Bovine Neosporosis:
A Review
In the past decade, the protozoan parasite Neospora
caninum has become increasingly recognized as an important
cause of reproductive failure in dairy and beef cattle.
The parasite is widespread in the United States, but the
majority of case reports have come from California, where
Neospora caninum is considered to be the number one
cause of abortion in dairy cattle. In Indiana, neosporosis
has been sporadically reported as a cause of abortion and
neonatal mortality in cattle; however, the current seroprevalence
of Neospora caninum in Indiana cattle is unknown.
Neospora caninum is a protozoan parasite which
closely resembles Toxoplasma gondii. Wild or domestic
canids are the natural definitive host for Neospora caninum.
Infected animals may shed large numbers of oocysts in their
feces, which may then be ingested by intermediate hosts
such as cattle. Sheep, goats, horses, deer, and other animals
are also suitable intermediate hosts. There is no direct
transmission between cows; however, the parasite can be
maintained by vertical transmission of the organism from
the dam to the fetus in utero. Vertical transmission, which
may occur over several generations, is a major factor contributing
to the persistence of Neospora to their offspring.
At birth, congenitally infected calves may have neurologic
signs, be underweight, unable to rise, or have no clinical
signs. Congenitally infected calves have a higher rate
of abortion, particularly during their first pregnancy,
and a high rate of vertical transmission to their offspring.
Neospora abortion typically occurs in mid gestation
with a mean age of 5.5 months (range 3.5-8 months) and may
occur throughout the year. This is different from most
causes of bovine abortion which occur during late gestation.
Abortion storms may occur with multiple abortions over a
period of 1-2 months. The aborted fetuses are typically
autolyzed and usually do not have characteristic gross lesions.
If present, lesions are subtle, consisting of pale white
foci in the skeletal muscle and heart, and there may be
fluid in the pleural or peritoneal cavities. The dams typically
do not have clinical signs.
Neosporosis can be diagnosed by serology and/or histopathology.
One or more aborted fetuses, the placenta, and serum from
the dam should be submitted to the Animal Disease Diagnostic
Laboratory at Purdue University. Important organs from
which to identify the organism via histopathology and fluorescent
antibody (FA) testing include the brain, heart, liver, and
skeletal muscle. The most characteristic lesion in the
fetus is focal encephalitis, characterized by necrosis and
nonsuppurative inflammation. Although most aborted fetuses
are autolyzed, Neospora organisms and/or the characteristic
lesions may be found in the brain. The brain should be
fixed in 10% formalin and submitted for histopathology.
A herd diagnosis can be achieved by serology. By comparing
the Neospora status of aborting versus nonaborting
cows, it will become clear if seropositivity correlates
with the tendency to abort. During abortion storms, it
is best to collect blood immediately from all animals at
risk and then post abortion blood samples should be taken
later. This technique is better for evaluating seroconversion
to any abortifacient than paired serology, using samples
collected at abortion and 3 weeks later, because most abortions
occur several weeks after acute infection.
There is no effective treatment for Neospora infection
in cattle. Research is being conducted to develop a vaccine
to help prevent abortion in cattle; however, none are commercially
available at this time. Culling seropositive cattle and
their infected offspring has been advocated for the control
of Neosporosis in herds with a low prevalence of infection.
However, this approach would not be cost effective in high
prevalence herds. Control may be attained by using seronegative
animals as replacement heifers. Valuable seropositive cows
can be used as breeding stock by embryo transfer to seronegative
recipients. Since dogs are the natural definitive host
for Neospora caninum, control programs should start
with preventing access by dogs to infectious material such
as dead calves, aborted fetuses, stillborn calves, and fetal
membranes. Furthermore, cattle feed and water should be
protected from contamination by dog feces.
- by Scott Storandt, Class of 2001
- edited by Brad Njaa, ADDL Pathologist
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