Dopaminergic neuropathology in calves naturally infected with Neospora caninum.

Neospora caninum is a protozoan parasite that causes abortion and congenital neurological disorders in cattle. Although its reproductive effects are well established, the neurochemical alterations associated with congenital infections remain unclear. This study aimed to evaluate the spatial relationship between N. caninum antigen distribution and the expression of dopaminergic markers in the brains of naturally infected neonatal calves. Brain tissues from infected and healthy calves were examined histopathologically and immunohistochemically. Immunohistochemistry was performed using antibodies against N. caninum, tyrosine hydroxylase (TH), dopamine receptor D1 (DRD1), and dopamine transporter (DAT). Regional immunoreactivity was semi-quantitatively assessed and compared between groups. Antigen positivity was observed only in infected calves, mainly within neuronal cytoplasm, and was most prominent in the forebrain and cerebral cortex. TH expression was markedly increased in these regions, while DRD1 and DAT expression was moderately reduced or regionally altered. Tissue cysts were absent except in one case. These findings indicate a region-specific dopaminergic imbalance associated with parasitic localization. This pattern is similar to previously described changes in Toxoplasma gondii models and suggests that N. caninum may also impair dopaminergic regulation in the developing brain. Further behavioral and molecular studies are warranted to explore the neurochemical consequences of congenital neosporosis.