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 in creased susceptibility to chronic uterine infection (CUI) and three ma res considered to be resistant to CUI was investigated. Electrodes wer e surgically implanted in the myometrium of the mares and electrical a ctivity was monitored by a Grass polygraph. Oestrus was determined by transrectal ultrasonography of the reproductive tract and teasing of t he mares with a stallion. Findings were confirmed by blood progesteron e 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 s train of 5 x 10(6) Streptococcus zooepidemicus. Myometrial electrical activity was monitored for 1-4 h before the bacterial infusion and con tinued until a visual stabilization of the activity occurred. No stati stically significant differences in electrical myometrial activity wer e detected between susceptible and resistant mares before the infusion of bacteria into the uterus. A visible increase in myometrial electri cal activity was seen in all mares following the bacterial infusion. H owever, 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 observ ed in frequency (P < 0.005) as well as duration (P < 0.001) and intens ity (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 importan t part of the uterine defence mechanism in mares. Mares susceptible to CUI appear to have an impaired electrical myometrial activity in resp onse to an intrauterine bacterial challenge.