Endometrial gland remodeling and preserved luminal ultrastructure in postpartum cows infused with ozonated sunflower oil.

Postpartum uterine disease is a major cause of impaired bovine reproduction and is usually managed with intrauterine antibiotics. Alternative non‑antibiotic therapy is needed to support uterine health without contributing to antimicrobial resistance. The goal of this study was to characterize histomorphometric and ultrastructural changes of the bovine endometrium after intrauterine infusion of ozonated sunflower oil during early puerperium, and to assess its in vitro antimicrobial activity against uterine bacteria. Eighteen primiparous cows received a single intrauterine treatment with ozonated or non‑ozonated oil (O3 and non‑O3 groups; n=9/group) on day 10 postpartum. Endometrial biopsies were collected before and 15 days post-treatment (D0 and D15) for light microscopy and scanning electron microscopy. Uterine cytology was used to evaluate subclinical endometritis. Minimum inhibitory and bactericidal concentrations (MIC and MBC) of the treatments were determined against uterine isolates (Escherichia coli, Staphylococcus spp., Streptococcus spp. and Arcanobacterium pyogenes). At D15, the number of endometrial glands was higher in the O3 group than in the non‑O3 group (32.6±2.5 vs. 11.0±1.0; P<0.01), whereas glandular diameter, luminal area and cell number per gland were lower in O3 cows (P<0.01), indicating enhanced gland proliferation with reduced glandular dilatation. No degenerative histopathological changes, such as periglandular fibrosis or endometrial atrophy, were detected in either group, and scanning electron microscopy showed preserved apical ultrastructure without adverse effects of ozonated oil. Ozonated oil inactivated all uterine bacteria (MIC and MBC ≤0.18 µg mL‑1), while non‑ozonated oil showed no antimicrobial activity. Both groups showed a marked reduction in neutrophil percentage by D15, but histological evidence of persistent inflammation was more pronounced in non‑O3 cows. These findings indicate that intrauterine ozonated sunflower oil is a promising non‑antibiotic therapy to support postpartum uterine involution and endometrial recovery in dairy cows and may contribute to improved reproductive performance.