Discussion

Introduction:
Human chorionic gonadotropin (hCG) and dexamethasone play a role in the reproductive management of mares susceptible to persistent mating induced endometritis (PMIE). Interference between these 2 drugs has been hypothesised, potentially resulting in ovulation failure.

Applications of hCG to the reproductive management of mares:
Human chorionic gonadotropin was the first ovulatory agent to be used in the mare and remains the drug of choice by many equine practitioners to this day. Human chorionic gonadotropin improves breeding efficiency as it facilitates prediction of ovulation time and synchronism with breeding. Current literature reports that ovulation is expected to take place within 25 - 48 h of hCG administration, in 73 - 75% of the cases (Sullivan et al. 1973; Michel et al. 1986; Duchamp et al. 1987; Barbacini et al. 2000). An endogenous peak of LH appears to be responsible for ovulations occurring within 24 h of hCG administration (Freeman et al. 1991). Breeding close to ovulation, to apply minimum contamination techniques, is pivotal to the successful management of susceptible mares and aims at preventing the detrimental effects of repeated endometrial exposure to sperm. Predicting time of ovulation, through a timely administration of ovulatory agents, will ultimately reduce the number of breeding per cycle.

Corticosteroid applications in the management of mares susceptible to PMIE:
Mares susceptible to PMIE fail to 'turn off' the inflammatory response to sperm within physiological time limits and the subsequent persisting inflammatory condition impacts negatively on fertility, by creating a poor uterine environment and hampering embryonic survival. Corticosteroids (Dell'Aqua et al. 2006; Bucca et al. 2008), have recently been proven an effective means of modulating PMIE in susceptible mares, by improving post breeding uterine environment, without altering major defence mechanisms.
The administration of hCG and dexamethasone at breeding time, has been reported as an effective protocol in the management of susceptible mares (Bucca et al. 2008). Exogenous glucocorticoids in the horse have been associated with suppression of the hypothalamic-pituitary-adrenal axis (Toutain et al. 1986), immunosuppression (Tumas et al. 1994), laminitis (Johnson et al. 2002), suppression of sexual behavior (Asa et al.
1980) and have been reported to have adverse effects on reproductive function.
Asa and Ginther (1982), demonstrated that in the mare, administration of 30 mg of dexamethasone i.m. once a day for 20 days, from Day 10 after ovulation, inhibits follicular development and ovulation.
A decrease in LH and FSH has been recorded in ovariectomised pony mares following dexamethasone administaration (0.125 mg/kg bwt subcut. once per day) (Asa et al. 1980). However, McKinnon et al. (1997) administered 20 mg of dexamethasone i.m. twice a day, from detection of a 35 mm follicle and uterine oedema until ovulation, without adverse effects on ovulation rates or days to ovulation. More recently, Ferris and McCue (2010), administrated 50 mg dexamethasone twice a day to 6 mares in early oestrus and reported decreased uterine oedema, suppression of LH and a high rate of ovulation failure.
In contrast, a recent retrospective study (Bucca et al. 2011) reported no difference in ovulation patterns between control cycles and cycles where dexamethasone was administrated at breeding time. Efficacy of hCG to induce ovulation was not adversely affected in mares treated with dexamethasone.

Conclusions:
Human chorionic gonadotropin and dexamethasone have a role in the reproductive management protocols of mares susceptible to PMIE. A single dose of dexamethasone, administered at breeding time has been used as an effective modulator of PMIE in susceptible mares and does not interfere with efficacy of hCG to induce ovulation.