Also see Cattle.
Given the low diagnostic success rate, the high cost of laboratory work, and the low profit margin in both the beef and dairy industries, veterinarians should not attempt to make an etiologic diagnosis in every abortion. Instead, veterinarians should become concerned if fetal loss is >3%–5% per year or per month.
The actual incidence of abortions in cows due to genetic factors is unknown. Some genetically caused abortions may not have phenotypically recognizable lesions. Most lethal genes cause early abortion or early embryonic death.
Vitamins A and E, selenium, and iron have been implicated in bovine abortions, but documentation based on experiments is available only for vitamin A.
Heat stress causes fetal hypotension, hypoxia, and acidosis. High maternal temperature due to pyrexia may be more important than environmentally induced heat stress.
Although severe trauma may rarely result in abortion (the bovine fetus is well protected by the amniotic fluid), farmers undoubtedly blame too many abortions on the cow “getting bumped.”
A number of toxins can cause abortion in cows. Ponderosa pine needles can cause abortion if ingested in the last trimester; the cows may become moribund after delivery and hemorrhage excessively. The main abortifacient compounds in Ponderosa pine needles are isocupressic acid and labdane resin. Locoweed (Oxytropis or Astragalus sp) contains an indolizidine alkaloid that can affect the corpus luteum, chorioallantois, and neurons, resulting in abortion or deformities. Broomweed (Guttierrezia microcephala) ingestion can also cause abortion, as can coumarins from rat poison, many grasses, or moldy sweet clover. Sodium iodide, IV, has been contraindicated in pregnant cows, but no abortions or adverse effects occurred in pregnant cows treated with a single high dose in some studies. Mycotoxins, especially those with estrogenic activity, have been implicated in bovine abortions. Nitrates or nitrites have also been incriminated, but experimental evidence is controversial.
Neospora caninum is found worldwide and is the most common cause of abortion in dairy and beef cattle in many parts of the USA. Dogs and coyotes are definitive hosts for N caninum and can be the source of infection. Abortion can occur any time after 3 mo of gestation but is most common between 4 and 6 mo of gestation. Neospora can be associated with sporadic abortions or abortion storms, and repeat abortions in cows have been reported. Most infections result in an asymptomatic congenitally infected calf. Some infected calves are born with paralysis or proprioceptive deficits. Cows are not clinically ill, and placental retention is not common. The fetus is usually autolyzed or, in a few cases, mummified and rarely has gross lesions. Microscopically, nonsuppurative inflammation is common in the brain, heart, and skeletal muscles. Organisms can be identified in these tissues and the kidneys by immunohistochemical staining and PCR. Many late gestation fetuses have precolostral antibodies. They remain infected for years and possibly for life. Vertical transmission is common. During pregnancy, Neospora organisms can become activated and infect the fetus. This is thought to be the most common source of infection. There is no treatment. Strict hygiene to prevent fecal contamination of feed by dogs or coyotes may aid in prevention. A commercial vaccine is available. (See also Neosporosis.)
Bovine Viral Diarrhea (BVD)
In several surveys, BVD was the most commonly diagnosed virus in bovine abortion cases. The pathology of BVD in the developing fetus is complex. Infection before insemination or during the first 40 days of pregnancy results in infertility or embryonic death. Infection between 40 and 125 days of pregnancy results in birth of persistently infected calves if the fetus survives. Fetal infection during the period of organogenesis (100–150 days) may result in congenital malformations of the CNS (cerebellar hypoplasia, hydrancephaly, hydrocephalus, microencephaly, and spinal cord hypoplasia). Congenital ocular defects have also been seen (cataracts, optic neuritis, retinal degeneration, microphthalmia). After 125 days of gestation, BVD may cause abortion, or the fetal immune response may clear the virus. Diagnosis is by identification of BVD virus by isolation, immunologic staining, PCR, or detection of precolostral antibodies in aborted calves. The virus is present in a wide variety of tissues, but the spleen is the tissue of choice. Rising antibody titers to BVD in aborting animals or herdmates is diagnostic of recent infection. BVD virus is immunosuppressive and is found in many fetuses infected by other agents (eg, bacteria, N caninum). Outbreaks of abortions by organisms that normally cause sporadic abortion should raise suspicion of possible concurrent BVD virus infection. Prevention should focus on removal of persistently infected cattle and herd vaccination. (See also Bovine Viral Diarrhea and Mucosal Disease Complex.)
