Effect of calving season on metritis incidence and bacterial content of the vagina in dairy cows

Heat stress and uterine diseases, including metritis and endometritis, both reduce milk yields and reduce reproductive performance. Bacterial growth is promoted by elevated temperature while heat stress reduces host immune cell function, but it is not known whether increased environmental temperature promotes uterine disease by altering host immunity or bacterial growth. We hypothesize that seasonal variations in environmental temperature influence metritis incidence in the dairy cow independent of bacterial prevalence in the reproductive tract. To investigate how environmental temperature may impact metritis incidence, records of 3507 calvings in Florida over a 5-year period were evaluated. The incidence of metritis increased from 21.1% in the cool season (October through March) to 24.2% during the warm season (April through September, P < 0.05). To elucidate a link between environmental temperature and uterine disease, 102 cows were enrolled during the warm season (September 2017; n = 51) and cool season (February–March 2018; n = 51). Cows were maintained on pasture during the dry period and moved to free stall barns with fans and water soakers immediately prior to calving and remained in that environment after calving. Vaginal mucus was collected and scored on days 7 (to evaluate metritis) and 21 (to evaluate endometritis) postpartum to evaluate the incidence of uterine disease and quantify bacterial content and species using qPCR. Daily milk yield for the first 60 DIM was reduced during the warm season compared with the cool season (32.6 ± 1.62 vs 37.23 ± 1.60 kg, P < 0.05) consistent with effects of prepartum heat stress. Interestingly, more cows had persistent uterine disease on both d 7 and d 21 in the warm season compared with the cool season (58.0 vs 29.4%, P < 0.05). Regardless of calving season the total bacterial content in the vagina was greater on d 7 compared to d 21. While metritis incidence was increased in the warm season, the vaginal content of total bacteria, Escherichia coli, Trueperella pyogenes, Fusobacterium necrophorum and Prevotella melaninogenica were similar during the cool season and the warm season. Our data suggests that prepartum heat stress related to season of calving increased the incidence of metritis and persistence of uterine disease in the dairy cow independent of vaginal bacteria content. The possibility that prepartum heat stress perturbs host immune function and increases the risk of metritis when cows are exposed to an equivalent number of pathogenic bacteria requires further investigation.