Feline Mammary Hyperplasia Study Guide

Overview and Clinical Importance

Feline mammary hyperplasia (also known as fibroepithelial hyperplasia, fibroadenomatous hyperplasia, or mammary hypertrophy) is a benign, progesterone-dependent condition characterized by rapid, non-neoplastic proliferation of mammary ductal epithelium and stroma. This condition accounts for approximately 13-20% of all feline mammary masses, making it an important differential diagnosis on the NAVLE.

Understanding this condition is critical because over 80% of feline mammary masses are malignant. Differentiating benign hyperplasia from mammary carcinoma has significant implications for treatment planning and prognosis. The condition is highly responsive to appropriate medical therapy, making accurate diagnosis essential for optimal patient outcomes.

Etiology and Pathophysiology

Hormonal Mechanism

The primary driver of feline mammary hyperplasia is progesterone, which can be either endogenous (from ovarian corpora lutea during diestrus, pregnancy, or pseudopregnancy) or exogenous (from synthetic progestins such as megestrol acetate or medroxyprogesterone acetate).

Molecular Pathway

Progesterone binds to progesterone receptors (PR-A and PR-B isoforms) in mammary ductal epithelial cells. This triggers a cascade of events:

• Progesterone receptor activation: PR-positive cells in the mammary epithelium are stimulated
• Local growth hormone (GH) production: Progesterone induces local synthesis of GH within mammary tissue
• IGF-1 release: Growth hormone stimulates insulin-like growth factor-1 (IGF-1) production
• Proliferation: These growth factors drive rapid proliferation of both ductal epithelium and periductal stroma

Sources of Progesterone Stimulation

Board Tip - Memory Aid: "PROG" = Progesterone Rules Over Glands. Remember: PROGesterone causes PROliferation of Glands. When you see mammary enlargement in a young cat with recent estrus or progestin exposure, think PROG = Progesterone-mediated hyperplasia!

Signalment and Clinical Presentation

Typical Patient Profile

Clinical Signs

Primary Findings

• Mammary gland enlargement: Ranges from 1.5 to 18 cm in diameter; often affects multiple or all glands
• Bilateral symmetry: Paired glands often show similar degrees of enlargement (in contrast to neoplasia)
• Tissue consistency: Firm but turgid, with a regular edematous texture on palpation
• No milk production: Affected glands do not produce milk, even in pregnant animals
• Generally non-painful: Unless complicated by infection or ulceration

Secondary Complications

• Skin ulceration: Due to stretching and trauma; skin may appear moist, violet, or alopecic
• Mastitis: Secondary bacterial infection with pain, erythema, and purulent discharge
• Necrosis: Tissue death in severe cases, especially in axillary and inguinal glands
• Systemic illness: Lethargy, fever, anorexia in complicated cases

Diagnosis

Clinical Diagnosis

Diagnosis of feline mammary hyperplasia is primarily clinical, based on signalment, history, and physical examination findings. The combination of a young intact female cat with rapid-onset bilateral mammary enlargement following estrus or progestin administration is highly suggestive.

Key Historical Questions

• Reproductive status: Intact? Recent estrus? Pregnant?
• Medication history: Any progestin administration (oral, injectable)?
• Duration: How rapidly did the enlargement develop?
• Previous episodes: History of similar condition?

Differential Diagnosis

Diagnostic Testing

Cytology

Fine needle aspiration (FNA) can support the diagnosis but has limitations for differentiating hyperplasia from neoplasia.

Cytologic features of hyperplasia: Two distinct cell populations (epithelial cells and spindle-shaped mesenchymal cells) in abundant eosinophilic extracellular matrix. Epithelial cells are uniform with round nuclei, fine chromatin, and scant to moderate basophilic cytoplasm. Moderate anisocytosis and anisokaryosis may be present, with minimal criteria of malignancy.

Histopathology

Excisional biopsy provides definitive diagnosis and is recommended when clinical differentiation is uncertain.

