| Description:
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Bovine Leukemia is
a viral disease of cattle. This disease has several
names including BLV, Bovine Leukosis, Lymphosarcoma
or Malignant Lymphoma. BLV is associated with the B-lymphocytes
in infected cows. Bovine Leukemia virus is transferred from cow to cow
or from cow to calf by blood that contains the virus-laden
lymphocytes. Very small amounts of blood have been experimentally
shown to be capable of transmitting the virus. When
exposed to the virus, not all cows become infected.
However, those that do become infected remain infected
for the remainder of their life because the virus remains
hidden within the lymphocytes. Although many cows become
infected with BLV, only 2 - 5% of those cows eventually
develop tumours of the lymph nodes after a prolonged
incubation period. The sarcoma tumours that develop
are the most common malignant tumours found in cattle.
Most infections (90%) take place early in the life of
a calf when virus is transmitted in the milk, although
some calves are born infected. The mechanism for these
in utero infections is not known. Infections can also
take place by movement of blood from one cow to another
through routine husbandry practices. Also, there is
a risk of transmission from ear tagging, gouge dehorning,
tattooing, multiple animal needle use and rectal palpation.
Most cows infected with Bovine Leukemia virus do not show any clinical
signs of disease. There can be months to years between
infection and any evident clinical disease. Some cows
may develop a persistent lymphocytosis (increase in
white blood cell numbers). Other signs of infection
may include weight loss and decreased milk production.
Cows that develop lymphosarcoma are typically 4-8 years
of age and may show the following signs: enlarged lymph
nodes (70 – 90%), decreased appetite, persistent
diarrhea, melana (all due to digestive tract tumours),
weight loss, chronic moderate bloating (mediastinal
lymph node enlargement), heart failure signs including
jugular pulse, brisket edema and difficulty breathing.
Typically, cows with lymphosarcoma have numerous tumours
in many sites, accompanied by a wide variation in clinical
signs and syndromes. Some cows (5 – 10%) may die
suddenly without previous signs, due to rupture of abomasal
ulcer or spleen, followed by acute internal hemorrhage.
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| Diagnosis: |
It is important to
recognize that most cows infected with Bovine Leukemia
Virus do not show any significant signs, since only
5% of animals develop cancer (lymphosarcoma) it is impossible
to control this disease in a herd by only culling sick
animals. It is important to control this disease because
high levels may result in major productions losses and
the inability to ship product to other countries. In
the past, the most common method of diagnosing Bovine Leukemia virus was
through serological testing (ELISA). This method is
relatively accurate but has several shortcomings. Calves
up to six or seven months of age may falsely test positive
if they have received colostrum/milk with antibodies
against BLV. These passive antibodies gradually decay
during the first half-year of the calf’s life.
Additionally, ELISA testing is also prone to false negative
results which may occur under various situations including;
poor antibody response to infection, undetectable antibody
levels during early stage of infection, inconclusive
test results requiring concentration of the serum. Fortunately,
new DNA-based diagnostic testing procedures circumvent
the problems associated with traditional methods of
detection. Because DNA-based tests detect the actual
DNA of Bovine Leukemia virus, rather then detecting the antibody response,
none of the problems associated with serological testing
apply. Using the PCR (Polymerase chain reaction), unique
genetic sequences exclusive to BLV can be detected with
superior specificity and sensitivity, ensuring veterinarians
and livestock owners a definitive method of diagnosis
for Bovine Leukemia virus.
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