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Abortion in Horses


Abortion in the mare is the expulsion of a dead or living, nonviable fetus or embryo up to 300 days of gestation. Abortion after 300 days is termed premature or stillborn. The length of a normal pregnancy is approximately 340 to 342 days, and abortion can occur at any stage in this gestational period. The rate of abortions in mares is high, between 5 and 15 percent. Abortions occurring early in gestation, between conception and 90 days, often go undetected and are frequently confused with infertility. Research has found that the incidence of abortion between 20 and 90 days ranges from 7 to 30 percent.
In the mare, the majority of observed abortions occur after the fourth and fifth months of gestation. The incidence of abortions after this period accounts for approximately 2 to 12 percent of all pregnancies.


The agents involved in equine abortions at all stages of gestation are varied. Research into early embryonic and fetal loss, which includes reabsorption, is not conclusive, because many mares abort prior to pregnancy detection, but the use of ultrasonography has stepped up early detection and permitted observation of the early fetal environment. Early abortion, defined as fetal loss prior to 60 days, has been associated with maternal stress either from disease or environmental factors, age of the mare and her reproductive history, chromosomal abnormalities, progesterone deficiency, uterine infections, nutrition, twinning, impaired endometrial functioning and pathogens. Incidents of abortions after the fourth and fifth months can occur due to fetal, endometrial and placental abnormalities, twinning, incidence of maternal stress, and pathogenic organisms.

Causal agents associated with equine abortion

This paragraph focuses on some of causal agents associated with equine abortion. The discussion divides into sections on infectious and noninfectious agents with a brief discussion on implementing planned abortions. The discussion on infectious agents will include viral, bacterial, fungal and protozoal. The discussion on noninfectious agents is divided into maternal and fetal/placental factors including hormonal, uterine incompatibility, maternal age, trauma and physical insult, nutrition, chemical and plant agents, twinning, developmental and chromosomal abnormalities, and umbilical and placental abnormalities.

Equine herpesvirus 1 (EHV-1)
Equine viral rhinopneumonitis or equine herpesvirus 1 (EHV-1) is the leading cause of abortion in mares. EHV-1 is highly contagious and can result in an abortion epidemic in large herds of poorly managed mares. EHV-1 accounts for approximately 10 percent of all diagnosed abortions, approximately 10 abortions per 1000 pregnant mares at risk. The virus is widespread in the United States, Canada, Japan and European countries.
Horses are exposed to EHV-1 by inhaling the expelled breath of an infected horse or by contact or ingestion of infected material such as an aborted fetus. The virus manifest itself as a respiratory infection. Young horses, such as fall weanlings, with no previous exposure to the virus, are particularly susceptible to the respiratory infection. The symptoms are coughing, snotty nose, loss of appetite, depression, fever, and death when the virus invades the central nervous system. Older, previously exposed horses develop a type of immunity to the respiratory aspects of EHV-1 and may show few or no clinical signs of infection. Immunity is short lived after infection, usually two to three months, so horses are subject to reinfection, which in pregnant mares can lead to abortion. The EHV-1 virus is extremely hardy and survives due to its ability to maintain a carrier state within its host. Following the respiratory infection, the virus has a two to 12 week incubation period prior to infecting the fetus, damaging liver cells and affecting the lungs, brain, and other fetal organs. The cumulative effect is death and abortion, frequently without sign or warning. Because of the long incubation period, EHV-1 abortions rarely occur prior to the fifth month of gestation. Ninety percent of EHV-1 abortions occur from eight months to term. Infected foals delivered full term are septicemic and die shortly after birth or are stillborn.
The incidence of EHV-1 abortions can be lowered by thoughtful management of pregnant mares and a vigilant vaccination program. There are two vaccines available for EHV-1 abortions, killed and attenuated. The killed vaccine is marketed for the prevention of abortion in mares and for respiratory disease in young horses. The vaccination is administered at five, seven and nine months of gestation. Strict adherence to the vaccination regime will not eliminate the occurrence of EHV-1 abortions in a pregnant herd, but studies have shown that properly administered killed vaccine vaccination programs will lower the incidence. Attenuated virus vaccines are live viruses but altered to prevent infection. Attenuated vaccines are marketed for the prevention of the respiratory forms of the disease, but some farms are using the live virus to inoculate pregnant mares every two to three months throughout gestation with some reported success.
Fastidious broodmare management reduces the occurrence of EHV-1 induced abortions. If the agent is unknown, all abortions before day 150 of gestation should be considered an EHV-1 abortion until histological examination and culture of the virus rules out EHV-1. Mares who abort should be immediately isolated from the herd. Their bedding should be burned. If they abort in a paddock, all grazing horses should be kept out of the paddock for two weeks, pregnant mares for one month. The aborted fetus and placental membranes are highly infective and may cause an abortion epidemic in late term pregnant mares within days. All this material should be picked up, put in a plastic bag, and sent to a lab for diagnostic tests. Anyone contacting the mare should beware of spreading the virus with contaminated equipment, clothing and hands. Day-to-day management procedures for the prevention of EHV-1 infections include isolating pregnant mares from all other horses, especially weanlings, yearlings and two-year-olds, foals born septicemic and visitors. Stress reduction is also an important factor in reducing EHV-1 infections. Stress weakens immunological responses, so factors such as overcrowding, shipping, sales rings, nutrition, infection, pain, or any factor eliciting stress in a pregnant mare or herd should be minimal or avoided.
Barring complications unconnected with abortion of an EHV-1 infected fetus, the mare should suffer no damage to her reproductive tract. Usually, aborting mares recover as if they had undergone a normal parturition, and
they are no more susceptible to bacterial infections than normal foaling mares.

