Abortion in Sheep
Also see Management of Reproduction: Sheep.
Abortion in ewes, as in cows, is not always easily diagnosed. Although many of the toxins that cause abortion in cows also cause problems in ewes, others such as Veratrum californicum and kale seem unique to the ewe. Subterranean clover (Trifolium subterraneum) is a common cause of early embryonic death and abortion in sheep. The major infectious agents causing abortions in sheep are Campylobacter sp, Chlamydia sp, Toxoplasma sp, Listeria sp, Brucella sp, Salmonella sp, border disease virus, and Cache Valley virus.
Infection with Campylobacter fetus fetus, C jejuni jejuni, and C lari results in abortions in late pregnancy or stillbirths. The route of infection is oral. Ewes may develop metritis after expelling the fetus. Placentitis occurs with hemorrhagic necrotic cotyledons and edematous or leathery intercotyledonary areas. The fetus is usually autolyzed, with 40% having orange-yellow necrotic foci (1–2 cm diameter) in the liver. Fetuses may have accumulated serosanguineous fluid in the thoracic and peritoneal cavities. Diagnosis relies on finding Campylobacter organisms in darkfield or fluorescent antibody preparations or by isolation from fetal abomasal contents, liver, and lungs, or from placental smears or in uterine discharge. Identification of the species involved is important, because in some areas C jejuni is as common as C fetus, and some vaccines do not include C jejuni. Strict hygiene is necessary to stop an outbreak. Use of tetracyclines may help prevent exposed ewes from aborting. However, many isolates may be resistant to tetracycline. The disease tends to be cyclical, with epizootics occurring every 4–5 yr; therefore, vaccination programs, which help prevent outbreaks, should be consistently practiced. C jejuni is zoonotic and is a common cause of enteritis in people.
Chlamydia abortus is the cause of EAE, which is characterized by late-term abortions, stillbirths, and weak lambs. C pecorum is the cause of chlamydial arthritis and conjunctivitis of sheep. EAE occurs worldwide, except for in Australia and New Zealand, and is most important in intensively managed sheep. Abortions occur during the last 2–3 wk of gestation regardless of when infection occurs, and the fetuses are fresh with minimal autolysis. Gross lesions in the fetus are rare and may include ascites, lymphadenopathy, and liver congestion. There is placentitis with necrotic, reddish brown cotyledons and thickened brown intercotyledonary areas covered by exudate. Chlamydial elementary bodies can be found by examination of Gimenez- or Giemsa-stained smears of the placenta or vaginal discharge, but the organisms cannot be differentiated from Coxiella burnetii, which occasionally causes abortion in sheep.
Definitive diagnosis is by identification of C abortus by ELISA, fluorescent antibody staining, PCR, or isolation. Ewes seldom abort more than once, but they remain persistently infected and shed C abortus from their reproductive tract for 2–3 days before and after ovulation. Rams can be infected and transmit the organism venereally. Control consists of isolating all affected ewes and lambs and treating in-contact ewes with long-acting oxytetracycline or oral tetracycline. C abortus bacterins are available and reduce abortions. In parts of Europe, a modified-live vaccine is available for use.
C abortus is zoonotic, but human cases are rare. All have involved pregnant women, who developed life-threatening illness. Only in a few cases in which the fetus was delivered by cesarean section did the infant survive. Pregnant women should not work with pregnant sheep, especially if abortions are occurring.
Border disease occurs worldwide and is an important cause of embryonic and fetal deaths, weak lambs, and congenital abnormalities. It is caused by a pestivirus closely related to bovine viral diarrhea (BVD) virus and classical swine fever (hog cholera) virus. Infection of susceptible animals occurs after introduction of persistently infected animals. Venereal transmission from shedding rams is possible. Abortion can occur at any stage of gestation. There are no clinical signs in the dam, except for a mild fever and leukopenia in a few cases. Live infected fetuses usually are undersized, and they often have congenital tremors and an abnormally hairy coat (hairy shaker lambs). Diagnosis is by identification of border disease virus in the placenta or fetal tissues (kidneys, lungs, spleen, thyroid glands, abomasum) by fluorescent antibody staining, virus isolation, or demonstration of precolostral antibodies. No vaccines are available. Inactivated BVD virus vaccines are sometimes used in sheep, but their effectiveness is unproved. (See also Border Disease.)
Cache Valley virus is a mosquito-transmitted cause of infertility, abortions, stillbirths, and multiple congenital abnormalities in sheep. The virus is endemic in most parts of the USA, Canada, and Mexico. Often, there are epizootics affecting sheep over a wide geographic area that can include several states. The most noticeable effects are stillborn lambs and the birth of live lambs with congenital abnormalities affecting the CNS and musculoskeletal system. Hydranencephaly, hydrocephalus, cerebral and cerebellar hypoplasia, arthrogryposis, scoliosis, torticollis, and hypoplasia of skeletal muscles are common. Infection before 32 days of pregnancy results in early embryonic death. Infection between 32 and 37 days of pregnancy results in musculoskeletal and CNS lesions. Infection between 37 and 48 days results in primarily musculoskeletal lesions. At the time of abortion or birth, the virus is usually no longer viable, and diagnosis is by demonstration of antibodies in precolostral serum or body fluids. Vaccines are not available.
