Timing of Fasciola hepatica infection selectively impairs antibody avidity after foot-and-mouth disease vaccination in cattle

Introduction Fasciola hepatica is a trematode parasite of ruminants that induces potent immunoregulatory responses, potentially compromising vaccine-induced immunity. This study evaluated the effect of F. hepatica infection and its timing on immune responses elicited by foot-and-mouth disease virus (FMDV) vaccination in cattle. Methods Fifty-two one-year-old calves free of maternal FMDV antibodies were allocated to five experimental groups (10–11 animals/group). Animals were experimentally infected with F. hepatica either before or after FMDV vaccination and compared with antiparasitic-treated and non-infected control groups. Growth performance, parasitological parameters, hematological and biochemical indicators of liver function, and vaccine-induced humoral and cellular immune responses were evaluated throughout the study. Results Although total anti-FMDV antibody titers and IgG1 levels were not significantly affected by F. hepatica infection, calves infected after vaccination developed significantly lower IgG1 antibody avidity indices, indicating impaired antibody maturation. This reduction was accompanied by diminished vaccine-induced cellular immune responses and persistent biochemical evidence of hepatic dysfunction during chronic infection. Triclabendazole treatment restored antibody avidity and partially recovered productive performance, although some metabolic alterations persisted. Conclusions F. hepatica selectively impaired the qualitative maturation of the immune response to FMDV vaccination rather than the magnitude of antibody production. These findings highlight the importance of infection timing and effective parasite control to optimize vaccine efficacy in cattle and suggest that antibody avidity may represent a more sensitive indicator of helminth-induced vaccine interference than antibody titers alone.