Listeria monocytogenes meningoencephalomyelitis most likely due to septic spread as a rare cause of neurological disease and fever in an adult horse

A 14-year-old warmblood mare developed progressive neurological signs including ataxia, nystagmus, impaired mental status as well as decreased cranial nerve reflexes, and finally went recumbent. Postmortem examination and histology revealed severe chronic active multifocal suppurative meningoencephalomyelitis with multifocal fibrinoid degeneration and necrosis of the vessel walls, multiple microabscesses and pyogranulomas. Additionally, there was mild multifocal necrotising to pyogranulomatous hepatitis and moderate multifocal suppurative to pyogranulomatous interstitial nephritis with fibrinoid degeneration of the vessel walls. Routine diagnostic procedures did not reveal any viral or bacterial aetiology; however, listerial enrichment with subsequent matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis and 16S rRNA sequencing confirmed infection with Listeria monocytogenes. Listeriosis is uncommonly reported as a cause of sepsis in foals and rarely in adult horses. Furthermore it is rarely reported causing meningoencephalitis in foals as well as in adult horses. To the authors’ knowledge, this is the first case of meningoencephalomyelitis most likely due to septic distribution in an adult horse.