| Description:
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Neospora caninum is
a protozoan parasite which closely resembles Toxoplasma
gondii. In the past decade, Neospora caninum has become
increasingly recognized as an important cause of reproductive
failure in dairy and beef cattle. The life cycle of
this parasite comprises dogs as definitive hosts, and
many other mammalian species as intermediate hosts.
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. This situation
causes significant losses within the cattle industry,
thus N. caninum is a parasite of economical and veterinary
medical importance. Congenitally infected calves have
a higher rate of abortion, particularly during their
first pregnancy, and a high rate of vertical transmission
to their offspring. Infection with N. caninum in immune
competent hosts does not normally cause disease, but
when infection does occur it is usually chronic. Normally,
the very slowly proliferating N. caninum bradyzoite
stage is found encapsulated in tissue cysts, mostly
in the brain, and is surrounded by thick a cyst wall
which protects the parasites from immunological and
physiological reactions on part of the host. However,
upon pregnancy, parasites are reactivated, and transform
into the rapidly proliferating tachyzoite stage which
is capable of transplacental passage and infection of
the fetus.
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| Diagnosis: |
The principle method
of diagnosing Neospora caninum infection in aborted
fetuses is by histopathology of fetal tissues, followed
by specific identification of parasites within tissue
lesions by immunohistochemistry (IHC). IHC is relatively
insensitive as a confirmatory test for neosporosis because
parasite numbers in infected tissue can be very low,
possibly leading to false negatives. Fetal serology
has been used to confirm N. caninum abortion in individual
fetuses, but the assay is not highly sensitive, as demonstrated
by two separate studies where N. caninum-specific antibodies
were present in only 50 to 65% of confirmed N. caninum-infected
fetuses (B.C. Barr, 1995)(W. Wouda, 1997). Maternal
serology also is not consistently useful to confirm
N. caninum abortion in individual cows. Fortunately,
new DNA-based diagnostic procedures offer a sensitive,
specific and definitive method for diagnosis of Neospora
caninum. Using a technique known as PCR, specific regions
of N. caninum DNA are targeted and amplified, allowing
for accurate detection in the lab.
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