Mastitis is defined as inflammation of the mammary gland, most commonly caused by bacterial infection. While mastitis is a major disease in dairy cattle, it occurs relatively infrequently in mares compared to other domestic species.
Overview and Clinical Importance
Mastitis is defined as inflammation of the mammary gland, most commonly caused by bacterial infection. While mastitis is a major disease in dairy cattle, it occurs relatively infrequently in mares compared to other domestic species. The seemingly reduced incidence in mares can be partially explained by the smaller size and relatively concealed location of the mare's udder, coupled with a smaller storage capacity than cows and goats. Mastitis can affect lactating, peripartum, dry mares, mares at dry-off, or rarely prepubertal foals.
Understanding equine mastitis is essential for the NAVLE as it represents an important differential diagnosis for udder enlargement and can have significant impacts on both mare and foal health, including failure of passive transfer in neonates.
| Structure |
Description |
| Location |
Inguinal region, relatively concealed position between hind legs |
| Number of Mammae |
Two (one pair) |
| Teat Orifices |
Two per teat (each draining independent ductal tree) |
| Storage Capacity |
Smaller than cows and goats, leading to frequent emptying |
| Blood Supply |
External pudendal artery (via deep femoral artery from external iliac) |
| Lymph Drainage |
Superficial inguinal (mammary) lymph nodes |
Equine Mammary Gland Anatomy
The equine udder comprises one pair of mammae (two mammary glands), each with a teat. Each mamma is drained by two independent mammary ductal trees, unlike the cow which has four quarters each drained by one ductal system. Thus, each equine teat typically has two orifices through which the main ducts discharge.
Key Anatomical Features
High-YieldThe equine mammary gland has terminal duct lobular units (TDLUs) similar to the human breast, unlike rodents which have adipocyte-rich stroma. Each teat has two mammolobular-pilo-sebaceous units (MPSUs), each comprising a galactophorous duct, mammary hair, and sebaceous gland.
Why Is Mastitis Less Common in Mares?
| Factor Category |
Contributing Factors |
| Anatomical |
Smaller udder size
Concealed location between hind legs
Teats less prone to trauma and contamination |
| Physiological |
Small storage capacity leads to frequent emptying by foal
Shorter lactation period than dairy cattle
Possible endocrine or local immunity differences |
| Husbandry |
Mares rarely milked by hand or machine (reduced trauma)
Natural mare-foal nursing behavior
Possible under-reporting of subclinical cases |
Etiology and Pathogenesis
Bacterial Pathogens
Bacteria are the most commonly identified etiological agents. In a study of 28 mares with mastitis, aerobic bacteria were cultured from 71% of samples. Streptococcus zooepidemicus was the most common isolate (37%), with gram-negative species accounting for 42%.
NAVLE TipS. zooepidemicus is the MOST COMMON cause of equine mastitis. Remember: 'S. zoo = #1 in the Mare Zoo!' This beta-hemolytic Lancefield Group C streptococcus is also the most common bacterial pathogen in equine respiratory disease and can be zoonotic.
Pathogenesis and Risk Factors
Route of Infection
In lactating mares, bacteria typically enter via the teat canal (ascending infection). In non-lactating mares, infection may occur through the teat orifice secondary to trauma or insect feeding.
When Does Mastitis Occur?
High-YieldMastitis most commonly occurs during WEANING (summer/autumn months) when milk accumulates in the udder. Remember: 'Weaning = Warning for Mastitis!'
| Gram-Positive (more than 50%) |
Gram-Negative (approximately 42%) |
Other Causes |
| Streptococcus zooepidemicus (most common, 37%)
Streptococcus agalactiae
Staphylococcus spp. (15%)
Corynebacterium spp. |
Escherichia coli
Klebsiella pneumoniae
Pseudomonas aeruginosa
Actinobacillus suis (15%)
Enterobacter aerogenes |
Fungal: Coccidioides immitis, Blastomyces dermatitidis
Parasitic: Halicephalobus deletrix
Toxic: Avocado (Persea americana) |
Clinical Signs and Diagnosis
Clinical Presentation
The majority of mares present with unilateral disease, and sometimes only one ductal tree within a mamma is affected. Mastitis can be classified as acute or chronic, and clinical or subclinical.
