Bovine Abortion: Aetiology and Investigations

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FARM ANIMAL PRACTICE
Aborted bovine fetus.
Picture, R. D. Murray
Bovine abortion: aetiology
and investigations
EMMA CABELL
ABORTION in cattle is a significant cause of reproductive wastage and is of economic importance as a
single abortion event in a dairy herd is estimated to cost £630. Some abortions are due to infectious
diseases, several of which are zoonotic. Sporadic abortions occur in any herd, but once the incidence
exceeds 3 per cent, or several abortions occur in close succession, further investigations must be carried
out to try to determine the cause so that control measures can be instigated. This article reviews the
common causes of abortion in cattle and outlines an approach to the investigation of an abortion
outbreak, describing the tests available for diagnosis.
DIAGNOSTIC RATES FOR ABORTIONS
The diagnostic rate achieved for abortions is generally
low, averaging about 35 per cent of all cases where a
fetus and placenta are submitted for an abortion investigation. An important reason for this is that, in cattle,
there is frequently a delay between fetal death and expulsion. Autolysis occurs rapidly prior to expulsion, and can
have a significant deleterious effect on the sensitivity of
diagnostic tests.
The causes of abortion can broadly be divided into
non-infectious and infectious. In most cases where a
diagnosis is reached, the cause is infectious, although
non-infectious causes probably account for many un-
For the purposes of this article, abortion is defined
as the expulsion of a recognisable dead or nonviable fetus prior to the end of the normal gestation period and includes premature and stillborn
calves.
diagnosed abortions. The relative frequencies of diagnosis of the various infectious causes of abortion are shown
in the diagram below.
Neospora
species 26%
Other 30%
Emma Cabell
graduated from the
Royal Veterinary
College in 1993.
She spent nearly
five years in
predominantly farm
animal practice in
north Wales before
taking up a residency
in ruminant medicine
at Liverpool, where
she subsequently
became a lecturer
in the department
of livestock health
and welfare. Since
2003, she has worked
in the Veterinary
Laboratories Agency
(VLA) Surveillance
Centre at Liverpool.
She holds the
diploma in bovine
reproduction from
Liverpool, and the
RCVS certificate in
cattle health and
production.
Arcanobacter
pyogenes 8%
Bovine virus
diarrhoea 8%
Salmonella
Dublin 7%
Bacillus
licheniformis 21%
Autolysis of fetal internal organs. Note
the large volume of blood-stained fluid
within the thoracic and peritoneal
cavities. Picture, R. D. Murray
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Diagnosis of bovine fetopathy in 2006 as a percentage of all
cases of fetopathy where a diagnosis was reached (n=842).
From VIDA (2006)
455
In Practice (2007)
29, 455-463
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TOXIC
Safety precautions
Due to the zoonotic potential, care must be taken
when handling abortion material to ensure that
veterinary surgeons and their employees are not
at risk (Health & Safety at Work etc Act 1974). In
particular, pregnant women should avoid contact with aborted material. Protective clothing,
including waterproof overalls, rubber gloves and
Wellington boots, should be worn when handling
fetuses and placentas. Cut-proof gloves should
be worn if postmortem examination and sample
collection is carried out. All instruments, overalls
and other equipment should be cleaned and disinfected with an approved disinfectant at the appropriate dilution (see www.defra.gov.uk/animalh/
diseases/control/testing_disinfectants.htm for further information).
NON-INFECTIOUS CAUSES
PHYSICAL
Abortion can be caused by trauma, insemination, hyperthermia and twinning.
NUTRITIONAL
Iodine deficiency has been associated with stillbirths and
weak newborn calves. In affected cases, the fetal thyroid
gland is enlarged, usually weighing >30 g. The most common cause of iodine deficiency in farm animals is failure to provide iodine in the diet. Some parts of the UK
are iodine deficient, while secondary deficiency has also
been associated with high calcium intake, the feeding of
Brassica species and the application of human sewage
sludge on pasture. Micronutrients are not commonly implicated, but as well as iodine deficiency and toxicity, selenium/vitamin E deficiency has been associated with bovine
abortions in the UK (Mee 2004). An increase in the prevalence of abortions has been observed in herds where the
diet is deficient in vitamin A or its precursor β-carotene.
