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 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

Prevalence

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 cause of neonatal mastitis is not well understood, but may be related to high levels of maternal hormones prior to delivery. [7]

Symptoms

The symptoms of mastitis depend on the severity of the underlying infection. Common signs include: [2][3][6]

  • Swelling and firmness of the mammary gland
  • Hot and painful mammary tissue
  • Hindlimb lameness
  • Swelling of the lower abdomen
  • Prominent milk veins
  • Fever
  • Reluctance to let the foal nurse (if the foal is still nursing)

In most cases, only one of the mare’s two mammary glands develops mastitis. [1] Studies suggest between 59 – 78% of mastitis cases are one-sided. [1]

Effect on Milk Quality

The inflammation caused by mastitis can lead to alterations in the milk’s composition produced by the affected mammary gland. The main changes observed include: [4]

  • Increased white blood cells
  • Decreased lactose
  • Increased sodium and chloride
  • Blood in the milk from damaged capillaries

The appearance of the milk may also be affected mastitis. Changes in milk appearance may include: [3]

  • Thick or viscous milk
  • Changes to the milk’s opacity, such as becoming clear
  • Red or yellow-green tinge to the milk

Effect on the Foal

The bacteria causing mastitis in mares can adversely affect nursing foals. One risk is the transmission of these bacteria through the milk, potentially leading to septicemia, a serious condition where the bacteria invade the foal’s bloodstream [1] Foals may also develop pneumonia or joint infections due to the bacteria. [1]

If mastitis impacts the mare’s milk production or willingness to nurse, the foal may experience malnutrition due to insufficient milk intake. [1]

In cases where mastitis develops around the time of delivery, the foal may be at risk of failure of passive transfer if the mare is unwilling to let the foal nurse colostrum. Mastitis prior to foaling is rare. In some cases it is associated with stripping the udders for milk samples to test for signs of labor.

Mastitis is also associated with galactorrhea (milk production unrelated to pregnancy or nursing), but not all cases of galactorrhea develop mastitis. [12]

Diagnosis

Mastitis in mares is diagnosed based on clinical signs and physical examination of the udder. Milk samples can be taken for bacteriological culture and sensitivity testing to identify the causative organism and guide antibiotic therapy.

Bacterial Identification

Diagnosis of mastitis typically involves testing a milk sample to identify bacteria, indicating an infection. The veterinarian examines the milk sample under a microscope to look for bacteria and immune cells. [2][3][5]

This test also allows the veterinarian to identify the type of bacteria present, which can help guide antibiotic selection until bacterial culture is performed.

Another option is the California Mastitis Test (CMT), which was originally designed for diagnosing mastitis in cattle. There are reports of successful CMT use in horses, however use is not common. [3]

This test uses special detergents that bind to immune cells within the milk to form a gel. When combined with milk taken from an affected mare, the sample begins to clot, indicating the presence of mastitis. [3] This test only indicates the presence of infection, not the type of bacteria.

Bacterial Culture

A bacterial culture performed on a milk sample can also determine the type of bacteria present and which antibiotics will be most effective against that particular strain. [3][9] This testing is especially important in cases that do not respond to initial treatment efforts.

The veterinarian must be careful to avoid contamination of the milk sample for testing. This involves thorough cleaning of the udder prior to collection, milking using sterile gloves, and collecting the sample in a sterile container. [1]

This collection procedure is important as many of the bacterial causes of mastitis are normally found in the environment, and contamination of the sample with environmental bacteria may produce a false result. [1]

Ultrasound

Ultrasound examination is also common in diagnosing mammary disease. [1][3] Ultrasound can identify changes in the tissue structure, such as abscesses, cysts, or cavitation. [3]

The veterinarian can also identify scar tissue or blockages that may prevent normal milk drainage on ultrasound. [3]

Treatment

Treatment for mastitis has two main components: local therapy to reduce swelling, and antibiotics to treat the bacterial infection.

Local Therapy

Local therapy measures typically include: [1][3][5]

  • Regularly milking the mare to flush bacteria and pus from the gland
  • Hot packing and cold hosing to reduce swelling
  • Oxytocin administration to improve milk let-down

Mares may require sedation for local therapy, as their painful udders may make them unwilling to stand quietly for the procedure. [10]

Frequent milking during treatment of mastitis can cause dry skin on the teats. Cracks that develop in the dry skin can easily become infected, worsening the mastitis. To prevent this, some veterinarians recommend applying petroleum jelly to the teats after each milking. [1]

Medications

Non-steroidal anti-inflammatory drugs such as flunixin meglumine or phenylbutazone may also help reduce swelling and pain. [3][5] Mares that develop swelling of the abdomen also benefit from diuretics such as furosemide. [5]

Antibiotics

Antibiotic treatment typically involves intramammary antibiotics, such as those used to treat mastitis in cattle. [3] These antibiotics are formulated to be directly infused into the mare’s mammary gland through the teat canal.

Your veterinarian will follow these steps to administer intramammary antibiotics: [3][5]

  • Clean and disinfect the teat thoroughly to prevent introduction of new bacteria
  • Carefully insert the tip of the intramammary antibiotic product into the teat opening
  • Administer the antibiotic

Sedation is often necessary to ensure the safety of the person administering the medication.