Diagnostic Approach
Physical Examination Findings
- Asymmetric udder (one side larger than other)
- Heat, swelling, and pain on palpation
- Abnormal milk appearance (yellow-green, blood-tinged, or watery with flocculent material)
- Regional lymphadenopathy (superficial inguinal lymph nodes)
Laboratory Diagnostics
NAVLE TipA negative cytology does NOT rule out mastitis. An inflammatory cytological picture is useful when culture is negative. Always culture AND perform cytology for comprehensive diagnosis.
Differential Diagnosis for Udder Enlargement
High-YieldMammary NEOPLASIA can present identically to mastitis! Misdiagnosis as mastitis is a common cause of delayed cancer diagnosis. If an 'infection' doesn't respond to treatment, or if ULCERATION is present, pursue biopsy to rule out carcinoma.
| Timing |
Frequency |
Mechanism |
| During lactation |
44% |
Bacteria enter via teat canal; poor nursing leads to milk accumulation |
| Post-weaning (within 8 weeks) |
28% |
Milk accumulation increases intramammary pressure; dripping milk provides entry point |
| Dry/non-lactating (more than 1 year) |
28% |
Trauma to teat or udder; insect feeding; opportunistic infection |
Treatment
Treatment of equine mastitis involves antimicrobial therapy (systemic and/or local), anti-inflammatory drugs, and supportive care. Response to therapy is often rapid, and unlike cows, most mares do not suffer permanent fibrosis or loss of mammary function.
Antimicrobial Selection
Antimicrobial susceptibility testing showed that trimethoprim-sulfonamide would be active against more than 75% of isolates, while penicillin alone would be active against less than 60%. For broad-spectrum coverage, combine penicillin with an aminoglycoside.
Treatment Duration: At least 5 days minimum; continue based on culture/sensitivity results and clinical response.
Supportive Care
High-YieldWithout prompt treatment, abscessation or induration (fibrosis) of the gland can occur, leading to agalactia in affected quarters.
Complications and Prognosis
- Abscess formation: May occur with Corynebacterium, Streptococcus, or Staphylococcus infections
- Fibrosis/Induration: Can lead to permanent agalactia in affected quarter(s)
- Botryomycosis: Distinctive granulomatous response to Staphylococcus spp.
- Foal effects: Failure of passive transfer; sepsis if nursing from infected gland
Prognosis: Generally GOOD with prompt treatment. Response to therapy is often rapid, and unlike cows, most mares do not suffer permanent fibrosis or loss of mammary function. Complete resolution typically within one week.
| Common Signs (Local) |
Less Common/Systemic Signs |
| Swollen mammary gland (hot, firm, painful)
Abnormal mammary secretion (thick, discolored, blood-tinged, seroflocculent)
Pain on palpation (may react violently)
Ventral/preudder edema
Reluctance to allow foal to nurse |
Fever (greater than 38°C in 50% of cases)
Depression and anorexia
Hindlimb lameness (ipsilateral)
Stiff gait or standing with hind legs apart
Swollen mammary vein |
Prevention
Weaning Management (Critical Period)
NAVLE TipNEVER milk the mare after weaning! This stimulates continued milk production and delays involution. Remember: 'Milk accumulation initiates involution' - the pressure from accumulated milk signals the gland to regress.