Infectious Bovine Rhinotracheitis (IBR, Bovine Herpesvirus 1)
Infectious bovine rhinotracheitis (IBR) is a major cause of viral abortion in the world, with abortion rates of 5%–60% in nonvaccinated herds. The virus is widespread, causes latent infections, and can recrudesce; therefore, any cow with a positive IBR titer is a possible carrier. The virus is carried to the placenta in WBCs; over the next 2 wk to 4 mo, it causes a placentitis, then infects the fetus and kills it in 24 hr. Abortion can occur any time but usually is from 4 mo to term. Autolysis is consistently present. Occasionally, there are small foci of necrosis in the liver, but in a large majority of cases there are no gross lesions in the placenta or fetus. Microscopically, small foci of necrosis with minimal inflammation are consistently present in the liver. Necrotizing vasculitis is common in the placenta. Diagnosis can be made by immunologic staining of the kidney, lung, liver, placenta, and adrenal glands. IBR virus can be isolated from ~50% of infected fetuses (most successfully from the placenta). In most cases, maternal titers have peaked by the time of abortion. In abortion storms, rising titers can often be demonstrated in herdmates. Control is by herd vaccination; intranasal, modified-live virus, and killed vaccines are available. (See also Bovine Herpesvirus 1.)
The pathogenic leptospires were formerly classified as serovars of Leptospira interrogans, but they have been reclassified into 7 species with >200 recognized serovars. Leptospira serovars Grippotyphosa, Pomona, Canicola, and Icterohaemorrhagiae usually cause abortions in the last trimester, 2–6 wk after maternal infection. Serovar Hardjo is host adapted to cattle and can establish lifelong infections in the kidneys and reproductive tracts. In addition to third trimester abortions, serovar Hardjo reduces conception rates in carrier cows and cows bred to carrier bulls.
Although dams may show clinical signs of leptospirosis, most abortions are in otherwise healthy cattle. Abortion rates vary from 5%–40% or more. The leptospires cause a diffuse placentitis with avascular, light tan cotyledons and edematous, yellowish intercotyledonary areas. The fetus usually dies 1–2 days before expulsion and therefore is autolyzed. Occasionally, calves are born alive but weak. Fetuses infected with serovar Pomona may show icterus. There are no specific lesions, but placenta and fetus should be submitted to the laboratory for fluorescent antibody staining or PCR testing for Leptospira. Although maternal titers are probably waning by the time of abortion, an initial titer of >1:800 may be suspicious. Approximately one-third of cows aborting because of serovar Hardjo have titers of <1:100 at the time of abortion. Cows infected with serovar Hardjo can shed the organism in the urine throughout life. For other serovars, the dam's urine can be cultured or examined for leptospires within 2 wk of abortion.
For control, sources of infection (such as feed or water contaminated by dogs, rats, or wildlife) should be identified and eliminated. Vaccination with a five-way bacterin every 6 mo provides good protection against serovars Grippotyphosa, Pomona, Canicola, and Icterohaemorrhagiae but does not protect against infection and renal shedding by serovar Hardjo. New monovalent serovar Hardjo vaccines that prevent infection, but do not cure existing infections, are available.
The following treatments have been found to eliminate the renal carrier state: a single injection of oxytetracycline (20 mg/kg, IM), a single injection of tilmicosin (10 mg/kg, SC), ceftiofur (5 mg/kg/day, IM, for 5 days or 20 mg/kg/day, IM, for 3 days), or amoxicillin (15 mg/kg, IM, two injections 48 hr apart).
Leptospirosis is zoonotic, and urine and milk of dams may be infective for up to 3 mo, except for Hardjo, in which case they can be infective for life if not treated. (See also Leptospirosis.)
Brucellosis (Bang's disease) is a threat in most countries where cattle are raised. In the USA, active control programs, including test, slaughter, and heifer vaccination, have greatly decreased its incidence. Brucellosis causes abortions in the second half of gestation (usually ~7 mo), and ~80% of unvaccinated cows in later gestation will abort if exposed to Brucella abortus. The organisms enter via mucous membranes and invade the udder, lymph nodes, and uterus, causing a placentitis, which may be acute or chronic. Abortion or stillbirth occurs 2 wk to 5 mo after initial infection. Affected cotyledons may be normal to necrotic, and red or yellow. The intercotyledonary area is focally thickened with a wet, leathery appearance. The fetus may be normal or autolytic with bronchopneumonia. Diagnosis can be made by maternal serology combined with fluorescent antibody staining of placenta and fetus or isolation of B abortus from placenta, fetus (abomasal content and lung), or uterine discharge. Prevention is by calfhood vaccination of heifers.
Brucellosis is a serious zoonosis and a reportable disease, and the appropriate authorities should be contacted. (See also Brucellosis in Large Animals.)
