Managing mares with high-risk pregnancies (Proceedings)

Context

High risk mares are mares that are at significant risk of problems during pregnancy. This includes mares that are of advanced age, repeated history of unsuccessful foaling outcomes, mares with twin pregnancies, mares that have experienced a significant toxemia, or that suffer significant physical infirmaries including chronic founder, severe arthritis, or body wall injuries or mares that have placentitis,. In specific regions of North America mares carrying hypothyroid fetuses or that have been chronically exposed to fungal endophytes are also at risk of pregnancy and parturition related complications.

In this paper Abortion is defined as pregnancy loss in the period from the completion of embryonic development to just prior to the attainment of fetal viability (50 – 300 days). Preterm or premature delivery is defined as delivery of a foal from 300 – 320 days of pregnancy. Dysmaturity is delivery of a foal with signs of prematurity that has a normal or prolonged gestational period. A main topic of this presentation will be a discussion of placentitis in mares. As in other species the likelihood that a mare will experience complications related to pregnancy increases with age. Horses are one of the few species that are bred into the last years of their lives often past the age of 20 years. Mares over the age of 20 years are generally considered to be at high risk for pregnancy complications. For example the incidence of premature separation of the placenta (red bag) is higher in older mares. Mares with advanced age are at risk of colic, such as strangulating lipomas, neoplasia (melanoma), and pregnancy complications.

There a subset of brood mares that repeatedly experience problems at the time of parturition and deliver still born foals. There are potentially many reasons for this to occur but it may be related to abnormal patterns of labour contractions, prolonged fetal stress or placental separation. There are also inbred families where the mares may repeated have abnormal foals. Therefore the mare's age and the genetics of the fetus may influence the outcome of the pregnancy.

Mares that have a twin pregnancy that continue past early pregnancy usually abort in mid to late gestation. Twin abortion is the most common cause of non-infectious pregnancy loss. Today most twin abortions occur in mares that were not evaluated for twins early using transrectal ultrasound, or the examination occurred at time when it was not possible to detect twins. Some mares however will deliver twins prematurely before 11 months of gestation. Typically these mares have a large abdomen, experience premature udder development, but do not usually have a vaginal discharge. Transabdominal ultrasound should allow a determination of the orientation of the fetus and the status of the twins. Mares that are "bagging up" usually have a twin in distress and abortion may be near. They usually have elevated calcium levels in their prefoaling mammary secretion. Tachycardia followed by bradycardia is common in the stressed fetus. times one of the fetuses will perish, which results in stress of the remaining fetus. The remaining fetus is unable to utilize the former space of its co-twin, and so it may also die. These mares are carrying a large load in terms of fetal, placental and fetal fluid mass., and may develop complication related to the large load. Many live born twins are growth retarded, and premature. The twin survival rates are 1/300,000 chance of both foals surviving the neonatal period.

Some mares colic during pregnancy, develop Salmonella, or have other serious systemic illnesses, that cause an endotoxemia or septicemia. The cytokines and prostaglandin that are released in association with the response to endotoxins and the possibility of bacterial invasion of the uterus are factors associated with pregnancy loss. Pregnant mares that have colic surgery frequently struggle with low: blood pressure, perfusion, and oxygenation problems during anesthesia which compromises the fetus.

Significant physical infirmaries also frequently accompany advanced age in mares. These include mares with chronic laminitis that may worsen with the increasing weight and hormonal environment of pregnancy, mares with serve osteoarthritis that may lead to other problems such as laminitis. Arthritis that affects one side may result in overloading and founder in the contralateral limb. Chronic recumbency in mares may develop from osteoarthritis. There are some mares will develop body wall injuries, some of whom have twins, a history or injury or hydrops, however in the majority of cases the body wall tears are idiopathic. Mares with body wall injuries may develop a progressive and severe myonecrosis resulting in tearing and herniation of the lateral internal abdominal oblique muscles or in advanced cases rupture of the rectus abdominus and prepubic tendon.

