Collection of uterine secretion samples
Cows were selected from a standardized dairy farm at Qianjin Farm (Zhangye, China). Rectal palpation and cervicovaginal mucus observation were conducted to identify healthy animals or cows with endometritis. The opening of the cervix could be felt distinctly during a rectal examination. In repeat breeder dairy cows within three months postpartum, a thick uterine horn, high uterine secretions, and a large amount of yellow or white pus discharged from the vagina were considered as evident symptoms of clinical endometritis (i.e., presence of purulent or mucopurulent vaginal discharge) (Figure 1A). However, the cows with endometritis did not show symptoms of other local or systemic diseases (Sheldon et al., 2006, Williams et al., 2005). In the chemical examination, the uterine secretion was boiled with 4% sodium hydroxide. After cooling, uterine secretion from healthy cows was colorless, whereas a slightly yellow color indicated positivity for endometritis. All cows were fed and managed routinely (Figure 1B) (Li et al., 2010).
A 0.1% potassium permanganate solution was used to disinfect the vulva of the cattle. Three milliliters of secretion from the uteri of cattle were extracted through a sterile, one-time-use fertilization tube and stored in aliquots at −20 °C until further analysis.