Studying the endocrinology of premature parturition in the mare is difficult because of the precipitous nature of this event with few signs of impending delivery. Except in a few specialised referral practices, scanning of high risk pregnant mares in late gestation is not routine and, therefore, many cases of feto-placental compromise remain undiagnosed until the onset of clinical signs or until abortion or premature delivery occurs. There are multiple causes of equine premature parturition and perinatal deaths; umbilical cord vascular compromise (39–46%) and placental abnormalities (10–25%) predominate in Britain, whereas feto-placental infections are the leading cause of late gestation losses in the USA (Giles et al. 1993; Smith et al. 2003). Umbilical vascular problems (such as excessive cord twisting) tend not to stimulate premonitory signs in the mare, whereas feto-placental infections and other placental pathologies stimulate maternal (and fetal) endocrine responses, and premature lactation and vulval discharge are frequently observed (Macpherson and Bailey 2007). Therefore, placental infections have been induced experimentally in pony mares so that the aetiology of this condition can be investigated.
Intrauterine infections (IUI) are a significant cause of preterm birth and infant mortality in pregnant women (Goldenberg et al. 2008). Using clinical data and animal models, the endocrinology of IUI is relatively well understood (Elovitz and Mrinalini 2004; Adams Waldorf et al. 2011). Prostaglandins and their interactions with inflammatory mediators are key responses to IUI and initiate the onset of preterm labour. Bacteria gain access to the normally sterile amniotic cavity most commonly by ascending infection from the vagina and cervix but other potential routes include haematogenous dissemination through the placenta, accidental introduction via invasive procedures or by retrograde spread via the Fallopian tubes (Goldenberg et al. 2008). Once bacteria are in contact with the placenta, a typical inflammatory cascade is mediated, through recruitment of leucocytes, promoting release of proinflammatory cytokines and increasing concentrations of IL-1β, IL-6, IL-8 and TNFα in the fetal fluids, amnion, decidua and possibly placenta (Keelan et al. 1999). These in turn stimulate release of prostaglandins from the amnion, placenta, decidua and myometrium, via upregulation of NF-kB and PGHS-2 (Dudley et al. 1996; Belt et al. 1999). The rise in amniotic proinflammatory cytokines is elicited within hours of experimental infection (intra-amniotic infection with Group B Streptococcus in primates) followed within 12–24 h by a rise in PGE2, which is maintained until premature delivery some 36 h later (Gravett et al. 2007). Intrauterine infections (IUI) initiates fetal HPA activity and fetal exposure to glucocorticoids; amniotic fluid concentrations of cortisol and dehydropiandrosterone (DHEA) are high in pregnant women with IUI, and these stimulate placental CRH and oestrogen production, respectively (Gravett et al. 2000). Proinflammatory cytokines and prostaglandins stimulate placental 11β-HSD-1 and inhibit placental 11β-HSD-2, which activities collectively lead to transfer of high maternal cortisol concentrations across the placental barrier, which then act in a feed forward loop to stimulate fetal HPA activity (Alfaidy et al. 2001; Kossintseva et al. 2006). The rise in prostaglandins, IL-1β and IL-6 also acts in a series of feed forward loops to increase placental production of CRH, which stimulates further rises in amniotic fluid prostaglandins, fetal cortisol and DHEA the precursor for oestrogen production (Jones and Challis 1989; Petraglia et al. 1990; Smith et al. 1998; Gravett et al. 2000). Therefore, in the face of overwhelming endocrine changes that stimulate prostaglandin release and uterine contractility, IUI activation of the fetal HPA axis enables fetal tissue maturation and preparation for early delivery so that the infant can survive outside of the infected uterus despite its small size and truncated period of intrauterine development (Laatikainen et al. 1988).
Over the last 2 decades, a series of studies have investigated the endocrine events associated with naturally occurring and experimentally induced placental infections in mares. There are similarities between the endocrine pathways described by these equine studies and those in other animals described above. Early research in Thoroughbred mares with naturally occurring placental disturbances correlated precocious activation of the fetal HPA axis with infection and an early rise in maternal plasma progestagen concentrations and onset of mammary development before 310 days of gestation (Rossdale et al. 1991). Similar maternal changes were observed after inducing IUI experimentally by placental separation at the cervical pole in pregnant pony mares. Direct stimulation of the fetal adrenal with exogenous CRH or ACTH also elicits a rapid rise in maternal progestagens, presumably through enhanced production of adrenal P5, demonstrating a clear correlation between precocious fetal HPA activity and maternal progestagen rises (Rossdale et al. 1995; Fowden et al. 2008). Subsequently, specific models were developed to induce IUI by inoculating bacteria (Streptococcus equi ssp. zooepidemicus) into the cervical canal of pregnant mares of <300 days gestation to create ascending placentitis, and measuring maternal and placental responses (LeBlanc et al. 2002; Ryan et al. 2009; Bailey et al. 2010). Examination of maternal progestagens in the infected mares demonstrated a rise in progestagens within 8 days in those pregnancies with live fetuses, with some delivering live foals with precocious fetal maturation of the HPA and other tissues, while, in other mares, progestagen concentrations decreased in association with fetal death and abortion (Morris et al. 2007). Further investigations of proinflammatory cytokines and prostaglandin production in mares with placentitis (Fig 5) showed that allantoic fluid concentrations of PGE2 and PGF2α were approximately 10 times higher prior to abortion or preterm delivery than concentrations in control mares close to full term (LeBlanc et al. 