Persistence of Campylobacter fetus subsp. venerealis in the reproductive tract of vaccinated heifers after experimental infection: Insights into vaginal mucosal immunity in heifers.

Bovine genital campylobacteriosis (BGC) remains a major cause of reproductive failure in beef cattle systems. This study evaluated the persistence of Campylobacter fetus subsp. venerealis (Cfv) and associated lesions in the reproductive tract of heifers four months after experimental infection following prior immunization. Thirty-three heifers were randomly assigned to three groups (n = 11 each): two vaccinated groups receiving inactivated bacterin formulations and a non-vaccinated control group. All animals were experimentally challenged intravaginally with a regional Cfv strain. At four months post-infection, cervicovaginal mucus and reproductive tissues were collected for bacteriological culture, histopathology, and immunohistochemistry. Cfv was recovered in all experimental groups. Although non-vaccinated heifers showed a higher frequency of positive cultures and immunohistochemistry detection, vaccinated animals also remained culture-and IHC-positive at four months post‑infection, suggesting that vaccination did not induce sterilizing immunity and did not prevent persistence of Cfv within the reproductive tract Immunohistochemistry localized Cfv in the vaginal mucosa, supporting its role as primary site of long‑term colonization. Histopathological lesions were detected across all groups and were mainly characterized by mild to moderate lymphohistiocytic infiltrates affecting the vagina, cervix, and uterus. Immunophenotyping in vaginal fundus revealed a predominance of CD68⁺ macrophages, with scarce CD3⁺ T lymphocytes and minimal CD79a⁺ B-cell labeling, suggesting a limited and poorly organized mucosal immune response with minimal involvement of adaptive immune cell populations. Overall, these findings demonstrate that Cfv persists in the bovine reproductive tract for at least four months despite prior vaccination. The predominance of macrophage‑dominated inflammation and the absence of sterilizing immunity highlight limitations of current bacterin vaccines and underscore the need for improved vaccination strategies capable of more effectively targeting vaginal mucosal immunity to enhance control of BGC.