NAVLE Reproductive

Canine Mastitis Study Guide

📅 March 28, 2026 ⏱ 25 min read

Overview and Clinical Importance

Mastitis is inflammation of the mammary gland, most commonly caused by bacterial infection. It predominantly affects postpartum lactating bitches but can also occur in non-pregnant dogs with galactorrhea (pseudopregnancy). The condition ranges from subclinical with minimal signs to severe gangrenous mastitis that can progress to septic shock and death. With a reported prevalence of approximately 0.71% to 13.2% in postpartum bitches, mastitis represents an important reproductive emergency requiring prompt recognition and treatment.

The condition threatens both the dam and her nursing puppies, making it a critical topic for the NAVLE examination. Understanding the pathophysiology, clinical presentation, and treatment options is essential for successful management of this common condition.

Mammary Gland	Primary Drainage	Clinical Significance
M1, M2	Axillary lymph nodes	Infection may spread cranially
M3	Axillary AND/OR inguinal	Variable drainage pattern
M4, M5	Superficial inguinal lymph nodes	Most commonly affected glands

Canine Mammary Gland Anatomy

Dogs typically possess five pairs (10 total) of mammary glands arranged in two parallel chains extending from the thorax to the inguinal region. The glands are designated as:

M1 and M2: Thoracic mammary glands (cranial and caudal thoracic)
M3 and M4: Abdominal mammary glands (cranial and caudal abdominal)
M5: Inguinal mammary gland

The caudal glands (M4 and M5) are larger and more commonly affected by both mastitis and mammary neoplasia. Each nipple contains 6-16 papillary duct orifices that serve as potential entry points for bacterial pathogens.

Lymphatic Drainage (Critical for Metastasis Understanding)

High-YieldThe lymphatic drainage pattern explains why infection can spread between adjacent glands. M1, M2, and M3 have lymphatic communications with each other, while M4 only communicates with M5. There are NO direct connections between left and right mammary chains.

Organism	Characteristics	Clinical Notes
Staphylococcus aureus	Gram-positive cocci; coagulase-positive	MOST COMMON isolate; produces alpha-hemolysin causing tissue necrosis
Escherichia coli	Gram-negative rod; coliform	Environmental; endotoxin causes severe systemic signs
Streptococcus spp.	Gram-positive cocci in chains	Normal skin flora; opportunistic
Staphylococcus intermedius	Gram-positive cocci; coagulase-positive	Common cause of subclinical mastitis

Etiology and Pathophysiology

Causative Organisms

Mastitis is primarily caused by ascending bacterial infection through the teat canal, though hematogenous and lymphogenous spread can also occur. The most commonly isolated pathogens include:

Routes of Infection

Ascending infection (most common): Bacteria enter through the teat orifice, especially with teat trauma from nursing puppies' nails or teeth
Hematogenous spread: Secondary to systemic infection (e.g., metritis, pyometra)
Lymphogenous spread: Extension from adjacent infected glands

Risk Factors

Milk stasis (galactostasis): Small litter size, sudden weaning, puppy death
Teat trauma: Sharp puppy nails, rough nursing, environmental injuries
Poor environmental hygiene: Dirty whelping boxes, infrequent bedding changes
Pseudopregnancy: Galactorrhea without puppies to nurse
Immunocompromise: Concurrent illness, poor nutrition, stress
Large litter size: Increased demand and teat trauma

NAVLE TipGangrenous mastitis is primarily caused by Staphylococcus aureus producing alpha-hemolysin. This toxin binds to ADAM10 (a transmembrane protein), forming pores in cell membranes that cause intracellular calcium influx, leading to severe edema, necrosis, and gangrene. Remember: S. aureus = most common pathogen = gangrenous potential!

Type	Description	Key Features
Acute Septic	Bacterial infection with systemic signs	Fever, anorexia, painful hot glands, purulent milk
Chronic/Subclinical	Low-grade infection with minimal signs	Puppies fail to thrive; milk may appear normal
Galactostasis	Non-septic milk accumulation	Firm, uncomfortable glands; dam relatively healthy
Gangrenous	Severe infection with tissue necrosis	Black/purple glands, septic shock, EMERGENCY

Classification of Mastitis

Test	Expected Findings	Clinical Utility
Milk Cytology	Increased degenerative neutrophils; may see intracellular bacteria	Rapid, confirmatory; differentiates septic from non-septic
Milk Culture and Sensitivity	Identifies pathogen and antibiotic susceptibility	Guides targeted antibiotic therapy; submit even if milk appears normal
CBC	Leukocytosis with left shift; may see neutropenia with sepsis	Assesses severity; monitors response to therapy
Biochemistry Panel	Hypoglycemia in sepsis; assess organ function	Evaluates systemic effects and concurrent disease
Mammary Ultrasound	Reduced tissue differentiation; decreased blood flow; abscess detection	Identifies abscesses; guides drainage procedures
Blood Culture	Positive if septicemic	For severely ill patients; obtain before starting antibiotics

Clinical Signs and Presentation

Local Signs (Affected Mammary Gland)

Swelling, warmth, and firmness of affected gland(s)
Pain on palpation (dam may refuse nursing)
Erythema progressing to purple/black discoloration (gangrenous)
Abnormal milk: blood-tinged, purulent, or thick/clotted
Ulceration, skin breakdown, or abscess formation

Systemic Signs

Fever (pyrexia) or hypothermia (septic shock)
Anorexia and lethargy
Dehydration
Vomiting (indicates bacterial toxin absorption)
Maternal neglect of puppies
Tachycardia, tachypnea, weak pulses (septic shock)

Signs in Nursing Puppies

Failure to thrive: Weight gain less than 5% of birth weight per day
Restlessness, excessive crying
Bloating (from ingesting abnormal milk)
Diarrhea

High-YieldSubclinical mastitis may present ONLY with puppies failing to thrive! Always examine the dam's mammary glands and weigh puppies daily (they should gain 5-10% of birth weight daily). This is a favorite NAVLE question trap.

