Causes of Abortion

CHAPTER 182 Causes of Abortion

Abortions in horses are a common occurrence. A comprehensive study of causes of abortion, stillbirth, and perinatal death in horses over a 6-year period revealed that infection of the fetus or placenta caused by bacteria, equine herpesvirus, fungi, or placentitis of unknown cause was the most common cause of abortion and accounted for 34% of the diagnoses. The second most common cause involved birth complications, including asphyxia, dystocia, and trauma (19% of cases). Other common diagnoses were placental edema or premature separation of the placenta, twins, contracted foal syndrome, and other congenital and umbilical cord abnormalities. No diagnosis was made in 16% of the cases.


Viral Infection

Viral causes of abortion in horses are few in number but are important in equine reproduction, largely because of the potential for abortion outbreaks within a herd.

Equine Herpesvirus-1

Abortion associated with equine herpesvirus-1 (EHV-1) infection, and rarely EHV-4 infection, is well recognized and can occur as a random, isolated occurrence on a horse farm or as an abortion outbreak affecting multiple mares over a short period. The medical history may indicate recent introduction of new individuals into the herd, or the herd may be closed with no new introductions or movement of horses. EHV-1 abortions typically occur without premonitory signs. Abortions usually affect mares in mid to late gestation, and the delivery is usually rapid. Gross lesions are fairly characteristic, and the fetus is typically fresh, well preserved, and in good body condition. Internally the fetus may have clear yellow edema fluid in the thoracic cavity, pericardial sac, mediastinum, perirenal area, abdomen, and subcutaneous tissues and along the facial planes of the neck. Hemorrhagic areas on the surface of the organs and tissues are common. The liver may contain multifocal 1- to 2-mm whitish foci on the surface and within the parenchyma, and the lung typically is firm. The placenta is usually passed shortly after the fetus and is usually devoid of gross lesions.

Microscopically, EHV-1-associated abortions cause lesions in multiple organs, including the lung, liver, adrenal glands, spleen, thymus, and allantochorion. The lesions consist of multifocal areas of necrosis with accumulation of cellular debris and eosinophilic intranuclear inclusion bodies within parenchymal cells around the necrotic foci. Because of the rapidity of the abortion, little to no inflammation is associated with the necrosis and placentitis is usually not involved.

Gross lesions are highly suggestive of herpesvirus abortion, and microscopic lesions with characteristic intranuclear inclusions are diagnostic. Laboratory confirmatory tests include fluorescent antibody or immunohistochemical stains on fetal tissues, polymerase chain reaction (PCR) testing, and virus isolation using susceptible cell lines. Maternal serology can be performed; however, the common practice of vaccinating mares repeatedly against EHV-1 and the overlap seen between titers resulting from vaccination and those from natural infection makes interpretation of a single sample problematic.

Equine Viral Arteritis

Equine viral arteritis usually arises as an outbreak of influenza-like illness in adult horses (see Chapter 35, Equine Viral Arteritis). Some outbreaks consist of only respiratory signs in the herd, but other outbreaks also include mares that abort because of the infection. Fetuses are usually aborted late in gestation, may be aborted in either an autolyzed or fresh state, and are infected by arteritis virus. Gross examination of the aborted fetuses may reveal pulmonary edema but usually no other lesions. Microscopically, aborted fetuses may have no lesions or there may be inflammatory changes in sections of brain, liver, spleen, heart, placenta, and other tissues. Lesions consist of a variable vasculitis with mononuclear cells and rare neutrophils seen within the vessel walls and forming small perivascular cuffs. Presumptive diagnosis can be made by detection of vasculitis in aborted fetuses during an outbreak of abortions. The diagnosis is confirmed by virus isolation from the fetal tissues and serum neutralization evidence of maternal seroconversion.

