Naturally Occurring Tyzzer's Disease in a Foal

Tyzzer's disease, an acute multifocal bacterial hepatitis caused by Bacillus piliformis, is a fatal disease of young animals that has been reported in a number of domestic, wild and laboratory animals. This article describes the finding of Tyzzer's disease in a foal.

Case Report: The reported history was as follows: An 8 day old, male foal was born seven days premature to a 5 year old maiden mare. A few days after birth, the foal was sluggish and had a temperature of 96° F. The foal was treated with intravenous dextrose, Lactated Ringer's Solution® (LRS), Bana-mine®, and Gentocin®. The foal appeared to respond well to treatment. The temperature returned to 100ºF and the foal was up and nursing. The following morning the foal became weak and died upon transport to the veterinary clinic.

The gross examination was performed by the referring veterinarian. The only gross lesion observed was severe icterus.

Multiple tissue samples including liver, kidney, heart, spleen, and lung from this foal were submitted to the Animal Disease Diagnostic Laboratory. Histologically, the liver contained multiple coalescing foci of coagulative hepatocellular necrosis scattered throughout the parenchyma. The centers of these foci were composed of pyknotic nuclei, karyorrhectic nuclei, pink fibrillar material, and rare erythrocytes. The periphery of the necrotic foci was infiltrated by moderate to large numbers of degenerate neutrophils. Hepa-tocytes bordering these necrotic foci contained intracytoplasmic basophilic filamentous bacterial rods, which stained faintly with hematoxylin and eosin (H&E). Giemsa and Warthin Starry stains revealed numerous filamentous bacilli arranged in parallel and interlacing bundles within the cytoplasm of hepatocytes. In addition to the hepatic lesions, moderate suppurative subendocardial inflam-mation was observed. The lung and kidney were microscopically unremarkable.

Discussion: The microscopic lesions and identification of filamentous bacilli (Bacillus piliformis) with H&E, Giemsa and Warthin Starry stains were consistent with a diagnosis of Tyzzer's disease.

Tyzzer's disease is not common in horses but has been reported in foals between 7 and 40 days of age. The pathogenesis is not well understood. The organisms are most likely picked up by the foal from the environment. The proposed pathogenesis involves an oral route of infection resulting in a primary enteric infection and subsequent dissemination via the portal circulation to other tissues, especially the liver. Rodents, rabbits, cats, and non-clinically infected horses may serve as reservoirs. Because the dam may act as the source of the infection, subsequent foals should be monitored carefully and treated appropriately. In conclusion, the disease is sporadic, and as such, specific control and preventative measures are not commonly indicated.

- by Lavun Anothayanontha, DVM

- edited by M. Randy White, DVM, PhD