Prevalence, risk factors, and effects on fertility of cytological endometritis at the time of insemination in Norwegian Red cows.

The present study aimed to assess the occurrence of cytological endometritis (CYTO), a nonsymptomatic inflammation of the endometrium, at first artificial insemination (AI) postpartum in Norwegian Red cows. Further, risk factors for CYTO manifestation and its effect on reproductive success and late embryo loss were evaluated. In total 1,648 cows located in 116 herds were included in the study. On mainly spontaneous estrus, endometrial cytology samples were collected using a cytotape technique, and a total of 300 representative epithelial cells and polymorphonuclear neutrophils (PMN) were counted at 400× magnification. Vaginal mucus obtained by Metricheck (Simcro) and body condition score were recorded. Milk samples for progesterone analysis were collected at AI and 21 d later. Pregnancy was diagnosed by rectal palpation or analysis of pregnancy-associated glycoproteins. Based on the constructions of a receiver operator characteristics curve, the cut-off level for PMN defined as CYTO was set to 3.0%, representing the level at which the PMN occurrence affected pregnancy outcome, with the highest summation of sensitivity (32.4%) and specificity (74.9%). Three logistic models with herd included as random factor were constructed. The outcome for the first model was the likelihood for CYTO based on the endometrial samples, in the second model pregnancy to first AI, and in the third model embryo loss. The proportion of CYTO was 28.0% (461/1,648). The average interval in days to first AI was 71.7 d (standard error ± 0.7) and the overall pregnancy incidence to first AI was 59.8% (866/1,449). The likelihood for CYTO at first AI was associated with AI personnel, calving to first AI interval, vaginal mucus characteristics, amount of red blood cells in sample, season, and barn type. Pregnancy to first AI was lower in CYTO-positive cows (odds ratio = 1.51, confidence interval = 1.17-1.94). Other factors affecting pregnancy to first AI were AI personnel, test day milk yield, barn type, and obstetrical conditions or fertility treatments before first AI. The proportion of late embryo loss and abortion was 8.6% (82/948) and 2.8% (24/866), respectively. Late embryo loss was associated with treatment against fertility disorders before first AI, but not associated with CYTO. Overall, our results suggest that even if Norwegian Red cows show a fairly high prevalence of CYTO in the endometrium at first AI, it does not seem to have a major effect on the reproductive performance. The Norwegian Red breeding program has emphasized fertility and health for decades, and a genetically advantageous uterine immunology might be one of the preserved mechanisms.