Abortion in the mare may be caused by infectious and noninfectious causes. The placenta and fetus are susceptible to both transplacental (hematogenous) spread and to ascending infection from the vaginal tract.
Abortions occurring prior to the 4th month of gestation are rarely observed (too small). Most observed abortions occur from 6-11 months of gestation. Early embryonic and fetal death in the equine which commonly occurs between 20-90 days of gestation are more often considered infertility problems rather than abortion problems.
Some of the etiologies of early pregnancy loss in the mare include:
Bacterial causes of equine abortion include.
Most of these organism gain access to the fetus via ascending infections through the cervix. Once through the cervix the organism invades the allantoic and amniotic cavities and the fetus. There are other of these organisms, such as Salmonella and Leptospira that enter the uterus via the blood stream (hematogenous). The majority of these bacterial agents cause abortion during the last trimester of gestation.
A beta-hemolytic streptococcus, the most common bacterial cause of abortion in the mareThis organism is commonly found on the external genital organs of mares and stallionsNot uncommon to culture from the genital tract of large percent of foaling mares for several days after parturition.
This is the most common bacteria associated With vaginitis, cervicitis and endometritis in "wind suckers", i.e mare affected with pneumovagina.
Strep. zoo abortions commonly occur on poorly managed farms where proper breeding hygiene, proper breeding management and veterinary care is not adhered to.
These abortions are also common where frequent services of mare at foal heat, and frequent breeding of older mares occur.
Abortion may occur without any history of illness. Most common to see abortion 7 to 21 day following mild illness in mare, (Fever, anorexia, depression and icterus).
Fetus too autolytic to successfully attempt culture, FAT or special histopath stains.
Maternal serology may be of some help. since the aborting mare shows a rising antibody titer.
6.3.1 Mycotic cause
A. Mycotic of fungal causes of abortion in the mare have been attributed to the organisms Aspergillus fumigatus Allescheria boydii and Mucor spp. These organisms cause sporadic abortions in mares. (5-30% of all infectious abortions)
The organisms gain entry to the uterus and fetus via ascending through the cervix, which accounts for the extensive involvement of the choriollantois. Abortion occur after the placenta has been infected for many weeks.
2. Usual History:
Abortions occur later in gestation most commonly, but can be seen any time during the last half of gestation. Some of these fetuses may be expelled alive.
3. Fetal Characteristics/Lesions:
Fetus usually small for gestational age - growth retardation (fetal dysmaturity) secondary to chronic placental disease.
- -- Mycotic skin lesions rarely occur
- -- Mycotic pneumonia commonly, especially in aborte foals infected with lucor sp.
4. Placental Lesions:
- Extensive involvement marked by thickening, edema, necrotic surfaces with ulceration, viscous exudate.
- Necrotic amniotic plaques
- a. Organisms easily cultured from fetal organs, placenta
The disease is limited to the state of Florida and is endemic in the tropics
The most common viral cause of abortion in the mare. The virus is widespread in the U.S.A., Canada and European Countries.
The virus commonly affects weanlings in the fall of the year and causes a mild febrile respiratory disease characterized by coughing, nasal discharge, anorexia and depression. The disease is highly contagious.
By inhalation primarily, also transmission from infected weanlings to broodmare. Mares aborting can transmit virus from aborted fetus, placenta and fluids. This virus can also be spread by anyone handling infective material
- a. The virus is often unapparent in adult horses, but pregnant mares exposed to the virus after the 7th month of gestation may abort.
220.127.116.11 Usual History
Most herpervirus 1 abortions occur sporadically in a herd. Maiden mares unprotected via vaccination are most often affected.
- a. Multiple abortions in all age groups of mares indicates a herd with no immunity.
- b. Mares abort without premontory signs, no mammary gland development, the fetus is fresh and still attached to the placenta.
18.104.22.168 Incubation Periods to Abortion
- a. Averages 20-30 days, but may extend up to 90 days.
- b. The long incubation period may be due to the persistence of the virus in the leukocytes of the mare. The virus then invades the chorion of the placenta and gradually invades the fetus, causing death and expulsion.
- c. Fetal death and abortion rarely occur before the 5th month of gestation with approximately 90% of abortions occurring from 8 months to term.
- d. Occasional infected foals are born at term but die within 2-9 hours.
22.214.171.124 Fetal Lesions
- a. Fetus expelled fresh - little to no autolysis
- b. Some aborted fetuses show no significant lesions
- c. Common gross lesions seen include:
- (a) mild icterus
- (b) hydrothorax and hydroperitoneum
- (c) pulmonary edema
- (d) numerous small, grayish-white necrotic foci (1-2 mm) in liver
- (e) scattered petechiae
- (f) focal necrosis in lymph nodes, spleen and adrenals
- (g) perirenal edema
Rarely are all these lesions found in the same fetus, but the most consistent are pulmonary edema, hydrothorax, and necrotic hepatic foci
- a. Histopathological - viral intranuclear inclusion bodies can be seen in necrotic tissues from the liver, lung thymus, lymph nodes and adrenals. Other histopathology findings include a necrotizing bronchiolitis and focal pneumonitis in aborted fetuses as well as in term foals
- b. FAT - on fetal liver, lung and thymus
- c. Viral isolation can be attempted from fetal lung, liver or thymus (frozen specimens)
