Mastitis in mares is condition characterized by the inflammation of the mammary gland, usually due to a bacterial infection. Mastitis typically affects lactating mares, often in the period immediately after giving birth or after weaning.

Mares with mastitis typically have swollen, painful udders and are reluctant to let foals nurse. The milk from a mare with mastitis can either appear normal or may be viscous with a yellow-green or red tinge. Evaluating the milk for bacteria confirms the diagnosis.

Treatment involves a combination of local therapy to reduce swelling, regular milking of the mare, and antibiotics to resolve the infection. Most mares respond to treatment quickly, with complete resolution within a week. Some mares develop scar tissue within the mammary gland that reduces their future milk production.

Early detection and preventive practices are key to managing this condition, ensuring the health and comfort of both mare and foal.

Mastitis in Mares

Mastitis is a relatively rare condition involving inflammation of one or both of the mammary glands in the mare’s udder. In horses, the most common cause of mastitis is a bacterial infection, often due to Streptococcus spp.

These bacteria usually gain entry through the teat canal, resulting in an inflammatory response. The affected mammary gland becomes swollen, hot, and painful, and there may be visible changes in the milk, such as a watery appearance, presence of blood or pus, or clots.

Systemically, the mare might exhibit signs of discomfort, reluctance to allow nursing, and in severe cases, symptoms of systemic illness like fever and lethargy.

Regular milking or stripping of the affected mammary gland is important to clear the infected milk and reduce pressure within the gland.

Normal Mammary Function

The mare’s udder has two separate mammary glands (left and right), each with its own teat. [3] Each mammary gland divides into lobes of glandular tissue that are responsible for producing milk to feed the growing foal. [3]

The hormone prolactin triggers milk production in the mammary glands. [2] Lobes in the glands storetproduced milk until the bunting and suckling behaviour of the foal stimulates the release of the hormone oxytocin. [3]

Oxytocin triggers the muscle-like cells surrounding the lobes to contract, squeezing the milk into the teat canal, allowing the foal access to the milk. [2], [3]

Lactation continues until the foal is weaned, usually around five to six months of age. After weaning, the mammary glands undergo involution, or a return to their resting state. [2]

The primary trigger for involution is the accumulation of milk, causing increased pressure within the mammary gland. [2] This increased pressure results in a loss of the milk-producing cells, and eventually stops lactation completely. [2]

How Mastitis Affects Lactation

Mares affected by mastitis may experience disruptions in lactation or difficulty nursing. In severe cases, mastitis can lead to a reduction or complete cessation of milk production in the affected gland.

Inflammation from the infection causes swelling, pain, and increased blood flow to the affected mammary gland and surrounding tissue. This can alter milk composition and production, potentially making it less suitable for the nursing foal. The infected milk may become thick, discolored, and contain pus or blood, indicating the presence of pathogens and immune cells.

Mammary gland discomfort may lead the mare to disallow the foal from nursing, negatively impacting the foal’s intake of vital nutrients and antibodies present in the milk.

In cases where the mare is still nursing when diagnosed with mastitis, the foal might be able to feed on the unaffected side, but the mare’s discomfort may prevent this. It’s important to ensure that the foal is getting sufficient nutrition, which may involve hand-rearing the foal until the mastitis is resolved​

Causes of Mastitis

The most common cause of acute mastitis is bacterial infection originating from these species: [3][1]

  • Streptococcus zooepidemicus
  • Staphylococcus
  • Klebsiella
  • Actinobacillus
  • E. coli
  • Pasteurella ureae
  • Pseudomonas aeruginosa
  • Enterobacter aerogenes

Bacteria typically enter the mammary gland through the teat opening, and then ascend the teat canal into the mammary tissue. [3]

Additional Causes

Other less common causes of mastitis include: [2][3][8]

  • Fungal infections such as Aspergillus or Blastomyces
  • Parasitic infections
  • Consumption of avocado plants
  • Traumatic injuries to the mammary gland
  • Frostbite
  • Mammary gland tumours


Mastitis is a relatively uncommon condition in horses, particularly when compared to other species. [1] Studies suggest that under 5% of breeding mares develop mastitis. [1]

Reasons for the low rate of mastitis in horses include: [1][2]

  • Small udder size in a relatively concealed location
  • Small capacity with frequent emptying
  • Relatively short lactation period compared to dairy cattle or goats

Risk Factors

Mastitis typically occurs during the normal lactation period, particularly if the foal does not nurse the mare properly. [1][3][4] One study reported that 42% of mares with mastitis were affected during their normal lactation period. [5]

During lactation, the onset of mastitis typically occurs within the first 2 to 3 days after foaling. [1]

Mastitis also commonly occurs during the first 4 to 8 weeks after weaning a foal, due to continued accumulation of milk in the mammary gland once the foal stops nursing. [3][6]

Dripping milk from the overfilled mammary gland may provide a route for bacterial entry through the teat opening. [4] In a study of mares with mastitis, 30% of the mares were affected during this period. [5]

Mastitis can also occur in non-lactating mares. [1][3] Estimates suggest that around 30% of mastitis cases are in non-lactating mares. [5]

There are also rare reports of mastitis in young mares, including neonates. [2][7] The