Fungal endophytes such as Neotyphodium coenophialum, may infect forages such as fescue grass, and cereal crops such as oat and barley (crop scab or ergot). These fungal infections are invisible and commensals that impart the plant with vigour to survive extremes of weather. Specific growing conditions lead to higher amounts of toxins produced by the fungi that cause "fescue poisoning,." or "fescue toxicosis." Exposure to ergot alkaloids associated with Neotyphodium coenophialum will produce intrapartum placental edema, and induces agalactia in the mare. In western Canada hypothyroid foals typically have prolonged gestations, limb contracture, are dysmature and are associated with an increased incidence of dystocia.

Placentitis is one of the most common causes of pregnancy loss in mid to late gestation. Placentitis has further economic impacts because it is a major cause of preterm delivery / prematurity, and neonatal sepsis. Premature and/or septic foals require additional intensive or supportive care to survive. These compromised neonates are generally subsequently impaired in their athletic ability. Mares with placentitis are at increased risk for retained fetal membranes and may develop medical complications such as myoendometritis, septicemia, shock, laminitis and death. Placentitis is more common in mares than cattle. Pregnancy losses in mares after 100 days should be low; less than 5%.

Routes of infection: Ascending cervical route

It is believed that bacteria present in the vagina may proliferate and breach the cervical barrier. Mares with poor external barriers such a sloping perineal conformation, or pneumovagina are predisposed to placentitis. The vagina is contaminated with excess skin and fecal contaminants that disturb the normal microflora of the vagina. Other clinicians believe that certain mares in late pregnancy experience intermittent uterine contractions that lead to cervical relaxation or dilation. In some mares stress - related cervical relaxation / dilation such as following extreme exhaustion may result in the opportunity for bacterial inoculation through the cervix. Mares that have surgery while pregnant and are positioned in ventral recumbency may have increased risk of infection as the vulva will swell and gape open in this position. Some mares in late pregnancy have a very soft cervix that may facilitate pathogen entry. The late pregnant mare may also have a cervix oriented vertically rather than horizontally which may predispose them to contamination. This is particularly pronounced in mares with a dependent uterine location at breeding as this condition seems to worsen as pregnancy advances. Mares with cervical injuries may also be problematic due to a compromised defense barrier. The placental lesions associated with ascending placentitis include: a focally extensive demarkated zone extending from the cervical region of the placenta, suppurative exudate on the affected areas, placental thickening and separation of the chorioallantois from the endometrium. There is usually thickening noted on transrectal examination of the mare's uteroplacental thickness.

Hematogenous Route

The hematogenous route of placentitis involves the spread of microbial organisms through the blood or lymphatic system to the uterus. Bacteremia in the mare is frequently associated with endotoxemia, which also has negative effects on the pregnancy. Endotoxemia is associated with prostaglandin release. The hematogenous route almost always follows systemic signs of illness in the mare. Examples of hematogenous bacterial placentitis include Leptospira and Salmonella infection. Leptospira may be a co-infective agent. Other disease causing organisms that cause maternal illness include: Brucella abortus, Listeria moncytogenes, Mycoplasma, Borrelia burgdorferi (agent causing Lyme's Disease), Erlichia risticci (the agent of Potamac Horse Fever, and a variety of fungi that cause systemic mycosis. Systemic mycoses are also associated with granulomatous placentitis. Granulomatous lesions generally accompany chronic infections. Mares with faulty perineal conformation are more prone to these infections. As these infections are often chronic allantoic plaques may be noted. Some bacteria presumably have a predilection for a low oxygen environment or other factors associated with the pregnant uterus or fetal fluids that allows them to selectively colonize the uterus.

Persistent Endometritis Intrauterine colonization of bacteria may be a source of placentitis. Mares that have a chronic uterine infection and are bred, or are bred and then became infected, have been reported to develop placentitis with the same organism that was cultured at the time of breeding. Breeding or insemination always introduces bacteria into the uterus and some mares maintain pregnancy with a smoldering infection that then flares up and causes pregnancy loss.

Bacterial causes of placentitis

The most common cause of bacterial placentitis is gram positive Streptococcus equi zooepidemicus. This organism is frequently associated with ascending cervical infection. Streptococcus equi zooepidemicus is a common skin and environmental isolate. It is always in the mare's environment. Other bacteria associated with placentitis include: gram negative organisms Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and gram positive Actinobacillus, Staphylococcus aureus, and Nocardia actinomycete.