Mares typically have 2 to 3 openings on each teat, so multiple doses of intramammary antibiotics may be necessary to ensure the entire mammary gland is treated. [1] Additionally, the teat canal of horses is much smaller than cattle, so the veterinarian must be very careful to not damage the teat canal during administration. [1]

Oral or Injected Antibiotics

Some veterinarians may also use either oral or injected antibiotics as part of the treatment plan, usually for a 5 to 7 day period. [3] Mares that exhibit dangerous behaviours during intramammary antibiotic administration may only receive oral or injected antibiotics. [3]

The most commonly used antibiotic for treating mastitis in horses is sulfamethoxazole and trimethoprim. This combination is effective because most bacteria that cause mastitis are susceptible to it. [1] Other antibiotics that can be used include penicillin, gentamicin, and cephalosporins, chosen based on the specific bacteria involved and their susceptibility patterns. [1][3]

Rarely, mares may develop large abscesses of bacterial infection during mastitis. These cases require surgical opening of the abscess to allow drainage, and long-term antibiotic therapy. [2][3]

Surgical Intervention

In severe cases, surgical removal of a mammary gland is necessary to prevent continued discomfort and pain for the mare. [3][10] For the procedure, the mare must go under general anesthesia at a surgical referral center. [1]

Surgical removal is usually reserved for mares with long-standing mastitis who does not respond to treatment, as the procedure has a high risk of complications. [1] Possible complications include: [1]

  • Massive hemorrhage
  • Poor healing
  • Surgical site infections
  • Failure of the wound to close properly

Prognosis

Most mares respond quickly to treatment. Typically, the milk returns to a normal appearance within 1 to 3 days, and the texture of the mammary gland normalizes within a week of starting treatment. [3][5]

Veterinarians usually reduce the frequency of milking once the milk has a normal appearance, while continuing to monitor the mare. [1] Recurrence of mastitis is uncommon in mares. [3]

In severe cases, scar tissue may develop within the mammary gland as the tissue heals. These mares may experience reduced milk production during future lactations. [3] Breeders who have a mare with significant udder scarring often retire the mare from breeding, as their milk production may be insufficient to sustain a foal. [1]

A rudimentary test to determine the severity of mammary scarring involves the veterinarian distending the mammary gland of a non-lactating mare with sterile saline, to determine the potential storage capacity of the udder. [1] Ultrasound examination can also identify the extent of scar tissue formation. [1]

Prevention

Mastitis is uncommon in horses, but there are still measures that breeders can take to prevent the condition from occurring. The most important strategy is routinely checking the udder during lactation to identify changes quickly. Assess the mammary glands for cleanliness and signs of injury. [2]

During weaning, moderating the mare’s feed intake can reduce her milk production to prevent overfilling of the udder once the foal is no longer nursing. [2] Typically both grain and hay rations are reduced, but grain reduction is the most important. [1]

Providing creep feed to foals prior to weaning can also help by gradually reducing their reliance on the mare’s milk for nutrition. This strategy naturally decreases the mare’s milk production. [2]

There is a variety of commercial creep feed products available for foals, but using a specialized product is not strictly necessary. You can also provide the foal with some of their dam’s feed or a vitamin and mineral supplement. Creep feed is usually provided to foals in a pen that their mother cannot access, or in a specialized feed tub with a small opening that prevents the dam from accessing the feed.

Routine milking of mares is not recommended for prevention of mastitis after weaning, because some degree of milk accumulation is necessary to trigger the mare to stop producing milk. [1]

Summary

  • Mastitis in mares is characterized by inflammation of one or both mammary glands, typically occurring post-foaling or during weaning.
  • This condition is most commonly caused by a bacterial infection.
  • Effective treatment usually combines local therapies to reduce swelling and antibiotic administration to combat the infection.
  • The prognosis is good with most mares showing significant recovery within a week.
  • In cases where mastitis affects milk availability, supplemental hand-rearing of the foal may be necessary to ensure proper nutrition.

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References

  1. Canisso. I. F. et al., Diagnosis and Treatment of Mastitis in Mares. Equine Veterinary Education. 2021. doi: 10.1111/eve.13228.
  2. Hughes. K., Development and Pathology of the Equine Mammary Gland. J Mammary Gland Biol Neoplasia. 2021. doi: 10.1007/s10911-020-09471-2. View Summary
  3. McKinnon. A. O. et al., Equine Reproduction. Wiley-Blackwell, 2010.
  4. Doma?ska. D. et al., The Indicators of Clinical and Subclinical Mastitis in Equine Milk. Animals. 2022. doi: 10.3390/ani12040440. View Summary
  5. McCUE. P. M. and Wilson. W. D., Equine Mastitis – a Review of 28 Cases. Equine Veterinary Journal. 1989. doi: 10.1111/j.2042-3306.1989.tb02687.x. View Summary
  6. Perkins. N. R. and Threlfall. W. R., Mastitis in the Mare. Equine Veterinary Education. 2002. doi: 10.1111/j.2042-3292.2002.tb01804.x.
  7. Gilday. R. et al., Mastitis in a Neonatal Filly. Can Vet J. 2015. View Summary
  8. Gratwick. Z. et al., A Case of Verminous Mastitis in a Mare. Equine Veterinary Education. 2021. doi: 10.1111/eve.13363.
  9. Dascanio. J. J. et al., Eds., Equine reproductive procedures, 2nd edition. Hoboken, NJ: Wiley-Blackwell, 2020.
  10. Smiet. E. et al., Equine Mammary Gland Disease with a Focus on Botryomycosis: A Review and Case Study. Equine Veterinary Education. 2012. doi: 10.1111/j.2042-3292.2011.00352.x.
  11. Menzies-Gow, N. et al., Mammary gland: mastitis. Vetlexicon.
  12. Canisso, I.F., Diagnosis and treatment of mastitis in mares. Equine Veterinary Education. 2021.