Diagnostic Aspects Of Bovine Leukemia Virus
Bovine leukemia virus (BLV), an oncogenic retrovirus, is
widely distributed and endemic in many cattle herds. Most
cattle infected with BLV do NOT exhibit clinical signs. BLV
infection is life-long in cattle so demonstration of serum
antibodies to BLV indicates persistent infection. Persistent
(or fluctuating) lymphocytosis(demonstrated in peripheral
blood films or complete blood counts) develops in approximately
30% ofBLV-infected cattle. Lymphoma (lymphosarcoma) develops
in approximately 3% of BLV-infected cattle but usually not
until they are at least six years-of-age. This form of lymphoma
is termed the "enzootic", "endemic", or
"adult" form of bovine lymphoma or bovine leukosis.
BLV is a C-type retrovirus (single-stranded RNA encoded for
reverse transcriptase). It is cell-associated and its genome
is inserted into the genome of host lymphocytes and monocytes.
Transmission to cattle probably occurs when they are less
than one year-of-age. Direct (horizontal contact) transmission
of BLV is probably the primary means by which cattle become
infected although both vertical (from dam to offspring) and
vector (e.g., biting insect) transmission can also occur.
In both BLV-induced persistent lymphocytosis and enzootic
bovine lymphoma, the proliferating lymphoid cell population
is derived from B-lymphocytes.
The most important disease caused by BLV is enzootic bovine
leukosis, essentially a form of malignant lymphoma (ML). ML
develops in only a small percentage of cattle infected with
BLV, but it is a fatal disease characterized by lymphomatousinvolvement
of multiple organs. These include lymph nodes, the heart,gastrointestinal
tract (especially abomasum), liver, spleen, uterus, and kidneys.
Clinical signs reflect organ involvement Peripheral lymph
nodes can be visually or palpably enlarged. When retrobulbar
lymph nodes are enlarged,proptosis or exophthalmos can result.
Cardiac involvement usually includes the right atrium and
can result in congestive heart failure with subcutaneous,
ventral-dependent edema. Poor reproductive performance
and palpable enlargement of the uterine wall or intra-pelvic
lymph nodes are also indicators of ML. Dairy cows are more
commonly affected with enzootic bovine leukosis than beef
cattle. Ante-mortem diagnosis of ML can often be established
by aspiration cytology of enlarged lymph nodes.
Other forms of ML occur in cattle. These are not caused by
BLV and are therefore called "sporadic forms" of
bovine lymphoma. They include the "calf-hood", "thymic",
and "cutaneous" forms. Calf-hood lymphoma usually
develops within by six months-of-age and is characterized
by muldcentric lymph node enlargement, lymphocyticleukemia
and pancytopenia (reflecting extensive bone marrow involvement),
and lymphomatous tumors in abdominal viscera. The thymic form
usually develops in yearling beef cattle and is characterized
by massive enlargement of the thymus with clinical signs related
to compression in this area (neck and brisket edema, dysphagia).
The cutaneous (or skin) form of bovine lymphoma, usually develops
in cattle between two and three year-of-age, and is the most
indolent form of ML in cattle.
Persistent lymphocytosis (PL), identified as absolute lymphocytosis
in two consecutive complete blood counts obtained three months
apart, occurs in approximately 30% of BLV-infected cattle.
Circulating lymphocytes can be morphologically normal or exhibit
morphologic features of "reactive" B-cells. When
bone marrow becomes lymphomatous in cattle with BLV-induced
ML, leukemia with lymphocyte counts as high as 100,000/ul
can occur. In leukemic cattle, circulating lymphoid cells
can appear "immature or blastic". Some cattle with
BLV-induced ML are lymphopenic, probably because circulating
lymphoid cells are "trapped" in lymphomatous tissues.
Lymphocytosis, usually in the 8,000 to 20,000/ul range can
also occur with other chronic infectious diseases, such
as Tuberculosis, Trypanosomiasis, or Brucellosis.
BLV-infection can be assumed when postmortem examination
of ADULT cattle reveals lymphomatous tumors in multiple sites.
These include peripheral and visceral lymph nodes, uterus,
abomasum, heart (especially right atrium), liver, spleen,
and kidneys. Grossly, these tumors are usually soft, gray-white,
and can include friable areas of necrosis. Microscopically,
are observed in abundance as they replace normal
BLV causes a persistent infection and demonstration of antibodies
can be used to identify infected animals. Seroconversion occurs
from three weeks to three months post-infection. Several methods
can be used to detect antibodies against BLV. The methods
most widely used to detect, control and eradicate BLV infection
are the agar gel unmunodifiusion test (AGID) and the enzyme
linked immunosorbant assay (ELISA).
Antibodies to several BLV structural proteins are present
in the sera of infected cattle. In most cases, antibodies
against gp51, the viral envelope glycoprotein, have the highest
titer and appear earlier than antibodies against p24, the
Agar Gel Immunodifiusion:
The first serologic test used for diagnosis of BLV infections
was an AGID test using the internal p24 polypeptide. When
it was found that the gp51 glycoprotein increased the sensitivity
of the test this antigen was either mixed with or used to
replace the p24 antigen. Most AGID antigens today contain
both gp51 and p24, but the test should be standardized for
the demonstration of antibodies against gp51.
The ELISA can detect antibody levels ten- to a hundred-fold
lower than those detectable using the AGID test This allows
use of the test to detect BLV infections in cattle herds by
evaluating pooled sera from a number of individuals at one
time, thus reducing cost and labor required if undertaking
a large scale epidemiological study. It also enables slaughterhouse
and tank milk surveys to be done.
False positive results:
Calves up to six or seven months of age may be positive if
they have received colostmm/milk with antibodies against BLV.
These passive antibodies gradually decay during the first
half year of the calfs life. However, not all seropositivecalves
are false positive - four to eight percent of calves from
seropositive dams in naturally infected herds are infected
with BLV at birth. These infections are probably acquired
Also, since attempts to develop a BLV vaccine have focused
on the gp51 antigen as an immunizing agent, diagnostic tests
that use this antigen to detect antibodies in the milk or
serum would not distinguish naturally infected cattle from
False negative results:
-Undetectable antibody levels during early phases of infection.
-Poor antibody response to infection.
-Infected cows tested two to six weeks before or after
-Inconclusive test results requiring concentration of the
- by ZuhairIsmail, J.U.S.T.
- edited by EvanJanovitz, DVM,PhD
- edited by Charles Kanitz, DVM,
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