Bit & Bridle

Equine Mastitis

Equine mastitis is inflammation of the mammary gland (udder) in mares, most commonly caused by bacterial infection. Although less frequently encountered in horses than in dairy cattle, equine mastitis can be a painful, debilitating condition that affects nursing mares, recently weaned mares, and occasionally non-lactating mares. Prompt diagnosis and appropriate antibiotic treatment are important for resolving infection, preventing abscess formation, and preserving the mare's ability to nurse future foals.

Other Names

  • Mammary gland infection in mares
  • Udder infection
  • Mammitis

What Is Equine Mastitis?

Mastitis is defined as inflammation of the mammary gland tissue. In mares, this typically involves one or both mammary glands (the equine udder has two glands, each with one teat). The condition is most commonly infectious in origin — bacteria gain entry to the gland through the teat canal — but trauma, teat obstruction, and other inflammatory processes can also be involved.

Equine mastitis can be classified by duration (acute, subacute, or chronic) and by whether it affects one gland (unilateral) or both (bilateral). Acute mastitis represents a sudden-onset, often severe infection, whilst chronic mastitis tends to be subtler, with gradual glandular changes that may only be detected on examination. Left untreated, mastitis can progress to abscessation, glandular fibrosis, and permanent loss of milk production in the affected gland.

Causes

The most common causative bacteria include:

  • Streptococcus equi subsp. zooepidemicus — the most frequently isolated organism
  • Staphylococcus aureus
  • Streptococcus dysgalactiae
  • Escherichia coli
  • Pseudomonas aeruginosa (less common but antibiotic-resistant)
  • Klebsiella pneumoniae

Risk factors for developing mastitis include:

  • Weaning: Sudden cessation of nursing causes milk accumulation and engorgement, creating conditions favourable for bacterial growth
  • Teat trauma or injury: Wounds or lacerations to the teat canal facilitate bacterial entry
  • Poor hygiene: Dirty stabling and paddock conditions increase bacterial challenge
  • Fly irritation: Flies around the udder can introduce bacteria and cause skin breakdown
  • Previous mastitis: Glandular scarring from previous episodes increases susceptibility
  • Tick infestation: Ticks attaching to the udder region can introduce infection
  • Systemic illness: Immunosuppression from another disease may allow haematogenous (blood-borne) spread of bacteria to the mammary gland

Symptoms and Signs

Acute mastitis is typically unmistakeable. Signs include:

  • Swelling of one or both mammary glands — the udder may be visibly enlarged, hot, and firm or fluctuant
  • Pain on palpation of the udder — the mare may kick, resist grooming, or show obvious discomfort when the area is touched
  • Abnormal milk: Discoloured (yellow, brown, or blood-tinged), watery, clotted, or foul-smelling milk
  • Stiff, wide-based gait — the mare walks with her hindlimbs held wide apart to avoid contacting the painful udder
  • Reluctance to allow the foal to nurse — kicking at the foal, moving away, or squeal-and-kick responses when the foal attempts to suckle
  • Systemic signs in severe cases: Fever, depression, anorexia, and elevated heart rate

Chronic mastitis may present more subtly with gradual hardening (induration) of one gland, reduced milk production on the affected side, or a mare that is intermittently off-colour. The foal may be thinner than expected or nurse excessively from the unaffected side.

Diagnosis

Diagnosis is based on clinical examination combined with milk sampling:

  • Physical examination: Palpation of the udder to assess heat, pain, swelling, and consistency of each gland. The overlying skin and teats should be inspected for wounds or teat canal discharge.
  • Milk examination: Visual assessment — milk from an infected gland is often discoloured, watery, or contains clots. A California Mastitis Test (CMT), whilst developed for cattle, can be used in mares as a rapid on-farm screening tool for elevated cell counts.
  • Milk culture and sensitivity: This is the most important diagnostic step. A milk sample is submitted to the laboratory to identify the causative organism and determine which antibiotics it is sensitive to. This guides targeted antibiotic selection and avoids treatment failure from resistance.
  • Cytology: Microscopic examination of milk for inflammatory cells (somatic cell count) confirms the presence of mastitis
  • Ultrasound: Useful to assess the degree of glandular involvement, detect abscess formation, and guide aspiration if needed
  • Blood tests: In systemically unwell mares, haematology and biochemistry assess the degree of systemic involvement

