaled. The mare foaled a healthy full-term colt with no complications, and lactation proceeded normally from the unaffected mammary gland. Approximately 24 hours following parturition, the owner noted that the mare started lactating from the previously affected mammary gland.

Discussion

Identifying the cause of mammary enlargement in the mare is often difficult. It is considered rare and often associated with an inflammatory condition (mastitis).¹ Mastitis can happen at any time in the mare’s life (dry, lactating, or weaning mares) and even in fillies.² Given that this is a rare and benign lesion, clinical differentiation from other conditions affecting the mammary gland is relevant.³ Descriptive information regarding mammary gland conditions is scarce, with most reports describing malignant neoplasms⁴ and mammary carcinoma being the most commonly diagnosed. This case report is unique, as the histopathology revealed a benign tumor: mammary gland fibroma. Only a few cases of adenoma have been previously reported in the horse as a benign mammary neoplasm.³ It is theorized that mastitis can lead to loss of functionality of the mammary gland due to fibrosis or obstruction,^1,2 but a relationship to fibroma development has not been proposed. In humans, fibromatosis (mammary fibroma) accounts for 0.2% of breast tumors and is considered locally aggressive; however, due to its lack of metastatic potential, it is classified as a benign process.⁵ Current research on human fibromatosis suggests the cellular origin of this tumor to be from fibroblasts and/or myofibroblasts.⁵ Since diagnosing this case in our hospital, a mammary gland adenoma with similar histomorphological features was diagnosed from an excisional biopsy specimen obtained from a 14-year-old Warmblood mare in Florida, suggesting that this condition may be more common than previously reported.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The authors have nothing to disclose.