Skip to main content
Doctoral thesis, 1999

Studies on equine Actinobacillus spp

Sternberg, Susanna


Studies on equine Actinobacillus spp. found in the normal flora of the oral cavity ofthe horse, as well as isolates from clinical cases, are summarised in this thesis.

The major disease caused by equine Actinobacillus spp. is septicaemia in foals. In adult horses, respiratory infections can be seen, but various other opportunistic infections are also associated with these bacteria. The taxonomic status of equine actinobacilli is still uncertain, and future revisions can be expected.

The prevalence of Actinobacillus spp. in the normal flora of the oral cavity of healthy horses was found to be high but varied substantially over time, and most horses were carriers. The bacterial population was complex with a large number of different strains present at the same time. No overall phylogenetic differences could be detected by ribotyping or biochemical fingerprinting of isolates from clinical cases and isolates obtained from healthy horses. So far, there is no evidence of any difference in pathogenic potential between various subtypes of Actinobacillus spp. or between different strains. However, studies on bacterial culture supernatants from some clinical isolates of equine Actinobacillus spp., indicated the presence ofleukotoxic metabolites causing a decrease in total number of granulocytes in neutrophil function studies. Immunodiffusion studies demonstrated that Actinobacillus spp. carried in the oropharynx ofhealthy horses provoke a humoral immune response that can be transferred from mares to their foals via colostrum. This finding indicates that prompt and successive passive transfer of colostral antibodies is of specific importance in protecting the neonatal foal from Actinobacillus infection. Antimicrobial susceptibility testing showed that resistance against commonly used antimicrobial substances, such as penicillin and trimethoprim-sulfa, is present among Swedish strains of equine Actinobacillus spp. However, most ofthe bacterial population is susceptible. The possible presence of plasmid-mediated p-lactamase may change this situation by causing a rapid spread ofpenicillin resistance.

The pathogenesis of equine Actinobacillus spp. infections is still largely unknown. This thesis demonstrates that the bacteria are present in most horse populations and that interactions between the infectious agent and the host animal is important for the transition from commensal to pathogen.


equine Actinobacillus spp.; Actinobacillus equuli; infectious disease; bacterial infection; horse; foal

Published in

Acta Universitatis Agriculturae Sueciae. Veterinaria
1999, number: 67
ISBN: 91-576-5904-4
Publisher: Swedish University of Agricultural Sciences

Authors' information

Sternberg, Susanna (Sternberg Lewerin, Susanna)
Swedish University of Agricultural Sciences, Department of Veterinary Microbiology

UKÄ Subject classification


URI (permanent link to this page)