Several poisonous plants, including hemlock, cypress
and juniper, have been associated with abortion in cattle
in the UK, although abortion is unlikely to occur in the
absence of other signs of poisoning. The consumption of
needles of the ponderosa pine, found in some parks and
experimental forestry plantations in the UK, is a recognised cause of bovine abortion, resulting in a reduction of
serum progesterone levels. Ergot alkaloids have a known
abortigenic action, although abortion is rare in poisoned
animals. Cattle frequently abort following an outbreak of
nitrate/nitrite poisoning; this is probably due to methaemoglobinaemia leading to hypoxia in the dam and subsequent fetal hypoxia. Other clinical signs of poisoning such
as severe dyspnoea, muscle twitching and brown mucosae
and blood are likely to be present. Common sources of
nitrate/nitrite on farms are fertilisers and certain plants.
GENETIC
Genetic abnormalities that result in congenital malformations may cause fetal death and consequent abortion.
An example is complex vertebral malformation (CVM),
which occurs in purebred Holstein cattle and is caused
by a simple recessive genetic defect where both parents
are carriers. Affected calves are rarely carried to term.
Other defects that may result in the birth of dead calves
include dyschondroplasia (bulldog calves) and chromosomal abnormalities.
INFECTIOUS CAUSES
Infections during the early stage of gestation may result in
early embryonic death by directly affecting the embryo.
Infections at a later stage may lead to abortion, stillbirths
or the birth of weak live calves. Some infections indirectly affect fetal survival by the impairment of uterine
function (eg, maternal endotoxaemia due to acute coliform mastitis). Endotoxaemia can evoke prostaglandin
synthesis, leading to subsequent abortion, or induce intravascular coagulation, which interferes with placental circulation, resulting in fetal hypoxia. Other infections may
(left) Normal thyroid gland and (right) thyroid gland hyperplasia. Pictures, R. D. Murray
456
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directly damage the maternal component of the placenta
(eg, Brucella abortus infection). Maternal conditions that
lead to fetal hypoxia (eg, severe pneumonia, circulatory
failure resulting from traumatic pericarditis, or dehydration and acidosis as a consequence of carbohydrate
engorgement) can also lead to fetal death and abortion.
Infectious causes can be divided into bacterial, viral,
protozoal and fungal agents (see table below), and are
discussed in the following sections.
BACTERIA
Brucella abortus
B abortus infection has caused enormous losses to dairy
and beef cattle industries worldwide, and eradication
schemes exist in many countries. Cattle become infected
by ingesting the organism from infected abortion material/genital discharges or contaminated pasture, food
or water. It causes endometritis, placentitis and abortion. Retained fetal membranes are a common sequela.
COMMON INFECTIOUS CAUSES OF ABORTION IN CATTLE
Agent
Epidemiology
Time of abortion
Pathology
Fetus
Clinical features
Abortion rate
Notifiable disease.
Abortion, birth of weak/
dead calves, retained
placenta, infertility
Up to 90% in
susceptible herds
6+ months
Necrosis of cotyledons
and leathery, opaque
placenta with oedema
May be pneumonia,
which is only detectable
on histopathology
Campylobacter fetus
fetus
Abortion
Sporadic
4 to 8 months
Mild placentitis
Mild fibrinous pleurisy
and peritonitis,
bronchopneumonia
Leptospira hardjo
Few cows exhibit signs
other than abortion
Up to 30%
6+ months
Avascular placenta,
atonic yellow-brown
cotyledons, brown
gelatinous oedema
between allantois
and amnion
Fetal death is common
Arcanobacter pyogenes
Abortion occasionally
preceded by systemic
illness and mastitis,
and/or followed by
endometritis
Sporadic or multiple
6+ months
Suppurative placentitis
Often associated with
fetal pneumonia (on
histopathology)
Salmonella species
Abortion, frequently
without other clinical
signs in the dam
Sporadic. May be
a herd outbreak
Any time, 6 to 9 months
No specific lesions
No specific lesions
Listeria monocytogenes
Abortion
Sporadic. Rare reports
of abortion storms
Late, towards end
of gestation
Placentitis, multiple
yellow or grey necrotic
foci in cotyledons
May be multiple yellow
or grey necrotic foci in
liver
Bacillus licheniformis
Late abortions or live
calves with evidence
of placental lesions
Sporadic
Late, 5+ months
Placentitis and dry,
leathery, yellow-brown
allantochorion. Oedema
of allantochorion around
cotyledons, giving
appearance of vesicles.
Haemorrhagic and
necrotic cotyledons
May be fibrinous
pleurisy, pericarditis
and peritonitis
Depend on the stage of
pregnancy at which the
cow becomes infected
(see table, page 460)
Usually low
From 2 months
No specific gross lesions
May be congenital
abnormalities of the
central nervous system
or eyes (eg, cerebellar
hypoplasia)
Often associated with
previous or intercurrent
respiratory disease in
the herd
Sporadic, but can
be up to 60%
4 months to term
No significant gross
lesions
May be foci of hepatic
necrosis
Abortion, stillbirths and
live births may occur
Usually associated with
sporadic abortions.
