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Infectious Causes of Abortion
Viral Causes
Equine Herpes Virus Type I (EHV-I)
Equine herpes virus type I is the most common cause of abortion in horses. Typically abortion occurs at seven months gestation or later, and the aborted fetus appears necrotic or . Necropsy of the fetus shows necrotizing bronchiolitis, icterus, hepatomegaly, hydrothorax and necrotic foci in the liver and lymph nodes. The mare shows no clinical signs or symptoms of illness.
Equine Viral Arteritis (EVA)
Equine viral arteritis causes inflammation of the placental resulting in stenosis, which decreases the nutrient and oxygen supply delivered to the fetal circulation. Hypoxia and decreased nutrition of the fetus triggers an immune mediated reaction in the mare that results in metritis and abortion. Abortion caused by EVA occurs six to twenty-nine days after clinical disease is seen in the mare, at about 5-10 months. The aborted fetus is , with myocardial vasculitis evident on necropsy.
Bacterial Causes
Leptospirosis
Leptospirosis is caused by infection of leptospira spp. Bacteria. It commonly manifests as recurrent uveitis in horses and abortion in mares. These bacteria enters circulation and causes inflammation of the placenta. The mare aborts between 6-9 months. The placenta is fibrotic, shows signs of hemorrhage, edema and a thick brown mucoid matter. Upon necropsy of the foal, icterus, liver enlargement, hepatitis and nephritis are seen.
Potomac Horse Fever
Potomac horse fever is caused by infection of the Ehrlichia risticii bacterium. This bacterium crosses the placenta to enter fetal circulation and the fetal tissue. The presence of bacteria in the placenta, the fetal fluids and the fetus triggers an immune response in the mare, placentitis and metritis resulting in abortion of the fetus. Abortion caused by Potomac horse fever usually occurs mid to late gestation and necropsy of the fetus reveals the
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