Toxoplasma gondii is a major cause of abortion in small ruminants throughout the world. Ingestion of sporulated coccidian oocytes early in gestation results in resorption or mummification; if ewes contract the disease late in gestation, abortions or perinatal deaths occur. Ewes do not usually appear sick. In an outbreak, there is usually a wide range in gestational age of aborted fetuses. In most cases there are no gross lesions, but in a few cases there are distinct, small, white foci, 1–3 mm in diameter, in some cotyledons. The fetal brain often has focal areas of nonsuppurative inflammation on histology. Fetal serology (indirect hemagglutination inhibition, latex agglutination, or fluorescent antibody) may also be used. Once infected, ewes are immune, so running unbred ewes with aborting ones may allow them to develop immunity. Transplacental transmission is possible. Preventing contamination of feed by cat feces may help reduce exposure. Toxoplasmosis is a zoonosis. (See also Toxoplasmosis.)
Abortion caused by Listeria monocytogenes in ewes usually occurs in late gestation. Aborting ewes show variable clinical signs, including fever, depression, and anorexia, and some may succumb to septicemia. There is some necrosis of cotyledons and the intercotyledonary areas, and the fetus is usually autolyzed. Mummification is possible. The fetal liver (and possibly lung) may have necrotic foci, 0.5–1 mm in diameter. Diagnosis is by culture. (See also Listeriosis.)
The major importance of Brucella ovis is as a cause of contagious epididymitis in rams. On a flock basis, brucellosis results in infertility, but it also causes late-term abortions, stillbirths, and birth of weak lambs. B melitensis is rare in the USA but causes abortion in areas where it is found. B abortus occasionally causes abortion in sheep. Brucella abortions occur late in gestation, resulting in placentitis with edema and necrosis of the cotyledons and thickened, leathery intercotyledonary areas. Many fetuses aborted due to B ovis are alive at the beginning of parturition, although fetuses can be mummified or autolyzed. Most fetuses aborted due to B melitensis or B abortus are autolytic. Culture of the placenta, abomasal contents, and the dam’s vaginal discharge are diagnostic. A vaccine for B melitensis is available in some countries. B melitensis and B abortus are zoonotic. (See also Brucellosis in Large Animals.)
Salmonella Abortusovis, S Dublin, S Typhimurium, and S Arizona have caused abortions in sheep. S Abortusovis is endemic in England and Europe but has not been reported in the USA. The other serotypes occur worldwide. Most ewes are sick and febrile before aborting. There are no specific placental lesions, and the fetus is autolyzed. Diagnosis is by culture of placenta, fetus, or uterine discharge. Salmonella spp are zoonotic. (See also Salmonellosis.)
Bluetongue virus infection is a cause of abortion, fetal mummification, stillbirth, and congenital brain malformation in lambs. The clinical syndrome, serotypes involved, and diagnosis are the same as for cattle (see Bluetongue). Most, if not all, reproductive failure is caused by attenuated vaccine viruses rather than field viruses. Serotype 8 bluetongue virus has been documented as a cause of abortions and congenital brain malformations in cattle in northwestern Europe, and this is widely regarded as the first confirmed outbreak of transplacental transmission of wild-type bluetongue virus in ruminants. Published studies from Europe have not found evidence that serotype 8 is a significant cause of fetal infection in sheep. However, there are a few anecdotal reports of abortions and congenital anomalies in sheep attributed to serotype 8, and experimentally the virus has been demonstrated to be capable of crossing the ovine placenta. The virus should be considered a potential problem in sheep until more is known.
Control of bluetongue is by management procedures to reduce exposure to biting midges and vaccination. Inactivated and modified-live vaccines are available and widely used, but the availability of each varies between countries. Modified-live vaccines are predominantly used in regions with a long history of bluetongue, such as the USA and Africa, but their use is controversial. If used, the vaccine should not be given to pregnant ewes. Also, the vaccine should not be given when Culicoides spp are active, because they can and do transmit the vaccine virus to unvaccinated animals, including pregnant females. Bluetongue virus has a segmented genome, and reassortment viruses are readily produced in animals simultaneously infected or vaccinated with more than one serotype. (See also Bluetongue.)
Akabane virus (where present) causes abortion and congenital anomalies in sheep and is a differential diagnosis for Cache Valley virus infection. Coxiella burnetii causes occasional abortion storms in sheep, with the clinical syndrome and fetal pathology the same as for goats (see below). Neospora caninum has been reported to cause occasional abortions in sheep, with the lesions resembling those of Toxoplasma gondii. Fungal placentitis also occurs but is not as common in sheep as in cattle or horses. Other organisms associated with sheep abortion include Schmallenberg virus, Rift Valley fever, Nairobi sheep disease virus, Wesselsbron disease virus, Francisella tularensis, and Leptospira sp.