Actinobacillus equuli peritonitis and septicemia in an adult horse

Josepha DeLay, Đurđa Slavić

Animal Health Laboratory, University of Guelph, Guelph, ON.

AHL Newsletter 2020;24(3):17.

A 5-year-old thoroughbred gelding was found dead following recent transient (1 day) pyrexia (40^oC).  Gross lesions at postmortem examination included fibrinous peritonitis localized to colonic serosa and severe bilateral nephritis.  Kidneys were swollen and renal capsule was adherent to the cortical surface.  On capsular and cut surfaces of kidney, numerous 1-3 mm tan-white nodules were present in cortex, with few similar lesions in renal medulla (Fig. 1).  Synovial fluid in single carpal and fetlock joints was opaque, with orange-pink discoloration. 

Histologically, renal cortical and medullary parenchyma was replaced multifocally by discrete 0.5-3mm aggregates of neutrophils mixed with clusters of coccobacilli and variable fibrin and hemorrhage (Fig. 2).  Aggregates of similar bacteria filled lumens of adjacent glomerular capillaries and cortical tubules.  Similar clusters of inflammatory cells and bacteria or a few intravascular bacterial aggregates were present in myocardium, brain, adrenal gland, colon, and pancreas.  Actinobacillus equuli ssp. equuli was isolated from kidney and synovial fluid, and A.equuli ssp. haemolyticus was isolated from peritoneum. 

Figure 1. Kidney, gross appearance, cut surface. Numerous tan-white nodules in cortex.	Figure 2. Kidney, histologic section. Nodules identified on gross exam correspond to multiple aggregates of neutrophils mixed with hemorrhage, fibrin, and bacteria. Hematoxylin and eosin stain.

Septicemia and peritonitis are unusual in adult horses, but both conditions have been occasionally described in association with A.equuli.  The organism is a normal inhabitant of the equine oral cavity and intestine.  Opportunistic infection from intestinal mucosal injury or larval nematode migration has been suggested as the pathogenesis of A.equuli peritonitis and septicemia in adult horses.  A.equuli may cause abortion in mares, as well as septicemia in neonatal foals, and has been associated with hemorrhagic diarrhea in foals.  Prominent renal lesions are often present in foals and adult horses with A.equuli septicemia, similar to the lesions described in this horse.

Among AHL pathology cases from 2010-2020, A.equuli septicemia was diagnosed in 12 adult horses, with concurrent peritonitis in 2 horses, and in 6 neonatal foals (1-3 days of age).  Three equine abortion cases during this time period were attritubuted to A.equuli.   AHL

References

1. Patterson-Kane JC, et al. Septicemia and peritonitis due to Actinobacillus equuli infection in an adult horse. Vet Pathol     2001;38:230-232.