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Home:Equine Diseases: Mastitis

Equine Diseases


Equine Mastitis is inflammation of the mammary glands/udder caused by a bacterial infection. Traditionally, it has been associated with lactating mares, especially at the time of weaning when the mare has a full udder. (If this is the case, symptoms of equine mastitis may develop any time up to eight weeks after the foal is weaned). However, a mare does not have to be lactating to develop equine mastitis. The infection results from a bacterial invasion that causes the affected half of the udder to become warm, hard and swollen. Infection also causes the milk (if the mare is lactating) to change consistency to a lumpy, viscous fluid or alternatively a more watery substance than normal.

In horses, anatomically, the udder is much smaller and protected than the more pendulous udders of cattle. The teat canals and openings are therefore also smaller, making infection less likely. Interestingly neither age, pregnancy, or past parturient status greatly effects the incidence.

In other known cases of equine mastitis, it develops over a period of time forming a fibrosis that slowly destroys the mammary tissue. If the mares’ immune system is strong enough to deal with the invading pathogens, the mastitis will clear up of its own accord. Most cases will however need medication.


• Hot, swollen an painful udder
• Heat and pain
• Oedema in the surrounding area
• Usually only one teat is affected, but sometimes both are
• Depression
• Pyrexic (fever)
• Loss of appetite

- A mare may kick at her foal when it tries to nurse

- She may stand off balance and rest one hind leg (trying to ease the swollen side of her udder)

- Holding stifle out away from body.

- Milk from the infected side changes consistency to either lumpy viscous milk or watery fluid

- Abscesses can form in the udder and lymph glands, which require surgical drainage.

- Occasionally mastitis completely destroys the udder tissue and can cause death of the mare.



• Udder injuries (kick from horse, insect bites)
• Contamination of the udder from dirty bedding
• Inherited susceptibility to infection
- In a non-lactating mare, equine mastitis may develop due to abnormally high oestrogen levels in her system. This excess hormone may be due to a tumour on the pituitary gland or high levels can be found in legume hay.
- If there is an excessive amount of oestrogen in the system mammary glands will enlarge and trigger the production of milk, even though the mare is not pregnant. If this occurs mastitis may occur.


In order to determine the pathogen causing the infection; a sample of the milk can be tested, and then antibiotics are used to treat equine mastitis.

Frequent hand-milking of the affected side of the udder will speed up the elimination of the infected milk. If the mare is nursing her udder will be painful and possibly try to prevent the foal from nursing. The infected milk will usually not harm the foal, so it is important to try and get the foal onto the affected side. This will drain the milk naturally rather than having to milk her again.

Hot packs and non-steroidal anti-inflammatory medications may also help reduce the pain and swelling.

Antibiotic treatment must be completed or the infection will return. Generally there will be a significant improvement within the first few days and within a week the udder should return to normal.

If equine mastitis is treated properly and quickly, the chances of causing permanent damage to the udder will be reduced. Longstanding or recurring cases however, may prove more difficult to clear up and may be due to abnormalities in the teats themselves.