Can occasionally cause
abortion storms (≥10%
of at-risk cows aborting
over a six-week period).
Repeat abortions from
the same cow are
common. Prevalence of
infection in herds may
be high. Congenital
transmission is the only
known method in cattle
3 to 8 months
(mean 5·5 months)
No characteristic lesions
in placenta
Inflammatory lesions
widespread in fetus.
Non-suppurative
encephalitis and
myocarditis common
with parasite
present, identified by
immunohistochemistry
More prevalent from
December to March
Sporadic, up to 5 to 10%
in some herds
4 to 9 months
Diffuse or multifocal
thickening, leathery,
yellow-brown
appearance of
cotyledons and
intercotyledonary
areas
May be small, raised,
grey-white soft lesions,
or diffuse white areas
on skin. Resembles
ringworm
Bacteria
Brucella abortus
Viruses
Bovine virus diarrhoea
virus
Bovine herpesvirus
type 1
Protozoa
Neospora caninum
Fungi
eg, Aspergillus species
mycosis
Placenta
Adapted from Miller (1986) and Radostits and others (2000)
458
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Placental lesions in a case of brucellosis. Picture, VLA Truro
sheep may be ‘silent’ carriers and also a potential source
of infection. Leptospirosis accounted for less than 2 per
cent of total abortion diagnoses from 2003 to 2006 (VIDA
2006) and appears to be declining in importance. This
may be a reflection of widespread vaccination.
Until recently, the diagnostic method of choice had
been the detection of leptospires in fetal kidney using
the fluorescent antibody test (FAT), which is relatively
insensitive. This has now been replaced by a PCR test
for pathogenic leptospires in fetal kidney. Early validation studies for this test report a significantly increased
sensitivity and specificity. Culture is considered the gold
standard, but is expensive, time consuming and requires
expertise, and is therefore not practical for routine diagnostic use. With the more frequent use of PCR testing
to investigate suspected cases, a more accurate indication of the true prevalence of abortion due to Leptospira
species in the UK should be obtained.
Bacillus licheniformis
Acid-fast rods typical of Brucella organisms on a vaginal
swab smear. Picture, VLA Starcross
In non-pregnant animals, the udder and supramammary
lymph nodes are colonised and infection of the placenta may occur during the next gestation. Infected cattle
often shed the organism in milk, which presents a serious risk to human health.
It is a legal requirement for cattle keepers to report
every abortion or premature calving to a divisional
veterinary manager (DVM) under Article 10 of the
Brucellosis (England) Order 2000 and the equivalent
legislation in Scotland and Wales. An abortion or premature calving is defined as ‘an abortion or calving that
takes place less than 271 days after service, or 265 days
after implantation or transfer of an embryo, whether the
calf is born dead or alive’.
Salmonella species
In the UK, Salmonella Dublin is endemic in cattle and
is the most common species of Salmonella causing abortion. Many other Salmonella species may be involved in
individual cases. Aborting cows are often pyrexic and
may be diarrhoeic although some may show no clinical
signs other than abortion. All salmonellae, including
S Dublin, are zoonotic. S Dublin rarely causes disease
in humans but, if it does, infection can be severe. All
Salmonella isolates must be reported under zoonoses
legislation (The Zoonoses Order 1989) to the appropriate nominated officer – that is, a veterinary investigation officer in regional VLA laboratories in England and
Wales, and the DVM of Animal Health (formerly the
State Veterinary Service) in Scotland.
Bacillus licheniformis is frequently associated with sporadic bovine abortion. During 2006, it accounted for 21 per
cent of VLA abortion submissions where a diagnosis was
reached (VIDA 2006). Experimental studies have demonstrated a predilection for the placenta, especially the fetal
membranes and the fetal side of the placentome (Agerholm
and others 1999). The organism is ubiquitous, although
silage, run-off water/liquor, foodstuffs and bedding that
become contaminated with silage effluent, and wet spoilt
hay are the most likely sources of infection. The pathogenesis is not known, but probably involves haematogenous
spread to the reproductive tract following ingestion.
Listeria species
Listeria monocytogenes is ubiquitous in the environment, being present in soil, sewage effluent, bedding and
foodstuffs. The organism gains entry by ingestion and
has a predilection for the placenta, causing a placentitis.
