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; however, Streptococcus equi, Streptococcus equisimilis, Streptococcus agalactiae, and Streptococcus viridans are also found. Multiple species of gram-negative bacteria have also been reported to cause mastitis in mares.
Marked, painful swelling of the affected gland and adjacent tissues develops, and the secretion is often seroflocculent (see mastitis in mare image). Fever and listlessness may be present. The mare may walk stiffly or stand with hind legs apart due to the discomfort. Hemimastectomy or mastectomy may be considered in severe cases.
Courtesy of Dr. Thomas Lane.
Treatment is similar to that in cows; however, when intramammary infusions are used, they should be inserted separately into both orifices of the teat. Systemic treatment has been suggested to include trimethoprim-sulfonamide (30 mg/kg of the combination, PO, every 12 hours) or a combination of penicillin (20,000 IU/kg, IM, every 12 hours) and gentamicin sulfate (7.7–9.7 mg/kg, IV, every 24 hours). Treatment should be continued on the basis of culture and antimicrobial 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.
For More Information
Morresey PR. Agalactia, dysgalactia, and nutrition of the postpartum mare. American Association of Equine Practitioners Annual Convention; December 5, 2012; Anaheim, California.
Brendemuehl JP. Mammary gland enlargement in the mare. Equine Vet Educ. 2008;20:8-9. doi:10.2746/095777308x265687
Mastitis in mares. Kentucky Equine Research. August 2, 2011. Accessed December 28, 2023.
Also see pet health content regarding mastitis in horses.
