Neospora-Like Encephalomyelitis in a Calf: Pathology, Ultrastructure, and Immunoreactivity

Abstract

Protozoal encephalomyelitis was diagnosed in a 3-day-old calf that was stunted, weak, and recumbent. Grossly, the calf had contracted tendons in the forelegs, a slightly domed skull, a porencephalic cyst in the cerebellum, ulcerative esophagitis, and abomasitis. Histologically, there was a multifocal nonsuppurative encephalomyelitis with clusters of protozoal tachyzoites and numerous protozoal cysts. The porencephalic cyst and gastrointestinal lesions appeared to be unrelated to the protozoal infection and were suggestive of a concurrent bovine virus diarrhea infection. A few groups of protozoal tachyzoites and numerous tissue cysts were found in neuropile, particularily in neurons of the spinal cord. By light microscopy, smaller tissue cysts were found in the brain (majority from 14 to 20 pm) than in the spinal cord (majority from 20 to 48 pm). The cyst walls ranged in thickness from < 1 pm to a maximum of 2 pm wide. Bradyzoites contained PAS-positive slender bradyzoites (5–8 × 1–2 μm). Tissue cysts reacted positively to anti-Neospora caninum sera; but unlike N. caninum, they were positive to 2 of 4 antisera against Toxoplasma gondii and to antisera to H. hammondi. Ultrastructurally, tissue cysts closely resembled a Neospora-like organism, including the finding of intemeuronal protozoal cysts, thick cyst walls, a lack of micropores in the bradyzoites, and the presence of numerous micronemes oriented perpendicular to the pellicle. Ultrastructural features in the calf protozoan that have not been reported for N. caninum in dogs included the presence of numerous tubulovesicular structures in the cyst ground substance and bradyzoite vesicles that contained small vesicular structures and short, flat membrane segments. The differences in antigenic reactivity and the minor morphologic differences between this bovine protozoan and N. caninum may reflect variations of a single organism dependent on factors such as the hosts species infected, the tissue infective stage studied, etc., or may indicate protozoal species/strain differences. The results also demonstrate antigenic cross-reactivity between this bovine apicomplexan, N. caninum, and T. gondii.