Treatment

Treatment should always be based on culture and sensitivity results where possible, but empirical treatment can be started whilst results are pending in acute cases:

  • Systemic antibiotics: The mainstay of treatment. Commonly used options include trimethoprim-sulphonamide combinations, penicillin (for Streptococcal infections), or appropriate broad-spectrum options pending culture results. Duration is typically 7–14 days. Always complete the full course.
  • Intramammary antibiotics: Used in cattle mastitis but less commonly in mares due to differences in anatomy — equine teat canals are much shorter and wider. May be used in some cases under veterinary guidance.
  • NSAIDs: Flunixin meglumine or phenylbutazone for pain relief and to reduce glandular inflammation
  • Frequent milking out: Manually stripping the affected gland 2–4 times daily reduces bacterial load, removes inflammatory mediators, and helps preserve glandular function — this is an important part of management
  • Abscess management: Abscesses may require ultrasound-guided drainage or surgical incision
  • Fly control: Apply fly repellent to the udder region and manage the stable environment to reduce fly pressure during treatment

If a nursing foal is present, monitor closely — a foal consuming infected milk may develop gastrointestinal signs. Your vet will advise on whether nursing should continue or the foal should be supplemented.

Prevention

Several management measures reduce the risk of mastitis in mares:

  • Gradual weaning rather than abrupt separation — allow milk production to reduce progressively
  • Maintain clean, dry bedding in stabling, particularly for mares with foals
  • Check the udder regularly during the weaning period for early signs of engorgement or infection
  • Control flies with appropriate repellents and environmental management
  • Inspect the udder and teats routinely during grooming — catching minor lesions or teat injuries early prevents bacterial entry
  • Ensure good nutrition and general health to support immune function

Prognosis

The prognosis for acute mastitis treated promptly with appropriate antibiotics is generally good. Most mares recover full glandular function. Chronic or recurrent mastitis, or cases complicated by abscess formation, fibrosis, or resistant organisms, carry a more guarded prognosis for preservation of milk production in the affected gland. Mares that have had severe mastitis may have reduced milk output from the affected side in future lactations.

When to Call the Vet

Contact your vet promptly if your mare shows:

  • A hot, swollen, or painful udder
  • Abnormal milk from either teat
  • A stiff, wide-based hind-limb gait
  • Reluctance to allow the foal to nurse
  • Fever or systemic depression alongside udder changes
  • A foal that appears to be losing condition or nursing from one side only

For related conditions, see Equine Colic for guidance on recognising abdominal pain in mares, and Equine Insect Bite Hypersensitivity (Sweet Itch) for information on managing fly-related skin irritation. Our horse terminology guide on the equine reproductive system provides useful anatomical context.

Frequently Asked Questions

Can a mare with mastitis continue nursing her foal?

This depends on the severity of infection and the causative organism. In mild cases, the foal may continue nursing from the unaffected gland whilst the infected gland is milked out manually. However, feeding infected milk to a foal can cause gastrointestinal problems in the foal. Your vet will advise based on the clinical situation — in some cases, the foal may need to be supplemented with mare's milk replacer temporarily.

How do I collect a milk sample for testing?

Clean the teat end with a disposable wipe and allow the first few squirts of milk to be discarded to reduce contamination. Collect approximately 5–10 ml into a sterile universal container. Label clearly with the mare's name and which gland was sampled (left or right). Keep refrigerated and submit to the laboratory within 24–48 hours. Your vet can assist with collection if you are unsure.

Is mastitis common in non-lactating mares?

It is less common than in lactating mares but does occur. Causes include trauma to the udder, tick infestation, haematogenous bacterial spread during systemic illness, or obstruction of the teat canal causing milk accumulation. Any mare with an abnormal udder — regardless of lactation status — should be examined by a vet.

Can mastitis be caused by viruses?

Bacterial causes account for the vast majority of equine mastitis cases. Viral mastitis is extremely rare in horses. However, certain systemic viral infections may predispose mares to secondary bacterial mastitis by temporarily suppressing immune function. Always seek veterinary diagnosis rather than assuming a cause.