Fungal causes of placentitis

Fungal placentitis includes infection with yeast such as Candida and fungal organisms such as Aspergillus, Mucor, or Allescheria. The incidence of fungal abortion varies by region. These organisms usually gain entry through the cervix. They cause placental thickening. Mare may require local and systemic treatment to clear the infection. Fetal lesions include growth retardation. The fetus may have skin lesions, but usually does not. The fungus may be cultured from the fetus.

Etiopathogenesis

Bacteria may be eliminated through innate immune mechanisms. One of the major innate immune mechanisms is the activation of Complement which is comprised of a number of serum proteins in an inactive form, which when activated in a defined sequence, initiate immune effector mechanisms. Uterine secretion contains complement. The Complement cleavage product C5a stimulates neutrophil chemotaxis. Antibodies bound to bacteria lead to activation of the classical complement pathway. Additionally if enough C3b accumulates, the alternate complement pathway may be activated. Opsonized bacteria are more likely to be phagocytosed. In addition active inflammation results in prostaglandin secretion which stimulates myometrial contraction and uterine clearance through expulsion of material, sometimes including the fetus, through the cervix. Over 50 cytokines have been identified and cytokines are known to regulate the balance of a host's defense mechanisms against pathogens, while limiting damage to the host. There have been a number of studies indicating that IL-8 and IL-6 are important cytokines in placentitis. Premature delivery in experimentally induced Strep zoo ascending placentitis is associated with increased expression of placental cytokines and elevations in allantoic fluid PGE2 and PGF2-α. These experimentally infected mares had higher levels of IL-6 and IL-8 at the cervical star, and IL-6 at the uterine body. Increased synthesis and secretion of IL-8 is believed to also drive neutrophil recruitment. In mares intrauterine treatment with recombinant human IL-8 (rh-IL-8) has been used to stimulate neutrophil influx. The cytokine IL-6 is expressed in the placenta and is considered to be a proinflammatory cytokine. Innate immune responses to bacteria and their products are also regulated by receptors such as Toll-like receptors (TLR). It is now established that TLR4 regulates the response to the gram negative bacterial product LPS, while TLR2 does the same for products of Gram positive bacteria. Further studies need to be done to determine the changes in cytokine patterns and their relationship to bacteria.

Prednisolone is a glucocorticoid that has recently been used to treat susceptible mares at the time of breeding. Glucocorticoids inhibit cytokine expression through transcriptional (inhibition of cytokine mRNA expression) and post transcriptional levels (mRNA stability; i.e decreased half life). Glucocorticoids also inhibit neutrophil and macrophage recruitment into inflammatory sites. Glucocorticoids bind to glucocorticoid receptors which modulate the expression and action of a number of genes. In vivo studies in the mare are presently needed under carefully controlled circumstances to evaluate the role of TLR and cytokines in placentitis/endometritis. This data will provide needed information on the immunopathogenesis of placentitis/endometritis.

Clinical signs of Bacterial Placentitis

Clinical signs of bacterial placentitis include in late pregnancy premature udder development and mammary secretion, cervical softening, and vaginal discharge/ itching/ tail rubbing. Earlier in pregnancy there may be no precocious mammary development. Mares may show signs of colic or labour such as restlessness, looking at the flank, sweating etc. Vaginal examination usually shows a hyperemic cervix, and a purulent vaginal discharge may be present. There are cases of open cervix and closed cervix infections. Vaginal discharge is never normal or desirable in a pregnant mare. The underside of a mare's tail should be free from crusting or any debris. Cervical discharge is either bloody, purulent matter, or both and is problematic. Bloody vaginal discharge in the absence of pus may be a marker for placental separation. Unless the mare has a systemic illness such as Salmonellosis, that results in endotoxemia / bacteremia, there are seldom signs of fever, depression, inappetence, dehydration etc. associated with bacterial placentitis.

