Granulomatous typhlocolitis, lymphangitis, and lymphadenitis in a horse infected with Listeria monocytogenes, Salmonella Typhimurium, and cyathostomes.
Authors: Nemeth N M, Blas-Machado U, Hopkins B A, Phillips A, Butler A M, Sánchez S
Journal: Veterinary pathology
Summary
# Editorial Summary A 15-year-old Quarter horse mare presented with severe, treatment-resistant diarrhoea that ultimately proved fatal, prompting pathological investigation of what transpired to be a complex multifactorial colonic disease. Necropsy revealed thickened caecal and colonic walls with scattered nodular lesions extending into enlarged colic lymph nodes; histopathological examination identified severe granulomatous typhlocolitis alongside lymphangitis and lymphadenitis, with PCR and immunohistochemical analysis confirming intralesional *Listeria monocytogenes* alongside concurrent *Salmonella enterica* serovar Typhimurium infection detected via both culture and PCR, plus a minor cyathostome burden. Whilst *Listeria* infection remains exceptionally uncommon in equine practice, this case demonstrates how polymicrobial enteric infections—particularly the synergistic combination of bacterial pathogens and small strongyle parasitism—can precipitate severe granulomatous inflammatory disease of the large colon that resists conventional antimicrobial therapy. Clinicians encountering horses with refractory colitis and poor therapeutic response should consider broadened differential diagnoses beyond primary salmonellosis, including opportunistic pathogens like *Listeria*, and recognise that concurrent parasitic burdens may substantially complicate pathogenesis and treatment outcomes.
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Practical Takeaways
- •Persistent diarrhea unresponsive to standard therapy should prompt investigation for unusual coinfections, including Listeria monocytogenes and concurrent parasitic infections beyond routine diagnostics.
- •Consider comprehensive diagnostic testing (PCR, culture, and histopathology) in cases of severe colitis with poor response to treatment, as multiple concurrent pathogens may be present.
- •Aggressive parasite control and sanitation protocols are critical, as concurrent parasitic infections may potentiate bacterial colitis and worsen prognosis.
Key Findings
- •A 15-year-old Quarter horse presented with severe diarrhea caused by concurrent infection with Listeria monocytogenes, Salmonella enterica serovar Typhimurium, and cyathostomes, resulting in granulomatous typhlocolitis.
- •Histopathology revealed severe granulomatous inflammation of the cecum and colon with nodules extending into enlarged colic lymph nodes, confirmed by PCR and immunohistochemistry.
- •Listeria monocytogenes infection in horses is rare; coinfection with Salmonella and small strongyles likely contributed to the development of the severe granulomatous inflammatory response.
- •The horse showed poor response to therapy and required euthanasia, highlighting the severe nature of this multi-pathogen gastrointestinal infection.