A substantial number of outbreaks have occurred following the start of silage feeding. Other manifestations
of listeriosis, including encephalitis and eye infections,
are rarely seen in conjunction with abortion. The disease
accounted for 3 to 4 per cent of abortion diagnoses from
2003 to 2006 (VIDA 2006). Other Listeria species, such
as Listeria ivanovii and Listeria innocua, also occasionally cause abortion.
Arcanobacter pyogenes
Arcanobacter pyogenes is sporadically isolated from
aborted calves and accounted for 8 per cent of abortion
Leptospira species
Leptospira interrogans serovar hardjo causes abortion
in cattle four to 12 weeks after infection, with or without
additional clinical signs of illness in the dam. The pathogen is transmitted venereally and via urine and abortion
material, and is a potential zoonosis. Watercourses can
become contaminated and be a source of infection, while
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Placentitis due to Bacillus
licheniformis (arrow).
Haematoxylin & eosin,
magnification x200.
Picture, M. Wessels
459
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diagnoses during 2006 (VIDA 2006). Experimental evidence suggests that it is a primary pathogen, directly
capable of causing abortion.
Campylobacter species
Two species of Campylobacter affect the reproductive tract of cattle. Campylobacter fetus venerealis is
commonly associated with infectious infertility and
early embryonic loss, while Campylobacter fetus fetus,
a commensal organism of the intestinal tract, causes
sporadic abortions.
Mycoplasma and Ureaplasma species
Mycoplasma species are occasionally isolated from
aborted fetuses. Mycoplasma bovis causes mastitis
and polyarthritis in cattle and, in some cases, extensive lesions in the uterus. Ureaplasma infections are
also occasionally implicated in bovine abortion, with
the main route of transmission thought to be venereal.
Mycoplasma and Ureaplasma species are not considered to be major infectious causes of abortion and routine testing is not undertaken.
Other bacterial infections
Other bacteria that are rarely isolated from bovine fetuses include Chlamydophila abortus and Coxiella burnetii
(Q fever), both of which have zoonotic potential.
VIRAL DISEASE
Bovine virus diarrhoea
Bovine virus diarrhoea (BVD) virus is widespread in
the UK, with approximately 95 per cent of herds having
serological evidence of BVD infection (Paton and others
1998). Reproductive losses may be the most economically important clinical consequences associated with
BVD. The potential reproductive outcomes following
infection of a naive cow with BVD virus are listed in the
table below.
BVD infection is maintained in a cattle population
by the development of persistently infected (PI) animals. These act as a potent source of infection for naive
animals and are estimated to make up between 1 and
2 per cent of the national herd, and potentially a much
higher proportion within an infected individual herd.
Conversely, some herds (eg, those in the Premium Cattle
Health Scheme) are free from infection. It is generally
POTENTIAL REPRODUCTIVE OUTCOMES FOLLOWING INFECTION WITH BOVINE VIRUS
DIARRHOEA (BVD) VIRUS AT DIFFERENT STAGES OF GESTATION
Potential manifestations of infection
in naive dams
Period of gestation when these are most likely to occur
Decreased conception rates,
early embryonic death
0 to 45 days
Abortions
Fetal death following BVD virus infection can occur at
any time during gestation, but is most common during
the first trimester. Fetal reabsorption, mummification
or expulsion can occur
Immunotolerance
Fetuses that survive infection with non-cytopathic
BVD virus at between 80 and 125 days’ gestation
invariably develop immunotolerance to the virus
and subsequently become persistently infected
Congenital defects
Fetal infection at between 100 and 150 days’ gestation
often results in the development of congenital defects
Fetal immune response to
BVD virus and virus cleared
Late gestation (125 to 285 days) when
immunocompetence and organogenesis are usually
complete
From Grooms (2004)
460
Cerebellar hypoplasia caused by mid-gestational bovine
virus diarrhoea virus infection. Picture, R. D. Murray
considered that fetal infection prior to the development
of immunocompetence (at days 90 to 120) results in
either fetal death or the development of a lifelong persistent infection. By day 125, the immune system is becoming fully functional and infection after this time usually
results in a competent immune response and subsequent
elimination of the virus, so that at birth the calf is BVD
antibody positive but antigen (virus) negative.
