It would be easy for many mare owners who use them as saddle horses rather than breeding stock to forget that their mares have udders.
Fortunately, problems with the horse's mammary glands are fairly uncommon. A mare who has never been pregnant usually has a very small udder, which is barely visible except for the two teats. However, there are a variety of conditions that can cause the udder to swell or even to discharge fluid.
Owners who notice changes in their mares' mammary glands usually think first of mastitis. Mastitis simply means inflammation of the udder and is usually caused by an infection. Mastitis may be caused by bacteria or fungi, and there is even one report of a rare type of parasite causing mastitis.
The hallmarks of mastitis are that the udder feels hot and is painful when touched. Fluid milked from the udder is usually clear with clumps of white to grayish material in it when viewed against a dark background, like a black jar lid. If these signs are not present, odds are the mare does not have mastitis. However, she should be checked by your vet to make sure.
Mastitis can occur both in mares that are actively producing milk and those that are not. It is least common in a mare that is nursing a foal, unless there has been some type of injury to the teat. Mastitis is often misdiagnosed in mares that have udder engorgement, which is a back-up of milk.
Udder engorgement is common when a foal is weaned when it is still nursing frequently. The udder is swollen and may be warm and sensitive to touch, but the fluid is obviously normal milk, with no clumps. Udder engorgement may also be seen right before foaling and in the first day after foaling if the foal is not nursing well. In fact, a full udder on a mare that has just foaled is an important clue to make sure the foal is up, alert and nursing, and that the mare is permitting him to nurse.
Mares with Cushing's disease, a benign tumor in the pituitary gland in the brain, often develop enlarged udders that may even leak milk. This is due to overproduction of the hormone prolactin. Overweight and insulin-resistant mares that do not have a pituitary tumor sometimes also do this, and may have erratic estrus cycles or be difficult to get in foal. It's not at all clear what is going on with these mares hormonally, but similar scenarios have been described in women.
Udder swelling without production of fluid can occur with viral diseases that produce edema along the belly and in the legs. It may also occur with anything that blocks the free drainage of blood or lymphatic fluids from the udder.
In a mare that is otherwise well, you should always check the area between the two sides of the udder and between the udder and the inner thigh for any signs of irritation if there is unexplained udder swelling. Sweat, dirt and dead skin cells often accumulate in these areas and can inflame the skin. Be careful to not get kicked, since mares not used to having their udders examined can be touchy, and the skin irritation may cause the area to be extremely tender. If you find this build-up of dirt and debris, treat the udder by cleaning gently with a sheath-cleaning product.