Equine Viral arteritis (EVA)
Equine Viral arteritis (EVA) is a viral disease also associated with abortion in mares. EVA was first reported as an abortive agent in the United States in 1957 when the virus infected 80 percent of the horses on a single farm with 50 percent of the pregnant mares aborting. EVA is not known to occur in England and is uncommon in the United States. Cases have been reported in Switzerland. EVA is unrelated to EHV-1, but it does manifest itself as a respiratory disease. EVA is transmitted as an aerosol and through direct contact. Unlike EHV-1, EVA produces symptoms prior to most abortions including fever, limb edema, conjunctivitis, colic, diarrhea, and a serous nasal discharge.The course of the disease is two to 15 days. Abortion is a complication of EVA and has a high probability of occurrence in infected mares. Abortion due to EVA usually occurs within two to 14 days of the disease's onset. The greatest incidence of EVA abortions occurs between five and 10 months. Horses who recover from the infection have a prolonged immunity to reinfection. A commercial vaccine is available, but its use is closely regulated.

Equine infectious anemia
Equine infectious anemia (EIA or "swamp fever") is caused by an RNA virus, and the disease has been found to be an abortive agent in mares. The natural transmission of EIA is through blood sucking insects. The virus is also spread by blood-contaminated equipment and is present in bodily secretions, urine, and stools. Symptoms of acute EIA infections are profound anemia, depression, circulatory collapse, and hepatitis. Chronic cases show milder forms of these symptoms. EIA abortions may be due to the virus infecting the fetus or by maternal anemia and stress associated with the illness.

Bacterial organisms cause abortion at any stage of gestation and constitute seven to 13 percent of all diagnosed abortions. Organisms associated with equine abortion are Streptococcus zooepidemicus (S. zooepidemicus), Streptococcus equi and other streptococci; Escherichia coli (E. coli); Pseudomonas aeruginosa (P. aeruginosa); Staphylococcus aureus (S. Aureus) and other
staphylococci; Klebsiella pneumoniae var. genitalium (K. pneumoniae); Leptospira pomona; and Salmonella abortus equi.

