Idiopathic systemic granulomatous disease with encephalitis in a horse.

This study describes a case of an adult horse suffering from a rare variant of Idiopathic Systemic Granulomatous Disease (ISGD) where, along with systemic inflammation, the brain was affected by severe encephalitis – an inflammation not previously reported in horses with ISGD.

The Case

• In this study, the subject was a 14-year-old standardbred mare showing symptoms of acute bronchitis.
• The mare had to be euthanized due to rapidly progressing central nervous disturbances.

Post Mortem Observations and Diagnosis

• Necropsy (autopsy for animals) conducted after the mare’s death revealed systemic granulomatous inflammation.
• This inflammation was found to also involve the blood vessels (vasculitis) in multiple parts such as the lungs, thoracic lymph nodes, ribs, and liver.
• In the brain, particularly the cerebrum, severe subacute bilateral encephalitis was observed with malacia mainly affecting the white matter, and vasculitis with perivascular infiltration of lymphocytes, macrophages, and giant cells.
• No causative infectious agent could be identified through various techniques, including staining with Ziehl-Neelsen, Grocott, or Giemsa stains, periodic acid-Schiff reaction of tissue sections and bacterial and fungal cultures.
• Suspecting an immune-mediated cause, the final diagnosis was made as Idiopathic Systemic Granulomatous Disease (ISGD), a condition where inflamed granulomas occur throughout the body without a known cause.

Significance and novelty of the case

• The inflammation of the brain or encephalitis in this case of ISGD has not been reported before in horses. This suggests the disease can manifest with varying symptoms.
• Usually, ISGD also involves skin symptoms, which were absent in this case. This further highlights the atypical nature of this specific case of ISGD.