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
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.
Leptospirosis
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.
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 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
Mycotic Abortion
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.
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.
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