Multiple site electromyography recordings of uterine activity following an intrauterine bacterial challenge in mares susceptible and resistant to chronic uterine infection

The electrical myometrial activity of three mares with a documented increased susceptibility to chronic uterine infection (CUI) and three mares considered to be resistant to CUI was investigated. Electrodes were surgically implanted in the myometrium of the mares and electrical activity was monitored by a Grass polygraph. Oestrus was determined by transrectal ultrasonography of the reproductive tract and teasing of the mares with a stallion. Findings were confirmed by blood progesterone concentrations < 0.1 ng ml^-1. At the third day of oestrus or when a follicle > 35 mm was detected, the uterus was infused with a genital strain of 5 x 10⁶ Streptocaccus zooepidemicus. Myometrial electrical activity was monitored for 1-4 h before the bacterial infusion and continued until a visual stabilization of the activity occurred. No statistically significant differences in electrical myometrial activity were detected between susceptible and resistant mares before the infusion of bacteria into the uterus. A visible increase in myometrial electrical activity was seen in all mares following the bacterial infusion. However, the myometrial response of susceptible and resistant mares was different. Resistant mares demonstrated a greater myometrial activity (P < 0.001) than did susceptible mares. These differences were observed in frequency (P < 0.005) as well as duration (P < 0.001) and intensity (P < 0.001) of the uterine activity. Differences were most marked between 10 and 20 h after the intrauterine inoculation of bacteria. It was concluded from this study that myometrial activity is an important part of the uterine defence mechanism in mares. Mares susceptible to CUI appear to have an impaired electrical myometrial activity in response to an intrauterine bacterial challenge.