2-month-old female Belgian warmblood foal (Equus ferus caballusThe foal had a sudden onset of diarrhea and fever (40°C). Blood ex-amination revealed elevated liver values and abdominal ultrasound showed edema of the colon wall. Despite treatment the foal died quickly.

Gross Description:  

The foal was admitted for necropsy and postmortem examination re-vealed a good nutritional condition, and moderate dehydration. Petechial bleedings were noticed on the pleura, the pericardium, the thymus, the splenic capsule and on the serosa of the intestine. The kidneys showed numerous cortical white to gray foci and congestion of the medulla. The intestines were dilated with an edematous wall and a mucoid gray content.

Histopathologic Description:

Kidney: Randomly scattered within the cortex and occasionally extending into the medulla, there are numerous embolic microabscesses (0.3-0.40 mm in diameter) that regularly center on and efface glomeruli. These abscesses are composed of abundant ne-crotic debris (karyorrhexis, karyolysis, and pyknotic nuclei), admixed with many degenerate and non-degenerate neutrophils, fewer macrophages, lymphocytes and plasma cells. Multifocally within these mic-roabscesses, there are large colonies of basophilic coccobacilli (1x2 µm). Abscesses occasionally extend into adjacent inter-stitium and tubules, with degeneration and necrosis of tubular epithelium. There are multifocal areas of congestion, hemorrhage, and fibrin thrombi within vessels.

Morphologic Diagnosis:  

Kidney: Acute, severe, suppurative, embolic nephritis with intralesional coccobacilli.

Lab Results:  

Bacteriology of the kidney: positive for Actinobacillus equuli subsp. Haemolyticus Parasitology of the feces: positive for strongyles.


Suppurative embolic neprhitis/Actinobacillus equuli

Contributor Comment:  

Kidney: Acute, severe, suppurative, embolic nephritis with intralesional coccobacilli. whether such strains are common in-habitants of the equine gastrointestinal and respiratory tracts.5 Two subspecies of Actinobacillus equuli have been identified: A. equuli subsp. equuli, and A. equuli subsp. haemolyticus.1 The former appears to be pathogenic, while the latter’s pathogenicity appears to be associated with its expression of a repeats-in-structural-toxin (RTX) called Aqx, which is cytotoxic for equine leukocytes.1 Typically actinobacillosis is a disease of newborn foals and the pathogenesis of the infection remains speculative. Infection is probably acquired in utero, during parturition, or shortly after birth as an um-bilical infection.1 Death may occur due to fulminating septicemia. In foals that survive for several days, microabscesses are seen in the kidney and other organs and a poly-arthritis can be present. These micro-abscesses have an embolic origin and are characterized by the presence of numerous, 1-3 mm, white pinpoint foci on the cut surface throughout the renal cortex. Micro-scopically, glomerular capillaries contain numerous bacterial colonies intermixed with necrotic debris and extensive infiltrates of neutrophils that often obliterate the glomerulus.4 The lesions in our case are classic for Actinobacillus equuli, and the foal was also positive for strongyles. It has been post-ulated that migrating strongyle larvae from the intestinal tract may play a role in infection.

JPC Diagnosis:  

Kidney, cortex and medulla:  Nephritis, embolic, suppurative, acute, severe, with large colonies of coccobacilli.

Conference Comment:  

Despite some slide variability, the histopathologic appearance of this lesion is a classic for suppurative and embolic nephritis caused by Actinobacillus equuli. This entity is the most common cause of suppurative and embolic nephritis in young horses.1 In pigs, embolic nephritis is most commonly caused by Erysipelothrix rhusiopathiae. In cattle, Trueperella pyo-genes from valvular endocarditis causes numerous septic emboli, which shower the renal cortex causing randomly distributed microabscesses and infarcts. Corynebacterium pseudotuberculosis is most com-mon in sheep and goats, Pasteurella multocida in rabbits, and Streptococcus moniliformis in mice.1,4 In dogs, Prototheca zopfii organisms have been identified as a common cause of embolic nephritis secondary to systemic protothecosis.1  Endotoxin expressed by gram-negative bacteria and Streptococcus sp. causes endothelial damage, vasculitis, and bacterial emboli.1,4 Most conference participants noted ectatic tubules containing necrotic and sloughed tubular epithelial cells, fibrin, hemorrhage, and proteinaceous fluid.  Participants also noted occasional fi-brin thrombi with colonies of coccobacilli within glomerular tufts, as well as parietal cell hyperplasia secondary to the effects of endotoxin. This case illustrates the characteristic appearance of the large colony-forming coccobacilli, Actinobacillus equuli, in tissue section. In addition to discussing causes of embolic nephritis in other species, conf-erence participants also reviewed other bacteria that form large colonies in tissue. These bacteria are difficult to distinguish from one another other on hematoxylin and eosin stain (H&E), and require special stains or bacterial culture.1 Gram-positive large colony forming bacteria include: Staphylococcus, Streptococcus, Actinomy-ces, and Corynebacterium spp.; while gram-negative large colony forming bacteria include Yersinia and Actinobacillus spp.1,4 Several conference members men-tioned the acronym, YAACSS, as a helpful mnemonic device to remember which bacteria form large colonies in tissue section.


1. Cianciolo RE, Mohr FC, Urinary system, In: Maxie MG,
ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016: 432-433.

2. Frey J. The role of RTX toxins in host specificity of animal pathogenic Pasteurellaceae. Vet Microbiol. 2011; 153:51-58. WSC 2012-2013.

3. Matthews S, Dart AJ, Dowling BA, et al. Peritonitis associated with Actinobacillus equuli in horses: 51 cases. Aus Vet J. 2001; 79:536-539.

4. Newman SJ: Urinary System. In: eds. McGavin MD, Zachary JF: Pathologic Basis of Veterinary Disease.  5th ed. St. Louis, MO: Mosby Elsevier; 2012:649.

5. Patterson-Kane JC, Donahue JM, Harrison LR. Septicemia and peritonitis due to Actinobacillus equuli infection in an adult horse. Vet Pathol. 2001; 38:230-232.

Click the slide to view.

3-1. Kidney, foal.

3-2. Kidney, foal.

3-3. Kidney, foal.

3-4. Kidney, foal.

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