Differential diagnosis

The differential diagnosis for signs of impending abortion or preterm delivery includes: non-infectious abortion/pregnancy loss due to twins, placental insufficiency, viral abortion, and bacterial/fungal placentitis. In terms of generalities if EHV-1 is involved mares will abort within 2-3 days of showing reproductive clinical signs. Mares with twins do develop premature lactation / udder development, but they seldom have a vaginal discharge, the problem with twins is a lack of nutrition that results in pregnancy failure.

Diagnostic approach to the high risk mare

Diagnostic evaluation of mares with premature udder development or vaginal discharge includes a complete history, physical examination, and then a detailed reproductive examination of the mare. A detailed reproductive examination includes: an evaluation of the mammary secretion, rectal palpation, transrectal ultrasound examination, transabdominal ultrasound examination, and a vaginal examination. Temperature, pulse and respiratory rates should be obtained before the rectal examination. Digital pulses and mucus membrane colour should be assessed. Mares with ascending placental infections generally have no systemic signs. The mares with systemic infections (Leptospirosis, Salmonellosis) are the ones which are febrile and dehydrated. In these cases serial CBC's may help you track improvement. Also important to remember that some of these diseases are zoonotic. Mares having uterine contractions threatening abortion will have an elevated heart rate and colic like signs. Mares with abdominal muscle involvement should have an ultrasound of the affected muscles, and mares with arthirits or founder should have radiographs taken. The aim is to make a diagnosis and develop a therapeutic plan

Prefoaling mammary secretion

Mares that are carrying stressed adrenal activated fetuses will usually show an increase in milk electrolytes including calcium and magnesium similar to what is noted near parturition. Near parturition the elevation of calcium above 200 mmol / l indicates parturition will occur within the next 3 days. The elevation in these electrolytes loses its ability to predict the time of parturition. What it does indicate is that a foal born to a mare with elevated calcium has about a 75% chance of survival (due to precocious organ maturation), but the premature foal to a mare without an elevation in calcium has only a 25% chance of survival. Also examine the specific gravity of the secretion, colostrum of good quality has a specific gravity of >1.060

Clinical pathologic findings

A CBC is useful to determine if there is elevated fibrinogen suggesting long standing inflammation. A stress leukogram may be noted, or in cases of endotoxemia a left shift with toxic change may be noted. Chemistry may show prerenal azotemia in systemically ill mares. In cases of body wall tears the Alkaline phosphatase and AST will be elevated. The changes in these enzymes lag behind the damage visualized with ultrasound. Urine in these mares may show myoglobinuria.

Progesterone

Progesterone after 100 days is a composite of luteal and placental production of progestagens. Fetal stress may cause premature adrenal activation and the production of increased amounts of precursors for the progestagens. Progestagen values measured as progesterone typically range from 2 - 6 ng/ml from 180 - 310 days of gestation and may increase or decrease + 2 ng / ml on a daily basis, hence the suggestion to monitor the levels over time. Progestagen levels (cross reactivity) over 10 ng/ml are considered elevated. Elevated progestagen levels indicate fetal stress such as placentitis. A sudden drop in progesterone is often associated with impending abortion. In mares with premature udder development and no vaginal discharge or obvious changes on ultrasound measurements of the placenta the elevated progesterone suggests placentitis. Synthetic progestagens such as altrenogest (Regumate) do NOT interfere or cross react in the progesterone RIA.

Transrectal palpation of the cervix and uterus including the broad ligaments is performed. The tone of the cervix / uterus, fetal orientation, and fetal activity are also noted. The pregnant uterus should feel soft and the fetal parts after 8 months should be palpable. Mares with a uterine torsion often present with colic like signs.

Ultrasound examination

Ultrasound Examination includes an evaluation of the fetal fluids, uteroplacental integrity, fetus and fetal activity pattern. Remember not to confuse a full bladder or its content with a pregnant uterus. Some fetal activity is normal, however all fetuses undergo periods or inactivity (Sleep). Hyperactivity and repeated prolonged periods of fetal inactivity are not normal. Fetal death must be confirmed through other means besides lack of activity, such as direct visualization of the fetal heart not beating. Both a transrectal and a transabdominal approach are used to complete the evaluation.

