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Mastitis in Mares

By Heather Smith Thomas

Mastitis is infection and inflammation of the udder. A mare’s udder is divided into halves, each half containing two quarters, like the udder of a cow. The two quarters on a side feed into a single teat with two openings. If you ever milk a mare you’ll notice two streams of milk from the nipple when you squeeze it.

Mastitis usually affects one or two quarters, on the same side. Only rarely are two quarters on opposite sides affected, or all four quarters. Infection from bacterial invasion causes the affected quarter to become swollen, hard and warm. The milk (if the mare is lactating) changes consistency; it may become lumpy, thicker, or more watery.

Mastitis most often affects lactation mares, especially at weaning when the mare has a full udder that may be painful and slow to dry up. Mastitis may develop anytime up to 8 weeks after the foal is weaned. But a mare doesn’t have to be lactating to develop mastitis. Even maiden and barren mares occasionally get udder infections due to trauma and,bruising, or insect bites. Mastitis has been seen in fillies as young as 15 weeks old.

In a 1989-1990 study at the University of California (Davis) Veterinary Hospital, looking at mares with mastitis, 28% of the mares were not lactating at the time of infection. Of the mares examined, 44% were lactating when infection began, and another 28% developed mastitis within 8 weeks of weaning foals. 71% of the cases occurred during summer fly season; flies and insects were a possible source of infection. The inflammation may be triggered by fly bites and other external irritations of the teat opening that open the way for infection.

Mastitis in a non-lactating mare may also be due to abnormally high estrogen levels in her system. It can result from a pituitary gland tumor, or lush spring pasture or legume hay that contains high levels of estrogen. High estrogen concentrations can cause mammary gland enlargement and trigger production and secretion of milk, even though the mare is not pregnant and has not foaled. If the non-lactating mare begins to secrete milk due to excess estrogen, she may become a candidate for mastitis.

Udder injuries (such as a kick), insect bites, contamination (bacteria finding their way into the teat openings) from dirty bedding, and inherited susceptibility to infection, may all be factors that can lead to mastitis.

Mastitis symptoms may be obvious, or very subtle, depending on severity of infection and amount of pain involved. A nursing mare may kick at her foal when it tries to nurse. An alert horseman may notice that a mare with mastitis doesn’t seem quite right. She may stand off balance and rest one hind leg continually, trying to ease the swollen side of her udder, or she may hold her stifle out away from her body, so the leg doesn’t touch the swollen udder. She may show a slight lameness in the hind leg next to the affected quarter, moving stiffly in an effort to avoid bumping the udder.

Signs of mastitis include swelling, heat and pain, and edema in surrounding areas. Usually only one teat is affected. Some mares become depressed, feverish, or off feed. The foal may avoid nursing the infected teat because the milk tastes different, and that side may become even larger, adding to the mare’s discomfort.

Acute mastitis causes a hot, painful udder, and in a lactating mare with a severe case, milk from the infected side may be just a small amount of watery fluid. Some cases form abscesses in the udder and nearby lymph glands, which require surgical drainage. Occasionally mastitis completely destroys the udder tissue and can cause death of the mare.

Other cases develop gradually, producing fibrosis (thickening and scarring) that slowly destroys the mammary tissue. Some milder forms of mastitis may clear up without treatment, if the mare’s body defenses are strong enough to combat and control the invading organisms. But most cases will need medication.

Tissue smears or cultures of milk samples from the affected quarter or quarters can help the vet determine the specific pathogen causing the infection, and antibiotic sensitivity tests can be done to find the most effective antibiotic. Treatment may consist of antibiotics given ‘systemically (such as intramuscular injections) and in severe cases medication can be squirted directly into the affected part of the udder through the teat canal.

Treatment will also include frequent milking of the affected side of the udder, to hasten elimination of infection and abnormal milk. Hot packs and non-steroidal anti-inflammatory medications (such as bute or banamine) can help reduce the pain and swelling, and temperature, if the mare has a fever. Antibiotics should be given at least 5 to 7 days. If halted too soon, the infection may recur. But with prompt and appropriate treatment, continued long enough to completely halt the infection, most cases of mastitis improve a lot within 2 or 3 days, and the udder will generally return to normal within a week.

A number of different bacteria can cause mastitis, including Streptococcus (which also causes strangles), S. zooepiddemicus (the same organism that causes many uterine infections), Staphlococcus, Pseudomonas, Actinobacillus, and Rhodococcus (which also causes foal pneumonia). Researchers have also found Klebsiella and E. coli bacteria in mastitis infections.

Penicillin was traditional treatment for mastitis, but vets now use newer drugs or a two-phase treatment that is more effective against the many types of bacteria that might be involved. Broadspectrum treatment with a trimethoprim sulphonamide combination is usually begun immediately to start fighting infection while waiting for the results of the culture, cell study and sensitivity tests. After the specific bacteria are identified and the best antibiotic selected for continuing treatment, the vet may switch drugs.

In addition to antibiotics, other treatments can help the mare recover more rapidly and comfortably. If lactating, she should be milked 5 or 6 times every 24 hours to get rid of infected milk and hasten production of normal milk. If a local antibiotic is given (infused into the teat) , it should be administered twice a day (put in immediately after milking out the teat) and the udder gently massaged to help distribute the medication through the quarter. Treatment should continue until the milk dries up (in a mare with a weaned foal) or returns to normal. Commercial preparations for dairy cattle can be used in treating the area with mastitis.

You can reduce the udder swelling and discomfort by applying hot packs or hot soaks to the udder 15 to 20 minutes at a time, 2 or 3 times a day. Usually a combined treatment of systemic and infused (into a teat) antibiotics, along with frequent milking and hot packs, is the most effective treatment for mastitis.

If a mare is nursing a foal and her udder is painful, nursing sessions should be supervised to keep her from kicking the foal. Infected milk will usually not harm the foal, and if he’ll nurse the affected side, it will save you the task of milking it out.

Proper and prompt treatment of mastitis will reduce the chance of permanent damage to the udder. Long-standing or recurring cases, however, may prove more difficult to clear up, and may be due to abnormalities in the teats themselves.

To help prevent mastitis, check udders often, and be aware of each mare’s udder appearance—and note abnormal changes, especially in lactating mares. Avoid any unnecessary handling of the udder and make sure your hands are clean (or wear sterile gloves).

If flies are especially troublesome, bring mares and foals inside during the worst fly periods of summer. After weaning, observe the mare’s udder closely for several weeks (and do not milk her out). Reducing or eliminating grain ration at weaning time may help the mare cut down more quickly on her milk production. Some horsemen limit a mare’s water intake to about 5 gallons a day for a few days to help decrease the milk flow and speed her drying up.

If a mare has a serious case of mastitis, you won’t know till her next foaling whether the affected quarter will produce milk again, even if you treat it immediately and clear it up. It all depends on whether the mammary tissue is damaged. As she nears her next foaling date and the udder fills, you will be able to tell. A damaged quarter that is not producing milk will not fill, and the udder will appear a bit lopsided. If more than one quarter is damaged you may have to find another milk source for the foal.