Lawsonia intracellularis and virulent Rhodococcus equi infection in a thoroughbred colt.

The study examines a case of a horse suffering from chronic diarrhea and weight loss and concludes that it was caused by the combined infections of Lawsonia intracellularis and Rhodococcus equi.

Background

• The subject of the study was a 26-month-old thoroughbred colt that had been experiencing continuous diarrhea and weight loss for four months.
• The colt was subject to necropsy examination post-mortem, a thorough examination of its body to determine cause of death.

Physical Examination Findings

• Mucosa, or the innermost lining of the intestines, was found to be thickened, particularly affecting the terminal ileum, a segment of the lower small intestine.
• Under a microscope, they found signs of epithelial hyperplasia, an abnormal increase in the number of cells that line the surface of organs, and granulomas, small areas of inflammation.
• They also observed a marked lymphocytic infiltration of the lamina propria, a layer of the mucosa. This indicates a significant immune response in the tissue.

Microscopic Examination Findings and Immunohistochemical Studies

• Argyrophilic bacilli, a type of bacteria that has gram-negative properties and a curved shape, were seen within the cytoplasm of affected enterocytes (intestinal cells).
• Granulomas, small areas of inflammation due to tissue injury, were formed by macrophages and epithelioid cells. Inside these cells, light eosinophilic, foamy cytoplasm and large vacuoles (cellular compartments) were observed, housing gram-positive coccobacilli.
• Immunohistochemical studies suggested the argyrophilic bacilli were Lawsonia intracellularis bacteria and the gram-positive coccobacilli were Rhodococcus equi bacteria.

Lab Testing

• The researchers ran a Polymerase Chain Reaction (PCR) test, a technique used to amplify selected sections of DNA, on the affected ileocaecal mucosa tissue. They found fragments of DNA specific to Lawsonia intracellularis, confirming its presence.
• They were also able to isolate pure cultures of virulent Rhodococcus equi, characterized as VapA positive, from liver and mesenteric lymph nodes, confirming its presence as well.

Conclusion

• The combined infections of the two intracytoplasmic organisms, Lawsonia intracellularis and Rhodococcus equi, had induced a severe disease state in the horse.
• The disease was characterized by multifocal proliferative and granulomatous enteritis (gut inflammation) accompanied by intense and widespread infiltration of lymphocytes, a type of white blood cell that is part of the immune response.