Transrectal Ultrasound monitoring of Changes in the Placenta

A transrectal palpation and ultrasound examination is performed. The area of the placenta adjacent to the cervix, which is commonly abnormal in cases of ascending placentitis may also be evaluated. The uteroplacental unit is a fluid filled organ. The ventral surface of the uteroplacental unit is evaluated. The combined thickness of the uterus and placenta (CTUP) is measured. After the 8 month of pregnancy the CTUP should match the month of gestation. That is to say at 10 months the CTUP should equal 10mm, 11 months should equal 11 mm. The area adjacent to the cervix is also evaluated for evidence of placental separation, or pockets of discharge. This is one of the most readily assessed areas to examine. The vaginal branch of the uterine artery is used as a marker of the correct location to perform the uteroplacental measurements. (see figs. below). The fetal allantoic and amniotic fluid compartments may be visualized. Some floating particulate material is considered normal, excessive matter usually indicates fetal asphyxia / fetal diarrhea, or bacterial growth in the fetal fluids. The fetal parts are then located with the aim of identifying the fetal head and subsequently the orbit of the fetus. The goal is to measure the orbital diameter (length and width) which is correlated with gestational age (to a month). Location of the head informs the examiner that the fetus is in anterior presentation. Thicker measurements signal trouble. It has been our experience that once a placenta is thickened it seldom decreases in thickness.

Transabdominal ultrasound scan

This is performed by systematically evaluating the quadrants of the abdomen. A 3.5 mHz probe is used. This is accomplished by wetting the hair of the mare using alcohol or clipping the mare and using alcohol and ultrasound couplant. The scan is usually most productive in the caudal quadrants near the udder. The abdomen is scanned from the udder to the sternum to locate the uterus. The body wall muscle layers (<1cm), the CTUP is measured (<2.2cm), and fetal fluids evaluated. The fetal orientation, and then the chest is located in order to observe the fetal heart rate. Fetal ribs shadows are used to help locate the fetal heart. The fetal and non-fetal uterine horns are identified (or not if there are twins). The mean FHR varies from 75- 85 bpm in a non-stressed fetus. Prolonged FHR <40 or >110 bpm indicates significant distress. Note that the fetal heart rate normally should vary and Fetal Activity is normally associated with transient increases in FHR. If the FHR does not increase with fetal movement this is also a sign of distress. Fetal viability may be determined by evaluating the fetal biophysical profile. Typical fetal heart rates are around 75 - 85 bpm. Sustained heart rate elevations over 110 bpm or rates below 40 bpm suggest fetal severe compromise. Fetal heart rate should increase when the fetus is moving (exercising) and should decrease during uterine contractions. A non-reactive heart rate is an indication of fetal hypoxia.

Vaginal examination

The pregnant mare should have good perineal conformation a pale dry vagina, a tight cervix with no discharge. The mucus is tacky and light coloured. The vaginal examination of a mare is performed using strict aseptic technique. If there is a discharge prepare a few slides and Diff Quik. A liquefying cervical plug will have few cells in it. Sample the discharge and submit for culture and sensitivity if inflammatory cells are present on the cytology. The presence of RBCs or neutrophils in a vaginal discharge is abnormal. If neutrophils are present examine them for intracellular bacteria (determine if cocci or rods) and Gram stain a slide to determine the class of bacteria. If bacteria are present better information is available to chose an antibiotic and serve as a starting point if the foal needs treatment. A careful digital examination of the cervix may be used to determine if the cervix is softened, open or closed. Remember mares carrying twins do not typically have a vaginal discharge. A transabdominal scan is necessary to determine if there are twins later in gestation. As most fetuses occupy the uterus so there is one fetus in each horn, the presence of a non-fetal horn often indicates there is only one fetus. Carefully evaluate the fetal horn to be sure a very small fetus is not lurking near the tip of the horn.

