Pyogranulomatous meningoencephalitis due to Actinomyces sp. in a dog.

Actinomyces sp. are commensal, filamentous, gram-positive, acid-fast-negative bacteria that can cause pyogranulomatous inflammation in animals and humans. Central nervous system (CNS) disease is a rare presentation of actinomycosis and is usually due to extension from infected wounds or seeding from distant sites. A dog with progressive, poorly localized neurologic signs had primary CNS actinomycosis without history or evidence of previous trauma or other organ involvement. Histologically, there was a severe pyogranulomatous meningoencephalitis with intralesional filamentous bacteria that were also visible on cytology of the cerebral spinal fluid (CSF) postmortem. Actinomyces sp. was cultured postmortem from the CSF, confirming the diagnosis. This case demonstrates that Actinomyces sp. can be a causative agent of primary CNS disease in dogs.