Fungal placentitis due to Aspergillus sp (septated fungi, 60%–80% of cases), or to Mucor sp, Absidia, Rhizopus sp, and a few other nonseptated fungi, is an important cause of bovine sporadic abortion. Abortions occur from 4 mo to term and are most common in winter. It is believed the fungi gain entry through the oral or respiratory tracts and travel hematogenously to the placenta. Placentitis is severe and necrotizing. Cotyledons are enlarged and necrotic with turned-in margins. The intercotyledonary area is thickened and leathery. Adventitious placentation is common. The fetus seldom is autolyzed, although it may be dehydrated; ~30% have gray ringworm-like skin lesions principally involving the head and shoulders. The diagnosis is based on the presence of fungal hyphae associated with necrotizing placentitis, dermatitis, or pneumonia. Fungi can also be isolated from the stomach contents, placenta, and skin lesions. Isolation must be correlated with microscopic and gross lesions to exclude contamination after abortion.
For control, moldy feed should be avoided. (See also Mycotoxicoses.)
Trueperella (Arcanobacterium) pyogenes causes sporadic abortion at any stage of pregnancy. Rarely, the incidence in a herd may reach epizootic levels. The bacterium is present in the nasopharynx of many healthy cows and in abscesses. It is not normally present, even as a contaminant, in fetuses or fetal membranes, and isolation is almost always significant. It gains entry to the bloodstream and causes an endometritis and placentitis, which is diffuse with a reddish brown to brown color. The fetus is usually autolyzed, with fibrinous pericarditis, pleuritis, or peritonitis possible.
Bronchopneumonia may be evident on histopathology, but T pyogenes is best cultured from placenta or abomasal contents. Abortion is usually sporadic, and no effective bacterin is available.
Tritrichomonas foetus infection causes a venereal disease that usually results in infertility but occasionally causes abortion in the first half of gestation. Placentitis is relatively mild, with hemorrhagic cotyledons and thickened intercotyledonary areas covered with flocculent exudate. The placenta is often retained, and there may be pyometra. The fetus has no specific lesions, although T foetus can be found in abomasal contents, placental fluids, and uterine discharges. Infected cows typically clear the organism within 20 wk, but bulls, especially those infected after 3 yr of age, can become lifelong carriers. There is no legal, effective treatment for individual animals. Herd treatment is based on identifying and segregating pregnant females from “at-risk" females for ≥5 mo and by identifying and culling all infected bulls. Prevention is by artificial insemination or natural insemination using noninfected bulls. A killed, whole-cell vaccine is available for use in cows. (See also Trichomoniasis.)
Campylobacter fetus venerealis causes venereal disease that usually results in infertility or early embryonic death but occasionally causes abortion between 4 and 8 mo of gestation. C fetus fetus and C jejuni are transmitted by ingestion and subsequent hematogenous spread to the placenta. Both cause sporadic abortions, usually in the last half of gestation. The fetus can be fresh with partially expanded lungs or severely autolyzed. Mild fibrinous pleuritis and peritonitis may be noted, as well as bronchopneumonia. Placentitis is mild with hemorrhagic cotyledons and an edematous intercotyledonary area. Campylobacter spp can be identified by darkfield examination of abomasal contents or culture of placenta or abomasal contents. Isolation and identification of the species involved is important if vaccination is to be instituted. Venereal campylobacteriosis can be controlled by artificial insemination and vaccination. Campylobacter spp are zoonotic, and C jejuni is an important cause of enteritis in people. (See also Bovine Genital Campylobacteriosis.)
Listeria monocytogenes can cause placentitis and fetal septicemia. Abortions are usually sporadic but may affect 10%–20% of a herd. Abortion is at any stage of gestation, and the dam may have fever and anorexia before the abortion; retained placenta is common. The fetus is retained for 2–3 days after death, so autolysis may be extensive. Fibrinous polyserositis and white necrotic foci in the liver and/or cotyledons are common. Diagnosis is by culture of Listeria from fetus or placenta. There is no available bacterin. Listeriosis is a reportable disease in many areas and is a serious zoonosis, with spread possible through improperly pasteurized milk. (See also Listeriosis.)
Chlamydia abortus, the cause of enzootic abortion of ewes, causes sporadic abortion in cattle. Most abortions occur near the end of the last trimester, but they can occur earlier. Placental lesions consist of thickening and yellow-brown exudate adhered to the cotyledons and intercotyledonary areas. Histologically, placentitis is consistently present, and pneumonia and hepatitis can be found in some cases. C abortus can be identified by examination of stained smears of the placenta or by ELISA, fluorescent antibody staining, PCR, or isolation in embryonated chicken eggs or cell culture. Organisms can often be identified in the lungs and liver but not as consistently as in the placenta. There are no vaccines for cattle, although they are produced for sheep (see Enzootic Abortion of Ewes (EAE)). The bacterium is zoonotic, occasionally producing life-threatening disease and abortion in pregnant women.
