Meegan Larsen

Animal Health Laboratory, University of Guelph, Guelph, ON

AHL Newsletter 2022;26(4):12.

Several ewes aborted approximately one month prior to the expected due date.  Twin fetuses and their placenta were submitted to AHL for postmortem examination.  No significant gross lesions were noted.   Microscopically, there was necrosis and neutrophilic inflammation in the placental chorionic villi with intralesional bacteria and evidence of vascular injury (Fig. 1). No significant microscopic lesions were seen in the two fetuses.  Large numbers of Actinobacillus seminis and Histophilus somni were isolated from the placenta, and with supportive microscopic lesions of necrotizing and suppurative placentitis with vasculitis, abortion was attributed to infection with these two bacterial species.

Actinobacillus seminis is recognized as a common cause of epididymitis and associated infertility in rams.  Metritis, mastitis, posthitis and polyarthritis have also been attributed to infection by this bacterium in sheep.  A. seminis was first documented as a cause of abortion in an outbreak on a sheep farm in the United Kingdom in 1999, and was recently identified as a small ruminant abortifacient bacterium for the first time in the United States in a goat herd (personal communication with Maria Spinato regarding a report at the 2022 AAVLD Conference).  Histophilus somni is a commensal bacterium of mucus membranes in ruminants and a possible, although uncommon, cause of abortion in small ruminants.  Placental lesions with A. seminis abortion tend to be those of suppurative inflammation of the membranes and cotyledons, whereas H. somni infection is typically associated with vascular injury.  Both microscopic lesions were seen in the placenta from the aborted lambs in this case, and thus both bacterial species are considered contributory etiologic agents.   AHL

Figure 1.  Fibrin and neutrophilic inflammation (black arrow) and inflammation of blood vessel wall (blue arrow) in histologic section of placenta.  H&E.

Reference

1. Foster G, et al. Actinobacillus seminis as a cause of abortion in a UK sheep flock. Vet Rec 1999;144:479-480.