Histopathologic features: Proliferation of mammary ducts with multiple layers of normal epithelial cells (1-2 layers of cuboidal cells with basophilic cytoplasm and small round nuclei). Prominent periductal stroma is loose, myxoid, and edematous. No inflammatory cells or necrosis in uncomplicated cases. Rare mitotic figures. Ductal structures may form pseudo-acinar or cystic patterns.

Ultrasonography

Mammary ultrasound findings: Well-circumscribed solid mass with granular, slightly hyperechoic texture and regularly delimited margins. Small cleft-like anechoic structures may be present (representing ductal dilation). Homogeneous appearance without evidence of malignancy. Two patterns described: solid type (scant fluid) and intraductal type (with fluid-filled spaces).

Laboratory Testing

• CBC and chemistry: Usually normal unless secondary infection present
• Serum progesterone: Not a sensitive diagnostic indicator; blood levels may be within normal range despite tissue hypersensitivity
• Thoracic radiographs: Recommended to rule out metastasis if neoplasia cannot be excluded

Treatment

The therapeutic approach focuses on removing the source of progesterone stimulation. Treatment options should be discussed with the owner based on breeding intent, severity of disease, and presence of complications.

Treatment Options Summary

Aglepristone Treatment Protocols

Aglepristone (Alizin, Virbac) is a progesterone receptor antagonist that competitively binds to progesterone receptors without activating them, blocking the action of natural and synthetic progesterone.

Treatment response: Reduction in mammary gland size is typically visible within 6-7 days of initiating treatment. Complete clinical remission is achieved in an average of 3-4 weeks (range: 3-6 weeks). Longer treatment duration may be needed for cats previously treated with depot progestins.

Supportive Care for Complicated Cases

• Antibiotics: Amoxicillin-clavulanate (12.5-25 mg/kg PO BID) for secondary infection or ulceration
• NSAIDs: Meloxicam (0.1 mg/kg PO q24h after initial 0.2 mg/kg) for inflammation and pain
• Wound care: Topical chlorhexidine, antibiotic ointments for ulcerated areas
• E-collar: To prevent self-trauma and excessive grooming
• Adjuvant therapy: Aloe vera gel for topical massage; Hypericum and neem extract for ulcer healing

Board Tip - Memory Aid: "ALIZIN BLOCKS" = Aglepristone Liberates Inflamed Zones by Inhibiting Normal progesterone Binding, Leading to Obvious Clinical Kidney (OK, it's mammary!) Shrinkage. Remember the key points: Aglepristone = progesterone receptor ANTAGONIST, 10 mg/kg SC, Days 1-2-7 protocol, response in ~1 week, full resolution in 3-4 weeks.

Prognosis and Follow-Up

Prognosis

The prognosis for feline mammary hyperplasia is excellent with appropriate treatment. Key prognostic points:

• Complete remission is expected in 94-100% of cases with appropriate treatment
• Spontaneous regression may occur without treatment (takes weeks to months)
• Cats treated with exogenous progestins may have longer recovery and higher recurrence risk
• Fertility is preserved when treated with aglepristone alone (without OVH)
• Post-treatment breeding: Queens successfully treated with aglepristone have produced healthy litters

Recurrence

Recurrence can occur if the cat is re-exposed to progesterone (subsequent estrus, pregnancy, or progestin administration). Cats with a history of mammary hyperplasia should be advised against future breeding due to high recurrence risk during subsequent pregnancies.

Special Considerations

Pregnant Cats

When mammary hyperplasia develops during pregnancy, treatment decisions must balance fetal viability with maternal health. Aglepristone will cause pregnancy termination if administered. For valuable breeding animals, conservative management (supportive care, monitoring for complications) until progesterone naturally declines at parturition may be considered, though kittens may need to be hand-reared if nursing is not possible.

Male Cats

Mammary hyperplasia in male cats is rare but documented. It typically occurs following progestin administration for urine spraying or skin conditions. In males with no known progestin exposure, accidental environmental contact or rarely, functional testicular tumors (Sertoli cell) should be considered. Treatment with aglepristone is effective.