Ureaplasma diversum Infection
Ureaplasma diversum is a common inhabitant of the vagina and prepuce of cattle that also causes abortions. Abortions are usually single, but severe outbreaks occur on occasion. The infection may also result in stillbirths and birth of weak calves. Most fetuses are aborted in the third trimester and are well preserved. The cows are not sick, but retained placentas are common. Placentitis and a necrotic amniotic membrane are common features. The intercotyledonary areas are usually thickened and sometimes contain areas of fibrin deposition and hemorrhage. There are no gross lesions in the fetus. Microscopically, there is nonsuppurative placentitis and pneumonia characterized by accumulations of lymphocytes around bronchi and by diffuse alveolitis. Diagnosis is by isolation of U diversum from the placenta, lungs, and/or abomasal contents.
Epizootic Bovine Abortion (Foothill Abortion)
Epizootic bovine abortion is localized to the foothill region surrounding the Sacramento/San Joaquin Valley and the Eastern Sierra Nevada range of California, Oregon, and Nevada. Epizootic bovine abortion usually causes a protracted abortion storm affecting primarily heifers or cows recently introduced to the geographic region; however, abortion can occur 3–5 mo after leaving the endemic area. Abortion is usually in the last trimester, and rates may be as high as 60%. The animals abort without illness, and the fetus is seldom autolyzed. Although the etiologic agent has not been definitively determined, it is transmitted by the argasid tick Ornithodoros coriaceus and is believed to be an unnamed bacterium in the Myxobacteria family (order Myxococcales). The aborted fetus may have hepatomegaly, splenomegaly, and generalized lymphomegaly. Microscopically, there is marked lymphoid hyperplasia in the spleen and lymph nodes and granulomatous inflammation in most organs. Fetal IgG is increased. Cows seldom abort in subsequent pregnancies, and heifers are often exposed to endemic areas before breeding age in an effort to prevent abortions.
Bluetongue is caused by an Orbivirus with 24 serotypes and is transmitted by biting midges of the genus Culicoides. Historically, bluetongue occurred from approximately latitude 35°S to 40°N, except in the western USA, where it occurs to 45°N. After introduction of an attenuated, live virus serotype 10 vaccine in the 1950s, abortion, mummification, stillbirth, and the birth of live offspring with CNS malformations occurred in cattle and sheep. Since then, multiple bluetongue serotypes have been identified as causes of similar reproductive losses in cattle and sheep. Attenuation of bluetongue virus can increase its ability to cross the placenta. There is evidence that before 2007, reproductive losses were caused by attenuated bluetongue vaccine viruses, either by vaccination of pregnant animals or by spread of vaccine virus in nature by Culicoides spp.
In 2006, serotype 8 bluetongue virus appeared, spread, and became endemic across northwestern Europe (north of 50°N), where bluetongue was previously unknown. Beginning in 2007, abortions and birth of “dummy” calves with brain malformations occurred in bluetongue-infected herds; affected calves were documented to have been infected in utero. Since then, many such cases have been reported. Diagnosis is by identification of precolostral antibodies to bluetongue or identification of the virus by PCR. Brain, spleen, and whole blood are the preferred samples from fetuses and neonates for PCR. Control of bluetongue is by vaccination and management procedures to reduce exposure to biting midges. Modified-live and inactivated vaccines are available, but their availability and use varies between countries. (See also Bluetongue.)
Other Causes of Abortion
Akabane virus (where present) causes abortion and fetal anomalies. Parainfluenza-3 virus causes abortion in experimentally inoculated seronegative cattle, but is seldom, if ever, diagnosed in field cases of abortion. Occasionally, Salmonella spp cause abortion storms. The cows are usually sick, and the fetuses and placentas are autolyzed and emphysematous. Salmonellae can be isolated from the abomasal contents and fetal tissues and from uterine fluids and the dams' feces. Mycoplasma spp, Histophilus somni, and a wide variety of other bacteria can also cause sporadic abortions in cattle. Schmallenberg virus, discovered in Europe in 2011, belongs to the Simbu serogroup and has been associated with infertility, abortion, and fetal malformation in several ruminant species.
Last full review/revision August 2015 by Ahmed Tibary, DMV, PhD, DACT