Cows can suffer abnormalities during pregnancy leading to mummification of the foetus or resulting from maternal or foetal abnormality. All cases where the pregnancy terminates early and the foetus is expulsed are called abortions.
As there are multiple causes of abortion and the detection of abortions in a herd can vary significantly depending on the husbandry system and calving pattern, the incidence of abortion at herd level also varies markedly.
As there are multiple causes of abortion and the detection of abortions in a herd can vary significantly depending on the husbandry system and calving pattern, the incidence of abortion at herd level also varies markedly.
It has been suggested that an abortion rate of 5% or more in a herd should be considered an indication of an abortion problem
Abortion may be classified to:
Infectious:
Infectious:
- Non-specific
- Specific
Miscellaneous:
- Drug-induced (prostaglandins)
- Insemination/intra-uterine infusion
- Hypothyroidism
- Trauma/stress (transport, noise, veterinary treatment etc.)
- High fever and endotoxins (toxic plants, nitrate/nitrite, fungal toxins, other disease)
- Nutritional (malnutrition, vitamin A/selenium/vitamin E deficiency, goitre)
- Twin pregnancy
- Genetic (malformation)
Both non-specific and specific infectious causes of abortion can lead to "abortion storms" in a herd, whereas the miscellaneous causes often result in sporadic, individual cases.
The most important infectious abortion agents are:
The most important infectious abortion agents are:
- Brucellosis
- Listeriosis
- Leptospirosis
- Neospora caninum abortion
- Bovine Viral Diarrhoea (BVD)
- Infectious Bovine Rhinotracheitis (IBR)
- Campylobacteriosis
- Fungal/mycotic abortion
- Epizootic/chlamydial abortion
- Trichomoniasis
Brucellosis
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.
Listeriosis
Listeria monocytogenescan 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. Leptospirosis
The pathogenic leptospires were formerly classified as serovars of Leptospira interrogans, but they have been reclassified into 7 species with >200 recognized serovars. Leptospiraserovars 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.
Neosporosis
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.
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.Infectious Bovine Rhinotracheitis
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. Campylobacteriosis
Campylobacter fetus venerealiscauses 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 peopleMycotic Abortion
Fungal placentitis due to Aspergillussp (septated fungi, 60%–80% of cases), or to Mucor sp, Absidia, Rhizopussp, 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.
Chlamydiosis
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 Trichomoniasis
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.
