What is inappropriate lactation?

Rarely, some mares not raising foals, maiden mares, or even young fillies will produce milk. In folklore, inappropriate milk production has been called “witch’s milk” because it was believed it was used for witchcraft, or drank by the familiars of witches. Usually, any udder secretions from affected mares cannot be appreciated unless the teats are milked. Mares with inappropriate lactation may have a history of nonpainful udder enlargement for months to years. Fortunately, inappropriate lactation almost never causes problems for the mare.

What are the clinical signs of inappropriate lactation?

• In most mares, the udder remains normal sized to slightly enlarged.
• Udder secretions from affected mares can range from milky to fairly clear or cloudy.
• The udder is usually not painful to touch or milk.

How is inappropriate lactation diagnosed?

• The most important rule out for inappropriate lactation is mastitis (inflammation and infection of the udder), yet both conditions are uncommon in equine medicine. Mares with mastitis are generally very painful about their udders, even to the point of being dangerous. Inappropriate lactation is almost always not painful. Milk from mares with mastitis may be clumpy and discolored yellow to blood-tinged. Mastitis is more common when mares are “drying-up”, such as at weaning or after losing a foal. Mastitis is also reportedly more common during summer when flies are more prevalent and flies have been speculated to spread infectious bacteria between mares.
• Cytology (looking at the types of cells in milk under the microscope) performed by your veterinarian greatly helps to differentiate inappropriate lactation from mastitis. Milk from mares with mastitis will contain large numbers of white blood cells and bacteria may also be observed. In inappropriate lactation, there will be few or no inflammatory cells (white blood cells) and there should be no bacteria. Bacterial culture of mastitis milk often isolates Streptococcus species bacteria, but several different bacterial species have been reported to cause mastitis. Udder secretions from mares with inappropriate lactation should be sterile (no bacteria).

What causes inappropriate lactation?

• In most cases, a cause for inappropriate lactation cannot be found.
• It is important to determine if a mare with an enlarged udder is unknowingly pregnant as pregnant mares may develop early udder enlargement with placentitis.
• Inappropriate lactation may be caused by a pituitary gland dysfunction or tumor releasing the hormone prolactin which stimulates the udder to produce milk, but this is very uncommon. Some reports have associated inappropriate lactation with pituitary pars intermedia dysfunction (equine Cushing’s Disease; PPID) but PPID affects a part of the pituitary gland that does not secrete prolactin.
• Estrogen-like compounds can sometimes be produced in legumes, such as alfalfa, and in some fungal toxins which may induce lactation.
• Young foals that are nursing mares ingesting these estrogen-like compounds from the grass may also develop enlarged udders and start producing milk. This phenomenon has also been reported in human infants.
• If mares are kept in groups with other broodmares, foals may sometimes nurse other cooperative mares and promote milk production in nonpregnant or weaned mares.

Can inappropriate lactation be treated?

• Treatment of inappropriate lactation is not especially rewarding and may resolve on its own. Most often, the overall health of the mare is not affected.
• Dietary changes, such as decreasing concentrates, are unlikely to decrease milk production.
• Bromocriptine is a dopamine agonist drug that inhibits (blocks) prolactin from being released from the pituitary gland, and decreases milk production. It has been reported to resolve inappropriate lactation in some cases, but is off label in horses, thus it is very rarely used.
• If the mare will never be expected to have a foal, then a chemical irritant an be injected into the udder by a veterinarian to essentially kill the glandular tissue. If this is performed, then the mare will be unable to ever produce milk. This treatment should be considered a last resort and should not be performed solely for cosmetic purposes or convenience, as there are many risks to the mare.
• The best stimulus to “turn off” the udder is the back pressure of milk which eventually induces the mammary gland to decrease milk production (don’t milk the mare).

What is the prognosis for mares with inappropriate lactation?

• In virtually all cases, inappropriate lactation causes no problem to the mare. Treatment to resolve inappropriate lactation can be difficult and is not often medically indicated. Some cases resolve without treatment.
• Affected mares are not predisposed to develop mastitis and can be expected to produce milk normally if they become pregnant.

A veterinarian who has examined your horse and is familiar with your situation is always your best source of medical advice.