Systemic humoral immune responses and metabolic parameters in chronically Neospora caninum-infected primiparous Holstein heifers during the transition period.

Bovine neosporosis has emerged as a major cause of reproductive losses in dairy cattle worldwide, representing a relevant concern for animal welfare and health. The transition period in dairy cattle is characterized by physiological, metabolic and immunological changes. However, the specific humoral immune response against Neospora caninum has been only partially characterized during this critical period, raising the hypothesis that transition-associated physiological changes may influence the humoral response to chronic N. caninum infection. The aim of this study was to characterize changes in N. caninum-specific total IgG, IgG1/IgG2 ratio, and avidity in serum samples from chronically infected Holstein heifers (n = 7) at prepartum, calving, and postpartum sampling times. Additionally, temporal profiles of NEFA, cholesterol, GOT, GPT; total proteins and albumin between chronically N. caninum-infected and seronegative Holstein heifers (n = 5) were described. Specific total IgG antibody levels declined progressively from prepartum to postpartum (p < 0.05), with intermediate values observed at calving. The IgG1/IgG2 ratio was significantly reduced at calving compared to prepartum (p < 0.05), indicating a transient shift in the IgG subclass balance around calving. No significant differences were detected in avidity. Regarding metabolic indicators during the transition period, no significant interactions between serostatus and time were observed for any parameters except for NEFA concentrations (p < 0.05). The present study demonstrates that chronically N. caninum-infected Holstein heifers undergo measurable changes in humoral immune parameters across the transition period. Total specific IgG declined progressively from prepartum to postpartum, and the IgG1/IgG2 ratio was transiently reduced at calving, indicating a shift in antibody subclass balance rather than a uniform decline across all IgG-related parameters. Antibody avidity remained stable throughout the sampling period. Noteworthy, this humoral response to N. caninum infection did not appear to modify the metabolic status characteristic of the transition period: significant serostatus × time interaction was observed for cholesterol, GOT, GPT, total proteins, or albumin. This indicates that the immunological changes associated with chronic N. caninum infection occur largely independently of the metabolic adaptations that dairy heifers undergo during the periparturient period, and do not appear to exacerbate the metabolic stress typically observed during this stage.