2002). Silver et al. (1979) also reported elevated concentrations of PGE2 and PGF2α in both fetal plasma and allantoic fluid during the final 6 days prior to abortion in late gestation, while maternal PGFM concentrations were unchanged. Moreover, placental expression of numerous cytokines (IL-1β, IL-6, IL-8, IL-15, IL-18, interferon-γ) is greater in mares with placental infection than in noninfected mares (LeBlanc et al. 2002; Lyle et al. 2009). These results suggest that proinflammatory cytokines produced by the allantochorionic tisssues in response to infection and recruitment of leucocytes (Fig 5), stimulate uteroplacental production of PGE2 and PGF2α and result in increased prostaglandin levels, within the uteroplacental tissues, which remain elevated for some days, if left untreated. The rise in maternal progestagens indicates activation of the fetal HPA but, unlike normal term labour when prostaglandin production is suppressed by progestagens until close to delivery, both prostaglandin and progestagen concentrations were raised for several days in these infected pregnancies. Further investigations are needed to identify the endocrine changes within the uteroplacental tissues of abnormal pregnancies. The proinflammatory responses of infected uteroplacental tissues also appear to be associated with elevated fetal glucocorticoids as high allantoic cortisol concentrations are observed with significant expression of placental IL-1β in experimentally infected mares (Lyle et al. 2010a). This indicates that 17α-hydroxylase may be activated in the fetal adrenal gland some weeks prior to term in mares with IUI. Alternatively, there may be increased transplacental cortisol transfer from the mother as a result of impaired placental 11β-HSD activity, although preliminary evidence suggests no significant relationship between maternal and fetal cortisol concentrations in mares with in utero infections (Chavatte et al. 1995a; Lyle et al. 2010b). The decline in maternal progestagens and early fetal death in some mares indicates that the fetal glucocorticoid response is an essential element for fetal survival. Precocious activation of the fetal HPA appears to stimulate organ maturation, although not skeletal maturity and thus provides an ‘escape route’ for the infected foal, which will aid in its survival ex utero. However, these foals are often small with poor bone ossification (predominantly tarsus and carpus). Such detrimental effects may persist into later life and have consequences for the foals' future athletic potential. Therefore, treatments to prevent preterm delivery and to maintain pregnancies to term are currently being investigated (Bailey et al. 2010; Christiansen et al. 2010).
Induced parturition (term and preterm)
Induction of delivery requires activation of uterotonic hormones, prostaglandins and oxytocin. Before 35 days of pregnancy, a single dose (250 µg) of exogenous PGF2α or its synthetic analogue (fluprostenol or chloprostenol) usually terminates pregnancy via their luteolytic action on the corpus luteum (Penzhorn et al. 1986). However, once pregnancy is dependent on placental production of progestagens, then prostaglandins are less effective at inducing parturition. During this period, single doses of prostaglandins or their analogues rarely induce delivery unless the mare is close to term; instead multiple, high doses of prostaglandins over hours or even days are usually required (Alm et al. 1975; Rossdale et al. 1979; Leadon et al. 1982). In mares that are induced in late gestation, maternal PGFM concentrations rise either rapidly (within 15 min) in mares that are close to foaling or more slowly (>60 min) if they are unready for birth (Barnes et al. 1978; Rossdale et al. 1979). If delivery does not occur or the foal is dead, then PGFM concentrations remain low (Silver et al. 1979; Leadon et al. 1982). Since prostaglandin production depends, in part, on oestrogen concentrations in parturient sheep and horses, PGFM concentrations are low and labour contractions are weak in the absence of raised oestrogen concentrations (Pashen and Allen 1979; Whittle et al. 2000). Thus, the late rise in oestradiol-17β concentrations in mares may provide the key to whether labour induction with prostaglandin is successful or not. Oxytocin appears to be more effective than prostaglandins at inducing delivery in the mare, although higher doses are required prior to term than those needed at term (Leadon et al. 1982). In women, oxytocin is a very potent uteronic agent and, when bound to its receptor, is capable of stimulating PGF2α production and upregulating PGHS-2 (Husslein et al. 1981; Terzidou et al. 2011). It may have similar actions in the mare and overcome the relaxant effects of progesterone and other relaxatory hormones.
Suppression of prostaglandins during delivery
Clearly, suppression of the production and action of the prostraglandins could be beneficial for the prevention of uterine contractions and preterm delivery in mares with IUI. However, reducing maternal prostaglandin concentrations does not prevent either spontaneous, or oxytocin-induced, labour in healthy mares. The PGHS inhibitor meclofenamic acid (500 mg per os) is effective at abolishing the rise in PGFM concentrations that occurs following surgery or during fasting in pony mares (Silver et al. 1979). However, administration of higher doses (1.5 g/day) to pregnant pony mares for 12 days before term does not prevent parturition or high PGFM concentrations during labour despite suppression of basal, prelabour PGFM levels in the mare (Pashen 1981). In mares given high doses of dexamethasone (100 mg i.m.) for 3 days daily, maternal plasma PGFM concentrations were suppressed but 2 mares foaled spontaneously within 48 h of their last dexamethasone treatment with low PGFM concentrations at delivery (Ousey et al. 2011). Moreover administration of the PGHS-2 inhibitor, flunixin meglumine to mares 8 and 1 h before induced parturition at term again suppressed PGFM concentrations but did not prevent labour (Vivrette et al. 1995). These data indicate that once the uterus is in an activated state and CAPs are initiated, it is difficult to reverse or halt myometrial activity.