Antibiotic	Dosage	Route/Frequency	Notes
Amoxicillin-Clavulanate	14 mg/kg	PO every 8-12 hours	First-line empirical choice; broad spectrum; beta-lactamase resistant
Cephalexin	15-30 mg/kg	PO every 8 hours	First-line empirical choice; excellent for staphylococcal infections
Amoxicillin	20 mg/kg	PO, IM, or SC every 12 hours	Less effective against beta-lactamase producing bacteria
Enrofloxacin	5-10 mg/kg	PO every 24 hours	Reserve for culture-guided therapy; avoid in growing puppies due to cartilage damage

Diagnosis

Physical Examination

Diagnosis is often made on physical examination alone. Carefully palpate all mammary glands, comparing size, temperature, consistency, and pain response. Express milk from each gland and evaluate color, consistency, and odor.

Diagnostic Tests

Differential Diagnoses

Inflammatory mammary carcinoma: Rapidly growing, warm, erythematous mass; typically older, intact females
Mammary hyperplasia: Non-painful enlargement associated with hormonal stimulation
Mammary gland tumor: Usually discrete masses; may become secondarily infected
Galactostasis: Milk accumulation without infection; dam remains healthy

NAVLE TipAlways differentiate inflammatory mammary carcinoma from acute mastitis! Key differences: Inflammatory carcinoma occurs in older non-lactating dogs, involves multiple glands with dramatic edema, and has a poor prognosis. Mastitis typically affects lactating or pseudopregnant females with localized gland involvement.

Scenario	Recommendation
Mild/Moderate mastitis	Continue nursing from affected glands if dam permits; helps clear infection through drainage
Dam on antibiotics	Generally safe to continue nursing; consider probiotics for puppies to prevent antibiotic-associated diarrhea
Gangrenous/Abscessed glands	DO NOT allow nursing from affected glands; wean puppies or supplement with milk replacer
Dam severely ill	Temporarily separate puppies; hand-rear until dam recovers

Treatment

Antibiotic Therapy

Begin broad-spectrum antibiotics immediately while awaiting culture results. Treatment duration is typically 2-3 weeks. Consider effects on nursing puppies when selecting antibiotics.

Supportive Care

Local Treatment

Warm compresses: Apply to affected glands every 2-4 hours to encourage drainage
Cold compresses: Alternative for pain relief; use between warm applications
Cabbage leaf therapy: Secure leaves to affected glands for 2-4 hours; shown to reduce inflammation
Hand milking: Strip affected glands every 6 hours to promote drainage and blood flow

Pain Management

Use analgesics cautiously due to transfer through milk. NSAIDs (meloxicam, carprofen) are generally considered safe but use with caution. Opioids (buprenorphine) may be used for severe pain in hospitalized patients.

Severe/Septic Cases

Hospitalization for IV fluid therapy
Injectable antibiotics (ampicillin-sulbactam, cefazolin)
Blood glucose monitoring and dextrose supplementation if hypoglycemic
Vasopressors if septic shock present

Surgical Intervention

Surgery is indicated for abscess formation or gangrenous mastitis. Options include:

Surgical drainage and debridement: For localized abscesses
Mastectomy (partial or complete): For gangrenous tissue or non-responsive cases
Negative pressure wound therapy: Post-debridement wound management

Management of Nursing Puppies

High-YieldContrary to common belief, research shows that the primary cause of neonatal septicemia is NOT bacteria in the mother's mastitic milk. Continuing nursing (when appropriate) actually helps clear the infection by promoting milk drainage. Only withhold puppies from gangrenous or severely abscessed glands.

Type	Prognosis	Expected Recovery
Acute/Chronic mastitis	GOOD	Clinical signs resolve within 2-3 weeks with appropriate treatment
Galactostasis	EXCELLENT	Self-resolving once hormone levels normalize
Gangrenous/Septic	GUARDED to POOR	May lose functional use of gland; potential for systemic sepsis and death without aggressive treatment

Prognosis

Prevention

Maintain clean whelping environment: Change bedding 1-2 times daily; scrub whelping box regularly
Trim puppy nails: Keep short and smooth to prevent teat trauma
Monitor nursing behavior: Ensure all glands are being nursed; rotate puppies if needed
Weigh puppies daily: Early detection of failure to thrive
Gradual weaning: Avoid abrupt cessation of nursing which leads to milk stasis
Examine mammary glands daily: Early detection of swelling, heat, or pain
Remove sharp objects: Eliminate potential sources of teat injury from whelping area

Memory Aids

Mnemonic: "STAPH MASTER" for Mastitis Management

S - Staphylococcus aureus is the #1 pathogen

T - Temperature check (fever indicates systemic involvement)

A - Antibiotics: Cephalexin or Amoxicillin-Clavulanate

P - Palpate ALL glands, not just obviously affected ones

H - Hot compresses promote drainage

M - Milk cytology for rapid diagnosis

A - Allow nursing (except from gangrenous glands)

S - Surgery for abscesses and gangrene

T - Two to three weeks of antibiotic therapy

E - Examine puppies for failure to thrive

R - Recognize gangrenous mastitis as EMERGENCY (purple/black glands)

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