The highest incidence of bacterial abortions occurs between the fifth and tenth months of gestation. Bacteria has been implicated in early abortions, but this theory is based on the isolation of organism from uterine cultures following abortion. Bacteria cannot be implicated by its presence; there must be evidence of autolysis and inflammatory changes determined by the examination of the aborted fetal tissue and the placenta.
Organism enter the fetal-placental unit either through the maternal bloodstream as a result of a systemic infection; by ascending through the cervix and infecting the placenta and fetus; or possibly, although there is no pathological evidence, from a local, deep-seated endometritis. The majority of clinicians and researchers agree that the cervix is the most common route.
Streptococcus zooepidemicus is the most common cause of abortion at any gestational stage. S. zooepidemicus is found on the external genitalia of mares and stallions, and it is the most common organism associated with vaginitis, cervicitis and metritis. S. zooepidemicus usually ascends through the cervix infecting the placenta resulting in placentitis. Streptococcal placentitis is a major cause of fetal abortion. Gestation continues until the specific infection so compromises the placenta that it can no longer maintain the pregnancy resulting in fetal distress or death which triggers abortion. The infection can also spread to the fetus causing septicemia, possibly death, and abortion. Streptococcus infections occur most frequently on poorly managed farms utilizing unsanitary breeding, foaling, and examination procedures; excessive breeding of older mares; and frequent
services at foal heat.
A diagnosis of an S. zooepidemicus abortion is based on the isolation of the organism from fetal tissue and a culture of the genital tract. S. zooepidemicus infections are limited to the genital tract. Late term S. zooepidemicus abortions are often accompanied by the retention of a portion of the placenta (retained placenta) within the uterus. Subsequent infertility due to chronic endometritis could be a problem.

E. coli, P. aeruginosa, S. aureus, and K. aerogenes also ascend through the cervix due to poor management and pneumovagina, and they can result in the placentitis associated with progressive placental insufficiency and abortion.
The prevention of bacterial abortions caused by agents ascending through the cervix begins with sanitary breeding and examination procedures, pre and postbreeding local uterine therapy when necessary, and careful management of problem mares.

Leptospira pomona has been identified as an occasional cause of abortion. Abortion is due to infection of the fetus with the highest incidence occurring in the seventh through eleventh months. L. pomona infects the fetus serologically, usually after a mild one to three-week systemic illness in the mare. Symptoms include elevated body temperature, anorexia, and a slight depression. Treatment of L. pomona abortions includes treatment of the systemic illness and vaccination prior to the fourth month of gestation which is repeated up to three times a year in a herd known to have an endemic problem. The bacteria is thought to infect horses through their contact with L. pomona infected cattle. The bacteria probably infect mares through abrasions in the mucus membrane.

Salmonella abortus equi was infamous in the late 1800s as the causal agent in contagious equine abortions, but, except for two small isolated outbreaks, salmonella abortion has not been reported sinds 1923. Rare, single cases are reported, but they involve mares with prolonged, severe systemic infections. The organism is spread by the ingestion of genital or fecal contaminated feed and water.

Abortions associated with fungal infections are sporadic and occur primarily between five and 10 months of gestation with late gestational occurrences most common. Fungal abortions account for three to 10 percent of all diagnosed abortions.Mucor spp. and Aspergillus fumigatus are fungal agents commonly associated with equine abortion.
In most cases fungal organisms appear to ascend through the cervix, but they can possibly enter the uterus by hematogenous spread from pulmonary or gastrointestinal lesions.Abortion attributable to fungal infection occurs as a result of placentitis. The infection so damages the placenta that the foal receives inadequate nutrition. Eventually, the organisms infect the fetus causing lesions on the liver and lungs. Upon physical examination, the aborted fetus is often small and emaciated. Diagnosis of fungal abortions depends on isolation of the agent and clinical signs of severe placentitis. Many mares may have a copious discharge for several days following the abortion, but future infertility is not usually an effect, and no therapy is indicated.

Protozoal induced equine abortions are rare. However, in many tropical countries, protozoaI abortions are epidemic. Dourine is a venereal disease of horses that has been reported to cause abortion; the protozoan Trypanosoma Equiperdum is the causal agent. Its presence is diagnosed by a complement fixation test. Babesia Equi or Babesia caballi have also been implicated in equine abortions.