With the recent development of a real-time reverse
transcriptase (RT) PCR test for BVD RNA, it was anticipated that the detection rate for the virus in aborted
fetuses would increase. In fact, fewer abortions than
expected have been attributed to BVD infection despite
the increased sensitivity of the RT-PCR test, which suggests that, at present, there is still no evidence that BVD
is a major cause of abortion. Vaccination against BVD
is likely to have influenced this as it reduces the occurrence of fetal infection. Even if BVD antigen is detected
in a fetus, which is a significant finding, it is still entirely
possible that BVD was not the cause of abortion, as many
PI animals are born and live for some time, with most
succumbing to mucosal disease at between six and 24
months of age. Some PI animals live long enough to join
the adult herd and produce PI calves, so it is advisable to
test the dam of a fetus found to be positive for BVD virus
to determine if she is a PI animal. The indirect fluorescent
antibody test (IFAT) for the detection of BVD antibody
can be used in fetuses of 125 days’ gestation or greater to
indicate BVD challenge in utero. Both type 1 and type 2
BVD virus are present in the UK, with type 1 being the
most widespread. RT-PCR is able to differentiate between
the two. Immunohistochemistry on fixed brain tissue is a
useful aid to diagnosis in fetuses in the first trimester of
gestation prior to the development of immunocompetence.
Infectious bovine rhinotracheitis
Infectious bovine rhinotracheitis (IBR) is caused by
bovine herpesvirus type 1 (BHV-1). It usually results in
respiratory disease, but abortion may be seen in cows up
to 100 days after infection. Abortion generally occurs in
mid- to late pregnancy and aborting cows may or may
not have shown respiratory signs previously. Where systemic and respiratory disease results in pyrexia, which
is often very marked, this can cause abortion. The diagnosis of IBR-related abortions is difficult due to the
rapidly developing necrotising placentitis and expulsion
of the fetus before it is infected (reported experimentally). Currently, a FAT is used to detect IBR antigen,
but, in common with other FAT tests and virus isolation, this test is much less sensitive in autolysed tissue.
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Immunohistochemistry on internal organs can be useful where lesions of multifocal necrosis are observed on
postmortem examination.
PROTOZOAL DISEASE
Cluster of Neospora
caninum tachyzoites
in a cardiac myofibre.
Peroxidase–antiperoxidase
immunostaining with
N caninum-specific polyclonal
antibody, magnification x100.
Picture, E. H. Gibney
Neosporosis
Neospora caninum was first recognised as a cause of
abortion in cattle in 1989 and is now the most commonly diagnosed infectious cause. Oocysts are excreted by
dogs and ingested by intermediate hosts, including cattle, sheep, goats and horses. Tissue cysts and tachyzoites
are the stages of the life cycle found in cattle. Tachyzoites
develop in a wide range of tissues, including placenta and
amniotic fluids, while tissue cysts (containing bradyzoites)
are found only within the central nervous system. The
vertical route is the major mode of transmission in cattle, with infection of calves in utero occurring due to
recrudescence of disease in chronically infected cows.
Horizontal cow-to-cow transmission does not occur.
4 µm
Cows of any age may abort from three months’ gestation to term, although most neosporosis-induced abortions occur at six to seven months’ gestation. Fetuses
may die in utero, be resorbed, mummified, autolysed,
stillborn or born alive with clinical (neurological) signs,
or born clinically normal, but chronically infected; however, congenitally infected calves are generally clinical-
Postmortem examination of a fetus and placenta
When it is not possible to submit a fetus and
placenta to a laboratory, the practitioner
can perform a useful postmortem examination and collect appropriate samples either
on the farm or back at the practice.
Technique
■ Examine the skin of the fetus and placenta for lesions. Gross lesions are seen
relatively infrequently and are usually
associated with mycotic and B licheniformis
infections. Where the fetus is full term
and stillborn, look for signs of dystocia
(eg, oedema of the head, neck and tongue,
widespread petechial haemorrhages and
rib fractures)
■ If possible, measure the crown-to-rump
length (CRL). Calculate the gestation
length in days (Y) using the formula:
Y = 2·5 x (21 + CRL)
■ Place the fetus on its back and incise into
the axillae. Reflect the scapulae and disarticulate the hip joints to stabilise the carcase
■ Open the abdomen by incising along the
midline and the edges of the ribs
■ Open the thorax by incising along the
costochondral junction on each side
■ Record any gross pathology
Gross pathological findings in the aborted fetus are frequently unremarkable, but
can include purulent or fibrinous inflammation, which are suggestive of an infectious insult.