6. Additional information and required reading, Morrow, Current Therapy in Theriogenology, pp 711-714.
This virus will cause abortion in the pregnant mare on these rare occasions when outbreaks occur. The Arteritis virus causes a generalized systemic and respiratory disease in horses which was characterized as a severe "shipping-fever" like disease. Affected animals may present with fever (103-1060F), conjunctivitis, photophobia, lacrimation, palpebral edema, depression, weakness, elevated respiration, anorexia, pulmonary edema, and edema of ventral abdomen and limbs.
The disease can cause death in horses - course of disease - 2-15 days.
Abortion in affected mares can occur at anytime within 1-14 days after the onset of the disease. Most mares abort during the last half of gestation (5-10 months). Incidence of abortion up to 50 percent.
The fetus is usually retained in the uterus several days after dying and thus is autolytic when expelled. No obvious gross lesions to be found in fetus but the literature reports that the virus can be recovered from fetal liver and lung.
Vaccine available but its use is closely regulated.
At this time the virus is rather avirulent such that obvious clinical signs are not noticed or very mild. The virus is reported to be endemic in standardbreds, however the last reported outbreak in the U.S.A. occurred in Thoroughbred stallions in Kentucky in 1984.
Pregnant mares infected with the EIA virus may deliver term virus-infected foals. The virus is known to be transmitted by intrauterine passage and through the milk. There appears to be some doubt as to whether actual abortions can be directly attributed to the EIA virus. It may be feasible to assume that pregnant mares may abort during the febrile phase of the disease as in other infectious diseases in the mare.
Some of the noninfectious factors suggested or suspected of causing abortion in the mare include:
- 1. The most common cause of non-infectious abortion in the mare, 20-30 percent of all observed equine abortions. It is reported that 60-80 percent of all twin fetuses that survive through the fourth month of gestation are aborted.
- 2. Stage of gestation - may occur at any stage, however the majority occur between 8 months and term.
- 3. Placental Lesions
- The characteristic placental lesion seen in mares with twin fetuses is manifest as a large area of the chorioallantois (Ca) which is devoid of villi. The affected area of the CA occurs as a result of the twin placentas in direct apposition to each other which does not allow normal villi to develop.
- 4. Twin Placental Classification:
- Three classifications or types of twin placentas.
- a. Type A - One twin fetus lies in one horn and nearly all of the uteine body while the other occupies most of the other uterine horn.
- b. Type B - Both twins share equally one horn and an equal portion of the uterine body. Most twins foals born at term that survive have type B placentation.
- Type C - The viable twin occupies all of one horn, body and most of the other uterine horn. The non-viable twin which is usually small and mummified is found in tip of the other uterine horn.
- 5. Fetal Lesions
- Growth retarded, small foal, due to placental dysfunction, lack of sufficient villous contact with the endometrium. One fetus may be of normal gestational size, other growth retarded fetus usually autolytic or mummified.
- 6. Usual History
- Mares which abort twins commonly are observed to suddenly show mammary development in mid to late gestation, milk will leak from the mammary gland and perineum relaxation will occur prior to the impending abortion.
- 7. Diagnosis:
- Based on finding typical CA lesion and twin fetuses.
Additional information and suggested reading. Morrow, Current Therapy in Theriogenology, pp 672 & 673, Roberts, Obstetrics and Genital Diseases.
6.6.2 Abnormalities of the Umbilical Cord:
Umbilical cord abnormalities are reported to cause abortion in the mare. Those abnormal conditions of the cord which can produce potentially lethal problems in the fetus are usually associated with excessive length and include:
- 1. Strangulation of the amniotic portion of the cord around parts of the fetus which causes deep grooves and local edema in the fetus. i.e. head, neck, back, thorax or legs.
- 2. Excessive torsion of the amniotic part of the cord with urachal obstruction and vascular obstruction.
This leads to possible rupture of the urinary bladder, rupture of the urachus. Vascular occlusion of the umbilical cord is reported to be a cause of death and abortion of an autolyzed fetus.
- a. Diagnosis - based on finding, the amniotic portion of the cord excessively twisted, with hematomas, aneurysms, thrombosis and edema present. This condition reported to be responsible for 1 percent of fetal deaths and abortions.
- State of Gestation:
- Between 5 1/2 and 7 1/2 months.
- 3. Long, umbilical cord with ischemic necrosis of the cervical star area of the Chorioallantois has been reported to cause abortion in the mare.
6.6.3 Uterine Body Pregnancy
- D. Chromosomal Aberrations are seldom a cause of abortion from 3 months to term, however death of the early embryo or fetus before 90 days of gestation may be caused by chromosomal aberrations.
- E. Maternal Malnutrition
- F. Severe Stress
- G. Maternal Pyrexia
- H. Endocrine Dysfunction