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 fever 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 Overview of Rodenticide Poisoning in Animals Through centuries past, a variety of methods, active ingredients, formulations, and control measures have been used for rodent pest control. These efforts become more apparent as urban areas... read more , many grasses, or moldy sweet clover Sweet Clover Poisoning in Animals Melilotus spp (sweet clover). In the 1920s, cattle in North America developed a fatal hemorrhagic disease. Various causes, including pathogenic organisms and nutritional deficiencies... read more . 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 Overview of Mycotoxicoses in Animals For discussion of mycotoxicoses in poultry, see Mycotoxicoses in Poultry. Acute or chronic toxicoses in animals can result from exposure to feed or bedding contaminated with toxins produced... read more , especially those with estrogenic activity, have been implicated in bovine abortions. Nitrates or nitrites have also been incriminated, but experimental evidence is controversial.
Bluetongue as a Cause of Abortion in Cattle
Bluetongue 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.
Bovine Viral Diarrhea as a Cause of Abortion in Cattle
In several surveys, bovine viral diarrhea Bovine Viral Diarrhea and Mucosal Disease Complex Bovine viral diarrhea/mucosal disease is a pestivirus infection of cattle and other ruminants. Infection leads to immunosuppression and can cause signs in multiple body systems in addition to... read more (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 made via identification of BVD virus by means of isolation, immunologic staining, PCR assay, 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 for testing. Increasing BVD antibody titers in aborting animals or herdmates is diagnostic for recent infection. BVD virus is immunosuppressive and is found in many fetuses with infections caused by other agents (eg, bacteria, N caninum). Outbreaks of abortions by organisms that typically cause sporadic abortion should raise suspicion for possible concurrent BVD virus infection.
Prevention should focus on removal of persistently infected cattle and herd vaccination.
Brucellosis as a Cause of Abortion in Cattle
Brucellosis Brucellosis in Large Animals (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 months), 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 weeks to 5 months 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 contents 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.
Campylobacteriosis as a Cause of Abortion in Cattle
Campylobacter fetus venerealis causes venereal disease that usually results in infertility or early embryonic death but occasionally causes abortion between 4 and 8 months 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 Bovine Genital Campylobacteriosis .)
Chlamydiosis as a Cause of Abortion in Cattle
Chlamydophila 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 Enzootic Abortion of Ewes (EAE) 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... read more . The bacterium is zoonotic, occasionally producing life-threatening disease and abortion in pregnant women.
Epizootic Bovine Abortion (Foothill Abortion) in Cattle
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 months 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.
A novel Deltaproteobacterium closely related to Myxococcales,Pajaroellobacter abortibovis, has been identified as the etiologic agent. It is transmitted by the argasid tick Ornithodoros coriaceus ("pajaroello tick"). 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.
Infectious Bovine Rhinotracheitis (IBR, Bovine Herpesvirus 1) as a Cause of Abortion in Cattle
Infectious bovine rhinotracheitis (IBR) is a major cause of viral abortion worldwide, 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 weeks to 4 months, it causes a placentitis, then infects the fetus and kills it in 24 hours.
Abortion can occur any time but usually is from 4 months 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 Bovine Herpesvirus 1 Bovine herpesvirus 1 infections are widespread in the cattle population. In feedlot cattle, the respiratory form is most common. The viral infection alone is not life-threatening but predisposes... read more .)
Leptospirosis as a Cause of Abortion in Cattle
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 weeks after maternal infection. The most common organisms in cattle are L borgpetersenii serovar Hardjo type hardjobovis, L interrogans serovar Hardjo type hardjoprajitno, L interrogans serovar Pomona, and L kirschneri serovar Grippotyphosa.
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 urine throughout life. For other serovars, the dam’s urine can be cultured or examined for leptospires within 2 weeks of abortion.
For control, sources of infection (such as feed or water contaminated by dogs, rats, or wildlife) should be identified and eliminated. There is very little or no cross-protection among serovars. Immunization is based on the use of multivalent vaccines. The primary vaccination requires two doses at a 4- to 6-week interval followed by an annual booster. In some areas, use of a bacterin every 6 months 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 hours apart).
Listeriosis as a Cause of Abortion in Cattle
Listeria monocytogenes can cause placentitis and fetal septicemia. Abortions are usually sporadic but may affect 10%–20% of a herd. Abortion occurs 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 Listeriosis is a reportable disease in many areas and is a serious zoonosis, with spread possible through improperly pasteurized milk.
Mycotic Abortion in Cattle
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 sporadic bovine abortion. Abortions occur from 4 months 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.
Neosporosis as a Cause of Abortion in Cattle
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 months of gestation but is most common between 4 and 6 months 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 means of immunohistochemical staining and PCR assay. Many late gestation fetuses have detectable precolostral antibodies. They remain infected for years and possibly for life. Vertical transmission is common.
During pregnancy, Neospora Neosporosis 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.
Trichomoniasis as a Cause of Abortion in Cattle
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 after abortion. A significant proportion of cows develop 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 weeks, but bulls, especially those infected after 3 years 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 months 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 to help reduce shedding. ( See also Trichomoniasis Trichomoniasis .)
Trueperella pyogenes as a Cause of Abortion in Cattle
Trueperella 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.
Ureaplasma diversum Infection as a Cause of Abortion in Cattle
Ureaplasma diversum is a common inhabitant of the vagina and prepuce of cattle that also causes abortions. Abortions are usually sporadic, 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.
Other Causes of Abortion in Cattle
Akabane virus Akabane and Related Simbu Serogroup Virus Infections in Ruminants Akabane and related Simbu serogroup viruses are transmitted to ruminants by Culicoides midges. They cause congenital abnormalities of the CNS and musculoskeletal system in ruminants.... read more (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.