Sample collection
FOR CULTURE
■ Fetal stomach contents collected aseptically into a Vacutainer, or using a 5 ml syringe
and 16 gauge needle for ‘thick’ contents
■ Liver or lung (4 cm cube approximately)
■ Placenta with several cotyledons
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Kit for sample collection
■ Rubber and cut-proof gloves
■ Postmortem knife
■ Side-cutting pliers
■ Hacksaw
■ Scissors
■ Rat-tooth forceps
■ Number 22 scalpel blade and handle
■ Vacutainer and needle
■ 5 ml syringe and 16 gauge needle
■ Disposable pipette for obtaining
fetal fluid
■ Bijou vial x 2
■ Sterile sample pots x 5
FOR NEOSPORA IFAT (AND BVD IFAT, IF REQUIRED)
■ 2 ml fetal fluid from thoracic, pericardial
or abdominal cavities using a pipette and
bijou vial
FOR BVD RT-PCR
■ Thymus (1 cm cube in bijou vial)
FOR PATHOGENIC LEPTOSPIRES PCR
■ Kidney (1 cm cube in bijou vial)
FOR HISTOPATHOLOGY
■ If possible, fix the following tissues in
10 per cent formalin:
– 1 to 2 cm cube of heart
– Caruncle/cotyledon with intercotyledonary membrane
– Liver, lung and kidney
– Whole brain. Unless the fetus is nearly full
term, the bones of the skull are soft and not
completely fused and it is easy to remove
them using scissors or side-cutting pliers. In
stillborn animals, the skull will need to be
sawed. First reflect the skin from the head,
and make the first cut caudal to the lateral
canthus of the eye and at 90° to the midline.
Two cuts should then be made approximately 2·5 cm either side of the midline through
the nuchal eminence, and extending cranially through the frontal bones and crossing
the first cut. Caudally, they extend towards
the occipital condyles. A bone chisel can be
used as a lever to remove the frontal bones.
The dura mater is incised and reflected, taking care also to cut the tentorium cerebelli,
a crescentic fold that occupies the fissure
between the cerebellum and cerebral hemispheres. The brain can be removed by tilting
the head and severing the olfactory, optic
and remaining cranial nerves.
STILLBORN CALVES
■ Remove the entire thyroid gland, which
consists of two lobes connected by a band
adherent to the thyroid cartilage. If possible, weigh the gland, then fix half in formalin and submit the fresh half for tissue
biochemistry, specifically iodine analysis.
FOR MATERNAL SEROLOGY
■ Dam serum or clotted blood
Further information
Fetuses and placentas should be disposed of in accordance with the Animal
By-Products Regulations 2005. For
further information in each of the
devolved countries, see:
■ ENGLAND. www.defra.gov.uk/animalh/
by-prods/default.htm
■ WALES. new.wales.gov.uk/topics/
environmentcountryside/ahw/animal_
byproducts_waste/?lang=en
■ SCOTLAND. www.scotland.gov.uk/
Topics/Agriculture/animal-welfare/
policies/PolicyInfo/AnimalByProducts/
Introduction
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Placenta showing typical lesions of mycotic abortion.
Picture, W. R. Ward
Mycotic lesions on the
skin of an aborted fetus.
Picture, A. P. Foster
ly normal (Dubey 2005). Cows that are seropositive to
N caninum are 3·5 times more likely to abort than seronegative cows (Davison and others 1999). Congenitally
infected retained/replacement heifer calves may themselves infect future offspring or abort. Replacement
stock should therefore not be bred from known positive
cows.
The mechanisms by which Neospora infection of the
dam results in abortion are not understood, but appear
to involve recrudescence of infection possibly associated with changes in local uterine immune status during pregnancy. Diagnosis of Neospora-induced abortion
relies on the demonstration of antibodies to N caninum
in fetal fluid by IFAT or in maternal serum by ELISA;
this must be supported by histopathology of fetal tissues
(particularly the brain and heart) to show multifocal
encephalitis and myocarditis, which are consistent with
neosporosis. Immunohistochemistry is required to demonstrate the organism in tissues and distinguish it from
other protozoa.
Other protozoal infections
Outbreaks of abortion involving other protozoa, such as
Sarcocystis species, are occasionally reported.
FUNGAL DISEASE
Abortion can be caused by fungal invasion of the placenta and fetus. In the UK, mycotic abortion is much
more prevalent from December to March than the rest
of the year, suggesting an association with housing and
feeding of preserved fodder. Mouldy hay, straw, silage
and brewer’s grains are important sources of infection.
