Uterine disease impacts the dairy industry by reducing milk yield and fertility. Approximately 7d after calving 30% of cows develop metritis due to bacterial infection, that may persist beyond 21d as endometritis. Although heat stress has a negative impact on milk production and immune function, little is known about its effect on uterine health. The focus of this study was to assess uterine disease incidence and quantify bacterial load in vaginal mucus samples of dairy cows during a cool (n = 51, avg max temp: 23°C) and hot season (n = 51, 31°C). We hypothesize that during the hot season uterine disease incidence and severity is increased, as well as the bacterial load in the reproductive tract. Heat abatement was provided to all postpartum cows. Vaginal mucus was scored according to abundance and odor of pus on d7 and 21 postpartum to characterize metritis or endometritis, respectively. Bacterial 16S rRNA was quantified by qPCR in mucus. Categorical variables were compared by χ² and GLMM was used for milk data. During the hot season more cows had persistent uterine disease at both d7 and 21 (58.8% vs. 29.4%) and fewer cows recovered from metritis when compared with those in the cool season (21.1% vs. 50.0%). Incidence of endometritis was increased during the hot season (64.7% vs. 43.1%) when compared with cows in the cool season. The concentration of bacterial 16S rRNA in vaginal mucus was higher in cows with endometritis compared with healthy cows at d21 (2.35 vs. 0.04 ng/mg mucus) but did not differ between the hot and cool seasons (2.12 vs. 2.68 ng/mg mucus). Severity of uterine disease was associated with reduced milk production in the first 60 DIM which was further compounded in the hot season (Table 1). Our data suggests that heat stress increases incidence of uterine disease despite similar vaginal bacteria load. This work was supported by NICHD R01HD084316. Table 1 (Abstr. 403). Average milk production (kg/d) for 60 DIM

Season	All cows	Mucus score (d21)
0	1	2	3	4
Hot	34.2 ± 1.3	39.8 ± 3.1x	36.3 ± 2.8x	35.7 ± 2.2x	36.2 ± 2.6x	22.8 ± 4.0y
Cool	39.5 ± 2.1*	47.6 ± 3.1a	44.6 ± 1.9a*	36.6 ± 3.0b	30.8 ± 2.6b	37.7 ± 8.9ab

^a,b or ^x,yP < 0.05 within season, *P < 0.05 between season.