Naturally-occurring Tyzzer’s disease (Bacillus piliformis infection) in horse foals.

This research discusses Tyzzer’s disease, a rare bacterial infection in quarter horse foals that caused sudden death without prior signs of illness. The study presents significant findings from autopsies, detailing observable and microscopic changes in the animals’ livers, intestines, and lymph nodes.

Introduction to Tyzzer’s Disease

• The study focuses on Tyzzer’s disease, a relatively rare infection caused by the bacteria Bacillus piliformis, in quarter horse foals. This condition emerged spontaneously and led to sudden death in the affected animals, without any visible signs of illness before their death.
• The disease is of interest due to its sudden onset and the severe impact it had on the young horses, causing serious concern for both breeders and veterinarians.

Autopsy Findings

• Autopsies revealed an array of gross findings, which included icterus (jaundice), focal pale areas in the liver, and catarrhal enterocolitis (inflammation of the digestive tract) in the deceased foals.
• In one foal, the small intestine contained focal dark red lesions, and in another, enlarged and hyperemic (increased blood flow) mesenteric lymph nodes were found.
• Upon histopathological examination, the researchers observed multiple signs of disease in the livers of the foals, including discrete and confluent foci of necrosis (dead tissue), fat accumulation, congestion of the sinusoids, and haemorrhaging.
• Within the liver cells at the margins of these lesions, bundles of bacilli (rod-shaped bacteria) were found, pointing towards a bacterial infection.
• Other changes included inflammation in the connective tissue areas of the liver (portal triads), along with degeneration and excessive growth (hyperplasia) of the ducts within the liver (biliary ducts).

Intestinal and Lymphatic System Findings

• The intestines showed similar signs of infection and damage, including the death of mucosal cells, inflammatory cell infiltration, an increase in mucus production, and growth of lymphatic tissue within the small intestine’s walls (submucosal lymphoid hyperplasia).
• In one case, submucosal haemorrhage was also detected within the small intestine.
• The spleen and mesenteric lymph nodes, components of the horse’s immune system, also showed signs of damage, including necrosis of the lymph follicles, congestion, and haemorrhaging.