Noninfectious abortions
Noninfectious abortions occur sporadically and can be due to maternal and fetal factors. Maternal factors associated with abortion are progesterone deficiencies, uterine incompatibility, maternal age, trauma or physical insult, nutrition, stress, and toxic chemicals and plants. Fetal and embryonic factors associated with abortion are twinning, chromosomal abnormalities, and umbilical and placental abnormalities.

Progesterone deficiency has been implicated as a causal agent in equine abortions occurring throughout gestation. Progesterone secretion maintains a delicate balance during gestation as production sites shift between the primary corpus luteum, the mid-cycle corpus luteum, and the fetal-placental unit. Any interruption in this balance can disrupt the process and result in abortion. Progesterone deficiency is particularly suspect in early embryonic loss, the involuntary termination of pregnancy in the first 60 days of gestation. Progesterone is critical for the survival of the embryo, and a low maternal blood level prior to maternal recognition of pregnancy at 14 to 16 days post-ovulation has been associated with early embryonic loss.
Three mechanisms may lead to embryonic loss prior to 14 to 16 days post-ovulation. The first is premature luteolysis. Research has found an association between endometrial irritants, such as endometritis, and premature luteolysis. It is hypothesized that inflammatory exudate triggers the uterine secretion of prostaglandin which prematurely shuts off the corpus luteum's progesterone production. Prostaglandin's ability to cut off the corpus luteum's secretion of progesterone and return the mare to heat is called luteolysis. The second factor involved in progesterone deficiency is the failure of the embryo to prevent luteolysis. It is unknown how the embryo inhibits the uterine production of prostaglandin, but researchers hypothesize that the endometrium's secretion of prostaglandin is blocked or reduced by the embryo's free floating motility within the uterus until 15 to 16 days post-ovulation. This motility is thought to play a role in the maternal recognition of pregnancy. Failure of the embryo to trigger maternal recognition could be due to poor motility or underdevelopment. The third factor that could affect progesterone production is primary luteal insufficiency resulting from decreased progesterone production by the
mid-cycle corpus luteum. This hypothesis is based on progesterone levels taken from subfertile mares 12 days post-ovulation. Researchers in this study state that it is unknown if the primary luteal insufficiency is due to a defective corpus luteum or to factors initiating the endometrium's premature secretion of prostaglandin.
Researchers and clinicians are divided on the use of exogenous progesterone therapy in the prevention of early fetal abortion or reabsorption. Objective studies are lacking not only in progesterone's role in abortion and early fetal loss but in the efficacy of progesterone therapy in preventing abortion and early embryonic loss. The advocacy of progesterone therapy is based upon the fact that some habitual aborters will carry foals to term when given exogenous progesterone. There is also a lack of research on the correct therapeutic dosage, and if serum progesterone levels are indicative of the effectiveness at the target site, the uterus. It is purposed that progesterone has a local effect on the uterus and that bloodstream levels do not accurately reflect local levels.

Uterine incompatibility is a maternal factor associated with both abortion and early embryonic loss. Endometritis is the major cause of infertility in mares, and it is frequently implicated in early embryonic loss. If inflamed and infected, the uterus is a hostile environment to the embryo when it descends from the Fallopian tubes at five to six days post-ovulation. Three hypotheses exist concerning endometritis and abortion. The first was discussed above, infection-induced premature luteolysis. The second, endometritis can induce abortion by stimulating an inflammatory response. And third, the embryo is effected by the pathogens causing the infection. It has also been found that a relationship exists between uterine infection and the recovery of abnormal embryos.
Perigandular fibrosis due to trauma or infection is the most common cause of uterine incompatibility, and a direct relationship exists between endometrial fibrosis and early embryonic abortion or fetal abortion. The incidence of abortion in mares with severe endometrial fibrosis is three times higher than normal mares.7 Studies of subfertile mares with moderate to severe fibrosis found that these mares have a high incidence of embryonic loss prior to day 14 post-ovulation. Researchers believe that the loss of glandular elements in the endometrium at a point where the placenta makes contacts may hinder the adequate exchange of nutrition necessary for a developing fetus. Periglandular fibrosis is seen most commonly in older, multiparous mares as a result of the progressive damage from
foaling and repeated contamination. Mares with extensive and severe endometrial fibrosis are virtually incapable of carrying a foal to term. Fibrotic mares can become pregnant but usually abort prior to 90 days post-ovulation.

