Recurrent Actinobacillus peritonitis in an otherwise healthy thoroughbred horse

A Thoroughbred gelding in North America was evaluated for Actinobacillus peritonitis on three different occasions over a 4-year period. At each presentation, peritoneal fluid had an elevated nucleated cell count (220,000-550,000 cells/µL) characterised by non-degenerate neutrophils, no visible bacteria, an elevated total protein (4.6-5.5 g/dL) and bacterial culture yielding Actinobacillus spp. Actinobacillus peritonitis appears to be a regional disease occurring in Australia and less commonly in New Zealand and North America. Recurrence, other than incomplete resolution, has not been previously reported. This case highlights the classical presentation, response to therapy and excellent prognosis despite the alarmingly abnormal peritoneal fluid characteristic of Actinobacillus peritonitis and questions the role of parasite migration in the pathogenesis. Finally, this case is remarkable because Actinobacillus peritonitis was recurrent over several years in an otherwise normal horse.