Authors: Hollyer Jennifer A, McGuinness Eamon, Bowers Lisa C, Didier Elizabeth S, Giudice Chiara, Perl Daniel P, Fogarty Ursula
Journal: Journal of equine veterinary science
Summary
# Editorial Summary A previously unreported case of *Encephalitozoon cuniculi* genotype II causing granulomatous meningoencephalitis in a horse prompted investigation when standard diagnostic panels for common equine neurological pathogens—including West Nile virus, EHV, EIA, *Toxoplasma gondii*, *Neospora caninum*, and *Sarcocystis neurona*—returned negative results. Post-mortem examination and molecular analysis (PCR with sequencing and immunohistochemistry) confirmed *E. cuniculi* genotype II in the affected cerebral cortex, marking the first documented case of this organism causing encephalitis in equines. Whilst *E. cuniculi* is primarily known as a pathogen in rabbits and other small mammals, this case demonstrates that horses can develop clinical neurological disease from this microsporidia and should alert practitioners to consider atypical infectious agents when conventional differential diagnoses are excluded. The identification of genotype II—distinct from other *E. cuniculi* strains—may have implications for transmission patterns and clinical presentation, though further epidemiological work is needed to understand the clinical significance and prevalence of this organism in equine populations.
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Practical Takeaways
- •Consider Encephalitozoon cuniculi as a differential diagnosis in horses with encephalitis of unknown etiology, particularly when common viral and parasitic causes have been ruled out
- •E. cuniculi genotype II is now confirmed as a potential equine neurological pathogen; contact with infected small mammals or contaminated feed/water may be a transmission route
- •Definitive diagnosis requires histopathology with immunohistochemistry and/or PCR testing of neural tissue, as serological titers alone may not confirm active CNS infection
Key Findings
- •Postmortem examination revealed nonsuppurative granulomatous meningoencephalitis in the right cerebral cortex hemisphere
- •PCR and immunohistochemistry confirmed Encephalitozoon cuniculi genotype II in affected brain tissue
- •Serological testing was negative for West Nile virus, equine herpes virus, equine infectious anemia, Toxoplasma gondii, Neospora caninum, and Sarcocystis neurona
- •This represents the first documented case of E. cuniculi genotype II associated with encephalitis in equine species