Foal heat is the first postpartum estrus occurring approximately eight days after foaling. Postpartum mares ovulate within four to 18 days. Mares bred in foal heat have been reported to have higher incidences of early fetal loss and abortion than mares bred in subsequent cycles; other researchers studying abortion rates and foal heat breeding have found no association. In the case of higher abortion rates and foal heat breeding, researchers purpose two factors, stress due to lactation and hormonal imbalances. However, management factors such as nutrition were not considered in these studies and could account for some of the disparity in the findings. Delayed endometrial involution at foal heat is a recognized form of uterine incompatibility associated with abortion. It is not unusual for some mares' endometrium not to be fully recovered until 12 to 14 days after parturition. Breeding a mare with delayed endometrial involution at foal heat would expose her to pathogens that could overcome her already compromised uterine defense mechanisms. Particularly, if the delayed involution is due to an acute case of endometritis. If delayed involution does involve endometritis, intrauterine infusion with sensitivity tested antibiotics or antiseptic drugs could be prescribed. An intravenous drip of oxytocin in sterile saline could be prescribed to hasten normal endometrial involution. Breeding suitability at foal heat must be determined by palpation, culture, and cytological examinations.

Trauma and physical stress
Maternal trauma and physical stress have been implicated as causal agents of equine abortions, but the occurrences are infrequent and difficult to prove. Events causing trauma and physical stress associated with abortion are injuries, physical pain, illness, shipping, sales rings, inadequate housing, changes in nutrition and water, natural cover or artificial insemination during the first three months, hard work, and weaning. Rectal exams have been accused of causing early fetal losses, but this assertion has not been supported by research findings. Low flying airplanes have also been accused of causing abortions in pastured mares. Researchers believe that stressful events cause hormonal changes within the mare. Stress can induce the maternal production of glucocorticoids which can initiate parturition. Maternal stress can also affect progesterone levels, especially from three to five months when levels from the corpus luteum are declining.16 Some mares are more easily stressed than others. Research has found that mares bred as yearlings have a high incidence of abortion. This has been attributed to their susceptibility to stress, but other factors such as immaturity, poor nutrition, and inadequate progesterone levels have also been implicated in abortion in young mares.

Maternal nutrition has been reported to cause abortion in mares.Researchers have observed that protein deprivation between days 25 and 31 of gestation may cause abortion. Low levels of nutrition and imbalanced diets are also associated with prolonged gestation, fetal developmental abnormalities, and low birth rates. In general, nutritional levels have to be extremely poor to cause abortion.

The administration or maternal consumption of chemical agents and some plants have been reported to cause equine abortion. Some anthelmintics, such as carbon tetrachloride, phenothiazine, and the organophosphates have been linked with abortions. Due to the supposed harmful properties of anthelmintics, some breeders are hesitant to deworm mares late in gestation. However, it is reported that most available anthelmintics are safe in all stages of gestation. The product label should always be consulted prior to use.
Some plant species have been reported to cause equine abortions.Sorghum and sudan grass has been reported to be toxic to the fetus, although no direct cause and effect relationship has been established. Mares grazing fungus infected fescue pasture have been reported to have high incidences of agalactia, stillbirths, and abortions.

Twinning, a fetal factor in abortion, is the most significant noninfectious cause of equine abortions. Twinning accounts for approximately 20 to 30 percent of all diagnosed abortions, although, these figures are considered low due to early, unnoticed fetal loss and a lack of diagnostic evaluation. Approximately 65 to 75 percent of mares conceiving twins abort. Fifteen percent of twin fetuses are delivered alive, nearly half die soon after birth. Twins are aborted throughout gestation but are especially common from eight to 10 months of gestation. Twins may be aborted spontaneously, without warning, or late gestational stage mares may exhibit signs similar to parturition, such as milk production and waxy teats, weeks prior to aborting.
Two theories have been purposed for the abortion of twins.