Therapeutic approaches

Placentitis

Treatment is initiated to decrease inflammation and cytokine formation, fight or eliminate infection, and decrease myometrial activity. An antibiotic should be chosen based on cytology, gram stain, and culture. A culture and sensitivity are used to confirm the organism and to chose the appropriate antibiotic. Tocolytics such as clenbuterol need to be given 3 times a day to decrease uterine contractions. This drug will not stop abortion or parturition. Anti-inflammatories such as flunixin meglumine are often used to decrease inflammation. Pentoxyphylline is used to decrease cytokine production. The evidence in this area is weak. Dexamethasone may also have the same effect and may be used in dosages from 25 - 50 mg SID. The ability to cross the placental barrier has not yet been established. Inflammatory products may lead to increased uterine contractions. None of these products would stand up to evidence based scrutiny. Progestagen therapy is controversial and should be used in cases where the cervix should be kept closed. There is some information that progesterone in other species aids in maintaining pregnancy closer to term. Altrenogest has been shown to be more effective in stopping cloprostenol induced abortion than progesterone in mares. The minimum duration of treatment is 2 weeks at which time a mare should be reassessed. In some cases mares may need to be maintained on therapy until delivery. There is debate on gentocin and penicillin crossing the placenta.

Antimicrobials

     • Trimethoprim sulpha 30 mg/kg BID PO, Gentocin 6.6 mg/kg SID IV or IM, Potassium penicillin G 22,000 IU / kg IV QID, Procaine Penicillin G 22,000 IU/kg BID

Anti-inflammatories

     • Pentoxyfylline 8.5 mg/kg PO BID, Flunixin meglumine 1.1 mg/kg BID IV, Phenylbutazone 4 mg/kg BID PO

Tocolytics

     • Clenbuterol 0.6 μg/kg IV drip then TID PO, Progesterone 0.6 mg mg/SID IM, Altrenogest 0.088 mg/kg SID PO (double dose)

Antivirals

     • Acyclovir, valcyclovir

Mares with repeated foaling losses should have milk electrolytes monitored and serial fetal heart rates observed. If the fetal condition deteriorates induction of parturition using oxytocin is sometimes indicated.

Twins

If the mare seems otherwise healthy the approach is to let nature take its course. Some advocate the use of double dose Regumate. In the rare cases where the pregnancy is interfering with the health of the mare induced abortion / delivery is recommended.

Exposure to endotoxins and septicemia in most situations the mares will have serial blood gas measurements to assist in fluid therapy decisions regarding electrolytes, blood culture / serology / pcr for antibiotic decisions, and NSAIDs. Constant rate infusion of lidocaine, plasma, and antiulcer medications are also sometimes used.

Approaches for mares with physical infirmities

Mares with laminitis should be treated with NSAIDs, They should have immediate symptomatic care (cold hosing, pads) and therapeutic farrier care as soon as possible. Contributing conditions such as osteoarthritis should be evaluated. Diet and access to green grass may need to be evaluated. Therapy for mares with osteoarthritis usually includes the use of NSAID such as phenylbutazone. Mares should have their BCS evaluated and excess weight avoided. They should be treated intraarticularly with long acting steroids if needed such as Depomedrol (methyl prednisolone acetate) along with IA antibiotics. These products are not associated with adverse outcomes even in cases where multiple joints are involved. Joint products (nutriceuticals) may be used. Mares with progressive body wall tears should be induced. They should receive symptomatic care including cold hosing, NSAIDs, and corseting. Prognosis is poor to guarded.

Exposure to fungal endophytes should be reduced by removing the mares from infected pasture 1 month before foaling. The mares should be fed good quality endophyte free hay and screened oats. If the mares cannot be removed from the pasture treatment with a dopamine antagonist ( dopamine inhibits prolactin), domperidone daily 1.1 mg/kg PO for 10 days (Equidone) or sulpiride 250 mg IM before expected foaling will decrease the prevalence of the problems. Foalings should be attended with the expectation that there will be some premature placental separations (red bag) deliveries.

Consequences

Veterinary Bills are large. The foal is often born septic or endotoxemic. Foal may have Neonatal Maladjustment Syndrome (NMS). Foals may never reach their full size, they may develop debilitating developmental orthopedic disease. NICU graduates often are suboptimal athletic performers. Preterm delivery is also associated with full or partial retention of the placenta which may trigger its own set of problems in the mare. The mare has impaired reproductive performance including: uterine subinvolution and difficulty conceiving that breeding season.

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