| Description:
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Anaplasmosis is a
form of tick fever carried by a specfic species of cattle
tick. Anaplasmosis in cattle is caused by an organism
called Anaplasma marginale that invades and
multiplies in red blood cells. As the disease progresses,
infected and even uninfected red blood cells are destroyed
mainly in the liver and spleen, resulting in an increasing
anaemia and even death in severe cases. Any stage of
the cattle tick’s life cycle can become infected
after feeding on an animal carrying Anaplasma organisms
in its blood stream. The organism multiplies in the
tick and will pass to later stages of the tick life
cycle. However, it does not appear the infection is
passed on to the eggs. Consequently, the next generation
of ticks will not be infected unless they also feed
on a carrier animal. Therefore an infected stage of
the tick must transfer to a susceptible animal for transmission
to occur. Because the adult male tick is more mobile
and lives longer than other stages, it is the most likely
stage to transmit the disease. Biting flies can transmit
the disease but are less efficient vectors than ticks.
Mechanical transmission via veterinary instruments (needles,
dehorners etc.) is also possible and the organism can
cross the placenta to the foetus.
Calves from immune mothers receive temporary protection
(maternal antibody) from the colostrum (first milk)
which prevents anaplasmosis. This protection lasts about
3 months and, in most cases, is followed by an age resistance
that lasts until the animals are about 9 to 12 months
old. Calves exposed to anaplasmosis when the maternal
or age resistance is high, rarely show clinical symptoms
but develop a solid, long lasting immunity. It is therefore
possible to have both Anaplasma marginale and
cattle ticks present on a property without animal losses
or clinical disease. If cattle are not exposed to Anaplasma
as calves, the age resistance gradually wanes and these
animals will become increasingly susceptible to the
disease. If susceptible adult cattle are mixed with
infected cattle in the presence of the cattle tick,
serious losses due to anaplasmosis can occur. Clinical
symptoms of infection include transient fever, weakness
and respiratory distress particularly after exercise,
depression and loss of appetite, jaundice, and brown
urine due to bile pigments. Cattle that recover from
anaplasmosis remain carriers of the organism and are
immune to further disease.
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| Diagnosis: |
Clinical signs and
post mortem lesions are often suggestive of anaplasmosis,
but it is impossible to make an accurate diagnosis based
on these signs alone. Serological testing is also avaliable,
however, some of these tests are limited due to the
influence of environmental factors (temperature, humidity)
on test results. A new PCR-based test for Anaplasma
marginale has made it possible to detect DNA from
low numbers of Anaplasma organisms in whole blood samples.
This PCR assay therefore allows for detection of clinical
or subclinical Anaplasma marginale infection.
Amplification of a unique genetic region from the Anaplasma
genome facilitates accurate, specific, and definitive
diagnosis of Anaplasmosis in cattle.
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