The fungi most frequently isolated are Aspergillus fumigatus and Absidia, Rhizopus and Mucor species. The
lesions on the placenta and calf are fairly characteristic
of mycotic infection, but need to be differentiated from
those caused by B licheniformis and, to some extent, C
abortus – hence the need for laboratory testing. It is possible for infected live calves to be born. The dam does
not usually show any other clinical signs associated with
fungal abortion. Fungal hyphae can be demonstrated in
smears of fetal stomach contents and deep placental cotyledon scrapes, and are cultured using selective media
(Sabouraud’s dextrose agar). Detection in placenta alone
requires histopathological demonstration of characteristic necrotising placentitis and the presence of fungal
hyphae using special stains (periodic acid–Schiff [PAS]
reaction or a silver stain such as Grocott’s).
462
Mycotic placentitis. Fungal hyphae (arrows) are clearly
visible. Periodic acid–Schiff reaction, magnification x200.
Picture, M. Wessels
APPROACH TO THE INVESTIGATION
OF ABORTION IN CATTLE
Investigating every abortion may be considered uneconomical. Caldow and Gray (2004) suggest an interference level
of 3 per cent for abortions that occur after pregnancy is
confirmed at around six weeks, based on reports that quote
unavoidable fetal losses at around 1·7 to 2 per cent. Each
abortion should be considered in the light of other clinical
findings, such as whether there has been a cluster of abortions or if the aborting cow(s) is/are ill. It must be remembered that many abortifacient agents are potential zoonoses
and, under the Control of Substances Hazardous to Health
(COSHH) regulations, a farmer has a legal responsibility
to protect staff from avoidable health hazards.
DIAGNOSIS
For a detailed examination to be undertaken, the entire
fetus and placenta should be submitted to the laboratory
for an abortion investigation. Appropriate tests will be
authorised at the discretion of the veterinary investigation officer carrying out the investigation. Placenta (or
placentome) is often not submitted, but is frequently very
useful in establishing a diagnosis, particularly for fungal,
B licheniformis and Chlamydophila infections.
The following information is also helpful in assessing
the extent of the problem and providing clues about possible pathogens:
■ Age of the dam (heifer/cow);
■ Approximate stage of gestation;
■ Approximate date of first abortion;
■ Number of abortions occurred so far;
■ Number of normal calvings;
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LABORATORY TESTS AVAILABLE FOR USE ON FETAL AND PLACENTAL SAMPLES
Test
Samples required
Significance of result
Bacteria and fungi
(eg, Salmonella Dublin,
Arcanobacter pyogenes,
Bacillus licheniformis,
Aspergillus fumigatus,
Brucella abortus,
Campylobacter species)
Bacterial and fungal culture
(all samples). Stained
smears for Brucella and
Chlamydophila abortus
and Coxiella burnetii,
and wet preparations
for fungal hyphae (fetal
stomach contents and deep
cotyledonary scrapes)
Fetal stomach contents (5 ml),
placenta/placentome, liver
or lung
Culture from fetal viscera is usually significant. Culture only from
placenta requires histopathological evidence of placentitis with
demonstration of the organism in lesions
Bovine virus diarrhoea (BVD)
antibody
IFAT
Fetal fluid (1 ml)
A positive result after immunocompetency has been gained
indicates fetal exposure. The significance is questionable as
late pregnancy challenge is not often fetopathic
Bovine virus diarrhoea
antigen
RT-PCR
Immunohistochemistry
Lymphoid tissue (eg, thymus)
or other viscera (eg, liver)
A positive result indicates that the fetus has been infected in utero
and, hence, BVD virus may be the cause of the abortion
Bovine herpesvirus type 1
antigen
FAT
Immunohistochemistry
Fetal liver
A positive result is significant, but the sensitivity of FAT is low.
Immunohistochemistry tends to be performed only when
suggestive inflammatory lesions are identified in fetal viscera
by histopathology
Neospora caninum antibody
IFAT
Fetal fluid (1 ml)
A positive result indicates fetal infection in utero (histopathology
is required to demonstrate lesions to indicate if it is the cause of
abortion)
Leptospira hardjo
PCR
Fetal kidney
PCR recently developed and sensitivity is much greater than
previously used FAT
Iodine deficiency
(stillbirths only)
Thyroid weight
Thyroid iodine concentration
Histopathology of thyroid
gland
Fetal thyroid gland
Half thyroid gland (fresh)
Half thyroid gland (fixed)
Thyroid gland weight >30 g may be significant, but must be
supported by iodine concentration <1000 ppm or characteristic
hyperplasia
FAT Fluorescent antibody test, IFAT Indirect fluorescent antibody test, RT-PCR Real-time reverse transcriptase PCR
■ Any illness in dams or other disease problems;
■ Whether animals are housed or at grass;
■ Diet;
■ Recent purchases, stock movements or feed changes;
■ Herd serological status (from bulk milk or individual
cow tests);
■ Vaccination status;
■ History of previous disease/infections in the herd.
