Acute mastitis occurs occasionally in lactating mares, most commonly in the drying-off period, in one or both glands. Streptococcus zooepidemicus is the most frequent pathogen, but S equi, S equisimilis, S agalactiae, and S viridans are also found. A variety of gram-negative bacteria has also been reported. Marked, painful swelling of the affected gland and adjacent tissues develops, and the secretion is often seroflocculent. Fever and depression may be present. The mare may walk stiffly or stand with hindlegs apart due to the discomfort. Hemimastectomy or mastectomy may be considered in severe cases.
Treatment is similar to that in cows, but when intramammary infusions are used, they should be inserted separately into both orifices of the teat. Systemic therapy has been suggested to include trimethoprim-sulfonamide (based on 5 mg/kg of trimethoprim, PO, twice daily) or a combination of penicillin (20,000 IU/kg, IM, twice daily) and gentamicin sulfate (2 mg/kg, IV, three times daily). Therapy should be continued on the basis of culture and antibacterial sensitivity testing. Without prompt treatment, abscessation or induration of the gland can occur. Little is known about the frequency and persistence of subclinical intramammary infections in mares.