Case Report Mammary botryomycosis and hemimastectomy in a post-partum Welsh Pony mare M. Ciccarelli , A. Tibary and C. Ragle Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, Washington,USA *Corresponding author email: michela.ciccarelli@wsu.edu Keywords: horse; udder; mastitis; reproduction; surgery Summary Botryomycosis is a chronic pyogranulomatous disease characterised by the formation of multiple abscesses containing bacteria surrounded by eosinophilic material which protect them from antibiotics and phagocytosis. Mammary gland botryomycosis is a rare but aggressive disorder that can jeopardise the ability of a mare to nurture a foal. Our report describes a case of unilateral botryomycosis mastitis in a Welsh Pony mare, treated by hemimastectomy. The mare was able to successfully carry and nurse foals post- surgery. Introduction Botryomycosis is a rare, chronic infection characterised by the formation of one or more abscesses which are often unresponsive to treatment with antimicrobials. This disorder was described for the first time in the horse in 1870 as a granulomatous lesion in the lung (Bollinger and Modlich 1870), although its relationship with bacterial infection was not clear at that time. The term ‘botryomycosis’ was used erroneously to describe the grape-like appearance of the granules (‘botryo’ from the Greek botrys meaning cluster of grapes) and the possible fungal cause (‘mycosis’). Historically botryomycosis in horses has been mainly recognised as a post-castration complication (Knottenbelt 2009). Botryomycosis can be divided into cutaneous and visceral forms (Bonifaz and Carrasco 1996). Few reports describing botryomycosis of the mammary gland of horses are available (Scott 1989; Smiet et al. 2012; Belli et al. 2018). In the majority of the cases, hemimastectomy was performed after unsuccessful antimicrobial treatment, and no follow-up information was provided describing the ability of the operated mares to produce enough milk to meet the demands of the offspring. Mastitis is observed most commonly immediately after weaning but it can also develop during pregnancy or lactation, and in nonpregnant nonlactating mares. Clinical signs for mastitis include swelling, pain and increased temperature of the mammary gland, ventral oedema, hindlimb lameness and less commonly, anorexia, sweating, increased pulse and respiratory rate, neutrophilia and fibrinogenaemia. The affected mammary gland can also produce bloody/purulent secretions. Mastitis in mares can be bacterial, mycotic, verminous and avocado toxicity- associated. The most common cause of mastitis in mares is due to bacterial contamination of the mammary gland. The bacteria growing on lacerations or insect bites enter the gland through the teat orifice. Half of the reported cases are due to Streptococcus zooepidemicus (McCue and Wilson 1989), and other frequent pathogens involved include E. coli, Klebsiella and Staphylococcus spp. (McCue and Wilson 1989; Perkins and Threlfall 1993; Motta et al. 2014. The present report describes a rare case of botryomycosis of the mammary gland in the immediate post-partum period in a mare caused by Staphylococcus aureus. We also provide a long-term follow-up of the operated mare documenting her ability to raise and meet the milk demands of several foals post-hemimastectomy. Case details A 9-year-old Section B Welsh Pony mare was presented to the Washington State University-Veterinary Teaching Hospital (WSU-VTH) Equine emergency service for unresolving mastitis and lethargy exhibited by the foal. The mare foaled uneventfully at 317 days of gestation and the foal stood up and appeared alert within 30 min after delivery. The entire placenta was delivered in less than an hour. The mare had normal mammary gland development, and she was nursing the foal. One day after parturition, the left side of the mammary gland became enlarged, tight and hot. The milk was thick, clumpy and bloody (Fig 1). The foal seemed to prefer to nurse off the right side of the mammary gland. At this time, the mare’s udder was evaluated by the local veterinarian using ultrasonography. Some abnormal fluid accumulation was noticed in the parenchyma but no mastitis was diagnosed and an adequate transfer of passive immunity was confirmed. The mare was treated with flunixin meglumine for 2 days (1.1 mg/kg i.v. s.i.d.) and then switched to phenylbutazone (4.4 mg/kg per os s.i.d.). Additionally, she received a 5-day course of oral sulfamethoxazole trimethoprim (30 mg/kg s.i.d.). The owner was instructed to milk the mare daily. After one week of treatment, a draining tract developed in the mammary gland and the mare’s condition remained unchanged. Ceftiofur sodium (2.2 mg/kg i.v. s.i.d.) was then prescribed and the owner was instructed to flush the open tract with a betadine solution. At 17 days post-partum, with clinical signs persisting and development of additional draining tracts exuding purulent material, the mare was referred to WSU-VTH for further evaluation and treatment. Initial examination The mare’s reproductive history included five previous foals. There was no history of complications during the deliveries or on post-partum examinations. © 2020 EVJ Ltd 1 EQUINE VETERINARY EDUCATION Equine vet. Educ. (2020)  () - doi: 10.1111/eve.13377