The table above lists the laboratory tests available for
the investigation of an abortion at the regional laboratories of the VLA and Scottish Agricultural College.
SEROLOGICAL TESTS
Bulk milk serology for BVD, IBR
and leptospirosis
Bulk milk serology provides an indication of exposure to
the pathogens responsible for BVD, IBR and leptospirosis at herd level for minimal cost. The results are of most
value if they are used to monitor the antibody status of a
herd on a regular basis. This is particularly important for
naive herds where the appearance of antibodies would
be significant. In herds where seroconversion rates are
high, the results are most useful in determining the risk
of infection in non-immune animals entering the herd.
Vaccination will influence the results obtained.
Individual maternal serology for BVD, IBR,
neosporosis and leptospirosis
Single or paired serology is of little use in the diagnosis of abortion and, at best, can only provide supportive
evidence. Rising titres to BVD virus are rarely demonstrated in cases of abortion because of the delay between
infection and abortion. High titres can be maintained for
many years after infection and, therefore, are not necessarily an indication of recent infection. Serology could
be considered to rule out BVD virus infection – if the
aborted cow is seronegative (and antigen negative), it is
unlikely BVD was involved.
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● SEPTEMBER 20 07
Rising titres to BHV-1 may be detected in paired
serology, but it is not possible to differentiate a naive
IBR infection from recrudescence in a latently infected animal. Positive N caninum serology confirms that
the dam is infected, indicating an increased likelihood
of abortion occurring. It is useful to support fetal histopathological examination, and for deciding on the
future breeding of the animal (as mentioned earlier).
Serology for Leptospira species is usually unhelpful
in individual aborting animals, as antibody titres are falling or absent at the time of abortion. Further investigation
of herd seroprevalence may be warranted (in unvaccinated herds) to determine if a significant level of infection is
present and, bearing in mind the risk of zoonotic disease,
to decide whether to vaccinate.
Use and
misuse of
serology
Serology provides
an indication of
exposure to a
particular pathogen.
It cannot confirm
the causal agent of
an abortion, but can
sometimes provide
supportive evidence.
Serology is generally
unhelpful where stock
are vaccinated.
Acknowledgement
The author would like to thank Dr Arthur Otter for his helpful
comments on this article.
References and further reading
AGERHOLM, J. S., JENSEN, N. E., DANTZER, V., JENSEN, H. E. & AARESTRUP, F. M.
(1999) Experimental infection of pregnant cows with Bacillus licheniformis bacteria.
Veterinary Pathology 36, 191-201
CALDOW, G. & GRAY, D. (2004) Fetal loss. In Bovine Medicine, 2nd edn. Ed A. H.
Andrews. Oxford, Blackwell Science. pp 577-593
DAVISON, H. C., OTTER, A. & TREES, A. J. (1999) Significance of Neospora caninum in
British dairy cattle determined by estimation of seroprevalence in normally calving
cattle and aborting cattle. International Journal of Parasitology 29, 1189-1194
DUBEY, J. P. (2005) Neosporosis in cattle. Veterinary Clinics of North America: Food
Animal Practice 21, 473-483
ESSLEMONT, D. E. (2005) The cost of abortion in dairy cattle. UK Vet 10, 49-56
GROOMS, D. L. (2004) Reproductive consequences of infection with bovine
diarrhoea virus. Veterinary Clinics of North America: Food Animal Practice 20, 5-19
MEE, J. F. (2004) The role of micronutrients in bovine periparturient problems.
Cattle Practice 12, 95-108
MILLER, R. B. (1986) Bovine abortion. In Current Therapy in Theriogenology 2.
Ed D. A. Morrow. Philadelphia, W. B. Saunders. pp 291-300
PATON, D. J., CHRISTIANSEN, K. H., ALENIUS, S., CRANWELL, M. P., PRITCHARD,
G. C. & DREW, T. W. (1998) Prevalence of antibodies to bovine virus diarrhoea virus
and other viruses in bulk tank milk in England and Wales. Veterinary Record 142,
385-391
RADOSTITS, O. M., GAY, G. C., BLOOD, D. C. & HINCHCLIFF, K. W. (2000) Veterinary
Medicine. London, W. B. Saunders
VIDA (2006) Veterinary Investigation Diagnosis Analysis. Addlestone, UK, Veterinary
Laboratories Agency
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Bovine abortion: aetiology and
investigations
Emma Cabell
In Practice 2007 29: 455-463
doi: 10.1136/inpract.29.8.455
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