The first suggests that twin fetuses die due to insufficient nutritional exchange. This is supported by small fetal size, signs of growth retardation, and by evidence that the fetuses die at different times. The second theory suggests that abortion is due to the fetuses' immunological rejection of one another.This theory is based on the presence of plasma cells at the contact point between the fetuses. This theory is still just a suggestion and requires much additional research.
Three types of twin placentations have been reported. In the first type of placentation, one twin occupies most of the uterine body and one horn. The smaller twin occupies only one horn and a small part of the uterine body. These pregnancies frequently end in late term abortion or the stillbirth of one or both twins. In the second type of placentation, the fetuses occupy equally the uterus and horns. Frequently, both fetuses are born alive with a good chance of survival, however, one or both are usually euthanized due to weakness and poor development. In the third type of reported placentation, one fetus occupies only a small portion of one horn. This smaller fetus usually dies early and mummifies. The larger twin is usually born alive with a high probability of survival.
Two approaches have been used in the management of twin embryos, prevention and elimination. Prevention is the management of mares with multiple ovulations. One course of prevention is to postpone breeding a mare with two mature follicles until her next cycle. It is worth noting that in a study of 107 sets of twins, 76 percent were associated with a single detected ovulation. The second is to delay breeding for 12 to 24 hours after the ovulation of one follicle but before the ovulation of the second follicle. This technique is based on the assumption that the ovum ovulated first has a fertilizable life span of six to 12 hours. A third course of prevention is to aspirate one follicle with a needle and breed to fertilize the second. The ability of the three management procedures to prevent twin embryos has not been determined. More research is needed before specific recommendations can be made for mares with multiple follicular development.
Eliminating one embryo is the second approach to managing twin embryos. The success of elimination procedures depends on early pregnancy diagnosis. If the pregnancy is beyond 50 or 60 days, it is difficult to distinguish between the two vesicles. Six procedures used to eliminate one or both embryos are: one, inducing the mare to abort and then rebreeding her prior to the formation of the endometrial cups; second, the needle aspiration of the smaller of the embryonic vesicles through the vaginal wall; third, manually crushing one vesicle rectally has been used to eliminate twin pregnancies but rarely without the loss of both embryos; fourth, attempting
to disrupt the endometrial contact of one fetus through repeated massage and gentle movement; fifth, severe nutritional deprivation of the mare has been reported to causing the resorption of one of the embryos, however, nutritional deprivation frequently results in the resorption of both embryos; and sixth, the surgical removal of the smaller embryo through the uterine wall via the abdomen.
Multiple ovulations and twin pregnancies occur with a high degree of frequency and repeatability within certain mares and family lines. Breeders should select against these heritable characteristics.

Developmental and chromosomal abnormalities of the fetus
Developmental and chromosomal abnormalities of the fetus have been implicated in abortion. Researchers purpose, although it has not been accurately established, that a high number of early embryonic losses are due to chromosomal abnormalities. Abortion prevents the survival of a defective embryo or fetus. In humans, chromosomal abnormalities were estimated to occur in 50 percent of conceptions and in 66 percent of spontaneous pregnancy losses between three to seven weeks.
Developmental abnormalities have been reported in aborted fetuses. Some have been incompatible with fetal life. Other developmental abnormalities have been incidental to the causes of abortions. Abortions due to fetal developmental abnormalities may occur from three months of gestation to term. Contracted foal syndrome occurs in three to four percent of all diagnosed abortions. Fetuses with this syndrome have contracted forelimbs and occasionally, skull deformities. Hydrocephalus and schistosomus reflexus are other rarely occurring fetal developmental abnormalities. The causes of these abnormalities are unknown, but genetic variables are suspect.

