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EQUINE VETERINARY EDUCATION / AE / FEBRUARY 2018


could have resulted from bacterial inoculation secondary to oropharyngeal trauma, lymphatic spread, or haematogenous dissemination. Detection of SHI titres ≥512 have been used as a


benchmark for identifying internal C. pseudotuberculosis infection; particularly in the absence of concurrent external abscesses (Knight 1978; Aleman et al. 1996; Pratt et al. 2005; Jeske et al. 2013). Internal abscesses carry a worse prognosis compared to external abscesses as mortality rates can reach 40% with treatment and 100% without treatment (Aleman et al. 1996; Pratt et al. 2005). Disseminated C. pseudotuberculosis infection may involve multiple organs including the lungs, liver, kidneys and spleen or, in pregnant mares, spread to the uterus may result in abortion (Poonacha and Donahue 1995; Pratt et al. 2005). Life- threatening, secondary complications of internal infection include purpura haemorrhagica, disseminated intravascular coagulopathy and colitis from prolonged antibiotic therapy (Pusterla et al. 2003; Pratt et al. 2005; Rand et al. 2012). Both moderate hyperfibrinogenaemia and a markedly increased SHI titre were reported in this case. During serial transthoracic and transabdominal ultrasound examinations, internal infection was not identified and the mare’s high SHI titre probably resulted from guttural pouch infection. Periodic monitoring of fetal heart rate and trans-abdominal CTUP measurements showed no indications of placentitis or fetal compromise. Similar to internal C. pseudotuberculosis


infection,


establishing drainage of abscessed retropharyngeal lymph nodes or guttural pouch empyema can be difficult. S. equi ssp. equi, a more common cause of guttural pouch empyema, often requires systemic antimicrobial therapy along with serial guttural pouch lavage to resolve infection. Furthermore, systemic antimicrobial therapy may hasten resolution of upper airway lymphadenopathy, limiting complications including dysphagia and aspiration pneumonia (Judy et al. 1999; Borges and Watanabe 2011). The initial course of therapy administered in this case was


similar to one frequently followed for guttural pouch empyema secondary to Streptococcus spp. infections. The mare was initially treated with ceftiofur as S. equi ssp. zooepidemicus and ssp. equi are commonly susceptible to third generation cephalosporins, (Verheyen et al. 2000; McClure et al. 2011). Following aerobic culture and antimicrobial sensitivity results, antimicrobial therapy was changed to TMS-SMZ and rifampicin as ceftiofur formulations at labelled dosages do not maintain sufficient plasma concentrations to meet the mean inhibitory concentrations (MICs) for C. pseudotuberculosis (Rhodes et al. 2015; N. Pusterla, personal correspondence). Based on sensitivity results, the ease of oral administration, and permeability of the placental–fetal unit, the combination of oral TMS and rifampicin was chosen to treat this mare (Rebello et al. 2006). Rifampicin was administered as TMS-SMZ alone may have reduced activity in purulent environments or well- encapsulated abscesses (Rhodes et al. 2015). Systemic antimicrobial therapy remains controversial in cases of external abscesses; however, long-term TMS-SMZ therapy was continued to limit possible complications associated with guttural pouch infection, reduce the risk of placentitis and abortion, and, in light of the mare’s increased SHI titre, internal C. pseudotuberculosis infection could not be completely ruled out. MICs against this isolate were not


© 2016 EVJ Ltd


determined on initial culture but could have better guided long-term antimicrobial therapy for this case; particularly as the MIC for potentiated sulfonamides can vary between C. pseudotuberculosis isolates (Rhodes et al. 2015). Based on sensitivity results, oral chloramphenicol or injectable penicillin would have also been appropriate choices. Follow up re- evaluations showed favourable response to TMS-SMZ and rifampicin therapy in this case. Unlike the south-western USA, cases of equine


C. pseudotuberculosis infection are rarely reported in the south-east. This particular case was one of 21 reports of equine C. pseudotuberculosis infection documented in South Carolina during the years 2013–2014; prior to 2013 the disease had not been reported by the State Veterinarian’sOffice (A. Eichelberger, personal correspondence). Similarly in 2012, Florida reported an unusually high number of equine cases while a marked rise in the total number of C. pseudotuberculosis cases was reported in horses throughout the USA between 2011 and 2012 (House 2012; Kilcoyne et al. 2014). During this 2-year period, the greatest number of cases occurred in Texas but numerous cases were reported in states outside of the western USA, many of which had not reported C. pseudotuberculosis infection prior to 2010 (Kilcoyne et al. 2014). C. pseudotuberculosis can survive in soil for over 8 months


and environmental factors such as increased rainfall and manure content in soil may facilitate the endemic status in some areas (Spier et al. 2004; Spier 2008; Kilcoyne et al. 2014). Although the route of infection is not completely understood, C. pseudotuberculosis


is probably inoculated through


mucosal abrasions, direct soil contamination of wounds, or transmission from insect vectors that damage the skin barrier or contaminate existing wounds (Aleman et al. 1996; Spier 2008). The horn fly(Haematobia irritans), the stable fly (Stomoxys calcitrans) and house fly(Musca domestica), which feed along the ventrum of the horse or contaminate wounds, have been identified as potential vectors while experimental transmission of C. pseudotuberculosis to ponies from the house fly vector has been documented (Spier et al. 2004; Kilcoyne et al. 2014; Barba et al. 2015). Climate changes favouring vector survival and breeding could conceivably propagate the overall numbers and distribution of cases across the USA, including regions outside traditional endemic areas (Spier et al. 2004; Spier 2008). Rainfall in 2013 were almost twice those recorded in 2012 for this area of South Carolina, particularly during the summer months. Dry conditions in 2012 followed by a relatively rainy 2013 summer may have contributed to the unprecedented number of C. pseudotuberculosis cases recorded in the state that year (South Carolina State Climatology Office, personal correspondence). Although blood cultures collected within 18 h of birth


yielded no bacterial growth, a SHI titre was not measured in the mare’s foal. In endemic areas, infection is uncommon in foals younger than 6 months, indicating a role of protective immunity from maternal antibodies


against


C. pseudotuberculosis (Aleman et al. 1996). Passive transfer was confirmed in this foal and no signs of illness were detected at birth or throughout the first month of life. Complete evaluation for internal abscess formation in this


mare should have included cytological analysis of peritoneal fluid; however, abdominocentesis was not pursued in light of the mare’s favourable response to therapy and due to


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