General Prevention Strategies
- Udder hygiene: Regular cleaning to prevent accumulation of dirt, sweat, and sebaceous secretions
- Insect control: Reduce fly populations; insects may act as vectors or cause teat trauma
- Trauma prevention: Minimize risk of teat injuries from environment or other horses
- Nursing support: If foal unable to nurse effectively, milk mare frequently to prevent accumulation
- Foal health monitoring: Death or illness of suckling foal = increased mastitis risk
| Test |
Expected Findings |
| Milk Cytology |
Myriad viable or degenerate neutrophils (72%), necrotic debris. Bacteria visible in approximately 33% of cases. |
| Aerobic Culture and Sensitivity |
Positive in 71% of cases. ESSENTIAL for guiding antimicrobial therapy. |
| CBC |
Neutrophilia (in systemic cases) |
| Fibrinogen |
Hyperfibrinogenemia (indicates inflammatory response) |
| Somatic Cell Count (SCC) |
Elevated (research tool; increased SCC and PMN percentage indicate inflammation) |
Memory Aids for NAVLE
MASTITIS = M.A.S.T.I.T.I.S.
- Mammary swelling (hot, painful, usually unilateral)
- Abnormal secretion (thick, discolored, flocculent)
- Streptococcus zooepidemicus (#1 cause)
- Trimethoprim-sulfa (first-line antibiotic, greater than 75% effective)
- Involution impaired by milking post-weaning (DON'T DO IT!)
- Two teat orifices per teat (treat both with intramammary antibiotics)
- Incidence peaks at WEANING (summer/autumn)
- Stripping udder (frequent milking) as supportive therapy
| Condition |
Distinguishing Features |
| Mastitis |
Hot, painful, abnormal secretion, often unilateral, neutrophilic cytology |
| Mammary Neoplasia |
Often painless mass; may have ulceration (suspicious for neoplasia); malignant cells on cytology/biopsy. SIGNIFICANT clinical overlap with mastitis. |
| Physiologic Udder Development |
Normal late pregnancy; bilateral, non-painful; 'waxing' of teats near parturition |
| Ventral Edema |
May extend to udder; associated with hypoproteinemia, viral disease (EIA, EVA), or dependent edema |
| Mammary Abscess |
Fluctuant mass; may follow mastitis. Consider C. pseudotuberculosis in endemic areas (pigeon fever). |
| Drug |
Dose |
Route/Frequency |
Notes |
| Trimethoprim-Sulfonamide |
30 mg/kg |
PO q12h |
First-line choice; effective against more than 75% of isolates; broad-spectrum G+ and G- |
| Procaine Penicillin G |
20,000-25,000 IU/kg |
IM q12h |
Effective for G+ organisms; often combined with gentamicin for broader spectrum |
| Gentamicin Sulfate |
6.6-9.7 mg/kg |
IV q24h |
For G- coverage; monitor renal function; avoid in dehydrated patients |
| Intramammary Antibiotics |
Bovine preparations |
Per label |
Insert into BOTH orifices of affected teat |
| Intervention |
Details and Rationale |
| Frequent Milking (Stripping) |
Remove secretions as often as possible if mare tolerates; reduces bacterial load and removes inflammatory debris |
| Cold Hosing/Hydrotherapy |
Reduces swelling and provides pain relief; apply for 15-20 minutes several times daily |
| Hot Packing/Massage |
Can alternate with cold therapy; helps promote drainage |
| NSAIDs |
Flunixin meglumine (1.1 mg/kg IV/PO q12-24h) or phenylbutazone (2.2-4.4 mg/kg PO q12h) for pain, fever, and inflammation |
| Foal Management |
If mare refuses nursing, provide supplemental feeding to foal; monitor for failure of passive transfer in neonates |
| DO |
DO NOT |
| Reduce mare's dry matter intake at weaning to decrease milk production
Provide creep feed to foals BEFORE weaning (encourages natural weaning)
Monitor udder frequently after weaning
Allow gradual involution of mammary gland |
DO NOT milk the mare after weaning (prolongs involution and milk production)
DO NOT overfeed mare during weaning period
DO NOT ignore udder engorgement or discomfort |