Placental and umbilical abnormalities
Placental and umbilical abnormalities are also causal agents in equine abortion. Two placental abnormalities discussed here are premature placental separation and body pregnancy. The causes of premature placental separation are unknown, but the frequency is much greater with hormonally induced parturition. When the placenta separates prematurely at parturition a full term foal can die of anoxia, a lack of oxygen. Premature separation can be due to tears in the placenta and placental detachment from the endometrium in the horns some time before the start of labor. Body pregnancies are rare but occur when the embryo/fetus develops in the uterine body and the placental membranes remain underdeveloped and unattached in the horns. Aborted fetuses are growth retarded and emaciated. Researchers believe that abortion occurs when fetal nutritional need can not be met due to placental insufficiency.
Umbilical cord abnormalities that are potentially lethal to the fetus are related to cord length and vascular occlusion. An excessive long cord can be strangulated by wrapping around a portion of the fetus or by excessive torsion. Both conditions result in fetal vascular occlusion, followed by death and subsequent abortion. Vascular occlusion by strangulation or excessive torsion of the umbilical cord accounts for one percent of fetal deaths and abortions and occurs most frequently between five and eight months of gestation. An excessively short cord may be predisposed to premature rupture which usually occurs near the abdomen prior to parturition. The rupture allows blood to pool within the amniotic cavity asphyxiating the fetus.
The intentional induction of abortion is uncommon, but it can be performed at various stages of gestation. Twinning is the most common reason for induced abortions. Other cases occur due to mismatching, maternal orthopedic injuries, change of ownership, a change in the mare's function, or teaching and research. Prior to the maternal recognition of pregnancy, the corpus luteum is maintained by uterine signals until approximately 10 to 11 days. Abortion during this period is relatively simple. Abortion results from several common methods that initiate the uterine secretion of prostaglandin, or by the process of luteolysis. Both are intramuscular
injection of prostaglandin or the intrauterine infusion of sterile saline will initiate abortion. Prostaglandin therapy is preferable because it is often more cost effective, the results occur rapidly, and it does not invade the genital tract. Researchers have found that mares are generally resistant to luteolysis in the first five days postovulation, so abortifacient therapy is recommended after seven to eight days. Mares usually return to an estrous cycle of normal length within approximately five days. Although not thoroughly documented, researchers believe that subsequent cycles will be normal.
After the maternal recognition of pregnancy at approximately 12 days postovulation, the corpus luteum is more resistant to luteolysis and inhibits the subsequent occurrence of a normal, functional estrus. Prostaglandin is still the preferable abortifacient after maternal recognition. One intramuscular administration between 12 and 35 days postovulation will induce abortion. An estrous cycle generally follows therapy within four to five days. The fertility of subsequent cycles is not well documented, but some researchers recommend that the mare be short cycled with further prostaglandin therapy to allow the uterus time to involute prior to rebreeding. Saline infusion aborts a mare in this gestational period, but researchers report that her resultant cycle is unreliable following saline therapy. The fetus can also be manually crushed via the rectum with no reported damage to the rectum or subsequent fertility. Abortions after manual crushing generally occur within 96 hours, but the corpus luteum can continue to function for 10 to 90 days. It is difficult, but certainly not impossible, to abort a mare while the endometrial cups are functioning, between 35 and 120 days postovulation. Abortions prior to this time are strongly recommended. Mares aborted when the cups are functional generally will not return to estrus until after the cups have regressed, approximately 100 to 120 days postovulation. Three to five daily injections of prostaglandin will generally abort a mare around the fifth day of therapy. Rarely, a cycle will follow two to three days postabortion, but the fertility of these cycles is not well documented. Large volumes of saline can also be used at this time, but frequently, treatments must be repeated at two or three 24-hour intervals. Abortion can occur immediately or within 24 hours. Since the uterus is invaded, some researchers recommend the addition of antibiotics into the solution. After 80 days, abortion can be induced by penetrating the cervical seal with a finger, gently dilating the cervix for approximately 10 minutes. The placenta is ruptured and the fetus removed.

After four months, prostaglandin will terminate pregnancy if administered repeatedly at two times the normal luteolytic dose. Some mares will abort in a week, the remainder in the following 21 days. Manual dilation and withdrawal of the fetus and placenta has been used to terminate pregnancies as advanced as 150 days. After seven months, abortion is similar to the induction of parturition.
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