Abstract. Several Actinobacillus spp. are common commensal bacteria of the oral cavity, gastrointestinal tract, and reproductive tract of horses and can cause disease in both foals and adults. The current retrospective study was designed to review Actinobacillus spp. isolated from clinical samples or necropsies of 99 horses during 1999-2011. The cases consisted of 43 foals (<6 months of age), 4 young adults (6 months-2 years), 39 adults (>2 years of age), 2 aborted fetuses, and 11 with unspecified ages. Clinical history, signs, bacterial species isolated, and associated lesions were documented. Actinobacillus spp. were isolated 111 times. The most common isolates were Actinobacillus equuli subsp. equuli (38.7%) and hemolytic Actinobacillus spp. (24.3%). Other isolates were Actinobacillus lignieresii (5.4%), Actinobacillus pleuropneumoniae (1.8%), and unclassified Actinobacillus spp. (28.8%). Actinobacillus equuli subsp. equuli was most commonly isolated from clinical and necropsy cases of septicemia and respiratory disease in both foals and adults. Embolic nephritis, the classical septicemic lesion of equine neonatal actinobacillosis, was also present in several adult septicemic actinobacillosis cases. Predisposing factors such as failure of passive transfer of colostral antibodies as well as concurrent pathogenic bacterial or viral infections were present in numerous actinobacillosis cases. There were many cases, however, for which a predisposing factor or concurrent infection was not documented or apparent, suggesting that Actinobacillus spp. can be primary pathogens under the right circumstances and in the right location.

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