Bacterial and Fungal Infection

Bacterial and fungal infections of the fetoplacental unit are an important cause of abortion in horses. In one study in central Kentucky, fetoplacental infection caused approximately a third of abortions and neonatal deaths over a 6-year period, and bacterial infection caused almost three fourths of the abortions or deaths in which an etiologic agent was identified. These can be infections involving only the fetal membranes, or there can be concurrent spread to the fetal fluids and fetal organs. Most bacterial and fungal abortions are associated with placentitis, and the placentitis can be ascending or hematogenous in origin.


Ascending Placentitis

Ascending placentitis is the most common type in the mare. The bacteria or fungal organisms gain access to the placental membranes by traversing the lower segments of the reproductive tract and passing through the cervix. The cervical star region of the chorion is initially affected, and the infection may extend cranially on the body of the allantochorion. Subsequently the infectious agent may gain access to the fetal fluids, other placental structures, and the fetal organs. One study reported that bacteria were cultured from both the placenta and fetal organs in about 60% of the cases, demonstrating that dissemination of infection from the membranes is common. Multiple species of bacteria are cultured from infected placentas, and because these are common isolates from the lower reproductive tract of the mare, it is believed that these bacteria are opportunistic. The most commonly isolated organisms are Streptococcus zooepidemicus, Escherichia coli, Pseudomonas aeruginosa, Streptococcus equisimilis, Enterobacter agglomerans, Klebsiella pneumoniae, and α-hemolytic streptococci. Fungal isolates are usually Aspergillus spp. and mucoraceous fungi.

Mares with ascending placentitis may spontaneously abort with no evidence of problems; however, some will develop premature mammary gland enlargement, and some will have vaginal discharge. With the increased use of transrectal ultrasound and the practice of routinely scanning pregnant mares, diagnosis of placentitis before abortion is possible. Ultrasound may demonstrate thickening of the membranes in the area of the cervical star, placental separation from the endometrium, and fluid accumulation.

Gross lesions are usually present and include irregular thickening of the allantochorion in the area of the cervical star with brown or tan discoloration and mild surface exudate. Sometimes the affected area is hemorrhagic or congested, resulting in reddish black discoloration. The cranial extent of the placentitis is often sharply demarcated from the unaffected chorion, and there may be a hemorrhagic zone. The remainder of the allantochorion is usually grossly normal (Figure 182-1). If the infection has gained access to the fetal fluids or extraembryonic coelom, there may be concurrent hemorrhages, plaques, or roughening of the allantoic membrane, amnion, or umbilical cord. There may be petechial or ecchymotic hemorrhages on fetal organs and membranes. The lungs may be firm, and the liver is sometimes swollen, soft, and brown or brownish yellow (Figure 182-1).

Histopathologic changes may be acute or chronic. The allantochorion is usually infiltrated by neutrophils, macrophages, and lymphocytes. Bacteria may be observed in the exudate or within the stroma. There may be histopathologic evidence of pneumonia, and the liver may have swelling and vacuolation of hepatocytes.

Specific diagnostic testing should include bacterial and fungal culture of the affected portion of allantochorion and culture of the lung, liver, and gastric contents.

Atypical (Nocardioform) Placentitis

In this atypical type of placentitis, the location and appearance of lesions differ from classical ascending placentitis, and the solitary nature of the lesion is not consistent with hematogenous distribution of pathogens. The pathogenesis and route of infection are unknown, but the likely route of entrance is via the mare’s lower reproductive tract. This form of placentitis is associated with the presence of gram-positive filamentous branching bacteria. Initially these organisms were unclassified, which led to the designation “nocardioform” on the basis of their morphology.

Nocardioform placentitis is not associated with a single bacterial species; rather, a group of similar organisms is commonly isolated. The most common nocardioform organisms that have been characterized and identified are Crossiella equi, Amycolatopsis kentuckyensis, Amycolatopsis lexingtonensis, and Amycolatopsis pretoriensis. Cases have been reported in which the gross appearance was identical to nocardioform placentitis but there was no evidence of nocardioform-type bacteria, although other species of bacteria were isolated, including Staphylococcus spp., Pantoea agglomerans, Cellulosimicrobium cellulans, Enterobacter

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Causes of Abortion

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