A Review of Enzootic Bovine Lymphosarcoma
D. Franklin Collins,
DVM; Heather L. Tarpley, DVM; Kenneth S. Latimer, DVM, PhD
Class of 2005 (Collins), Department of Pathology (Tarpley, Latimer),
College of Veterinary Medicine, The University of Georgia, Athens,
GA 30602-7388
Introduction
Enzootic bovine
lymphosarcoma is also known as Bovine Leukosis Virus (BLV)-associated
malignant lymphoma (lymphosarcoma) and is the most
common neoplasm of cattle. This disease affects both beef and dairy
cattle, but is more frequently seen in dairy herds than in beef cattle.
In 1996, 89% of dairy herds in the United States were said to be endemically
infected with BLV, according to the 1996 National Animal Health Monitoring
System’s Dairy 96 study. Producers suffer economic losses from
death of cattle, loss of milk production, costs associated with treatment
and diagnosis, replacement costs from early culling, and carcass condemnation
at slaughter. Because there is no treatment or effective vaccination
for this endemic disease, control must come from testing and culling
or improved management practices which reduce or eliminate transmission
of BLV from infected carriers to future replacement animals.
Transmission
BLV infections result from transfer of infected lymphocytes in blood
or colostrum. The virus does not persist outside the host, so contact
or environmental contamination has not been found to be a source of
infection. Farm practices, including gouge dehorning, ear tagging,
and tattooing, as well as natural mating and surgical instrument contamination
have been implicated in viral transmission. Increased rates of transmission
are observed in cattle under crowded conditions. BLV has been recovered
from multiple blood feeding insects following feeding on an infected
host; however, no host has been proven to transmit disease under natural
conditions. In utero infection occurs in about 10% of calves
born to seropositive cows. Additionally, infection may originate from
ingestion of colostrum contaminated with infected lymphocytes.
Pathogenesis
BLV is a member of the viral family Retroviridae, subfamily Oncovirinae.
Members of this family contain genetic information in the form of RNA.
Unlike other members of the Retroviridae family, BLV has remained
genetically stable. Similar to other retroviruses, BLV contains genes
for reverse transcriptase (pol), core proteins (gag),
and envelope proteins (env). The BLV genome also contains
a region of the env gene known as the X region that codes
for a viral regulatory protein, Tax. This regulatory protein increases
the rate of viral transcription by activating a promoter in a region
of the proviral genome known as the long terminal repeat (LTR). Viral
entry into target cells (B lymphocytes) and virus-cell fusion are mediated
by the viral envelope glycoprotein GP-51 and its transmembrane subunit
GP-30. After entry, reverse transcriptase of viral RNA is expressed
by the virally encoded reverse transcriptase in the host cell cytoplasm.
Viral DNA may then be integrated into the host cell DNA. Integrated
viral DNA is termed a provirus. Cellular mechanisms which result in
B-lymphocyte transformation are not fully understood. Detection of
only the X region of the viral genome in some neoplastic B cells suggests
that Tax gene expression may be the only viral genetic component required
to cause transformation.
Clinical Signs
An animal with
BLV infection remains infected for life, though the majority (95%)
do not develop clinical disease. Three to five years
after infection, persistent lymphocytosis (PL; a polyclonal increase
in lymphocytes for > 3 months) subsequently develops in about 30%
of seroconverted cattle. Mechanisms for PL include delayed apoptosis
of infected B-lymphocytes, and a combination of increased IL-2 alpha
receptor expression and increased production of IL-2 by infected lymphocytes.
IL-2 is a cytokine which promotes lymphocytic proliferation. Persistent
lymphocytosis is considered a subclinical manifestation of BLV infection
which in a small percentage (less than 5%) of infected animals may
progress to a fatal monoclonal or oligoclonal B cell lymphoproliferative
disease approximately 3 to 10 years after infection. Lymphoproliferative
disease usually manifests as lymphoma; only 5-10% of these lymphoproliferative
diseases result in lymphocytic leukemia (Figures 1-3).
 |
 |
| Fig.1. Aspirate
of a lymph node from a cow with lymphosarcoma. There are numerous
large lymphocytes which have a high nuclear:cytoplasmic ratio
and oval nuclei with fine chromatin. Nucleolar rings can sometimes
be discerned. The background contains many bare nuclei, cytoplasmic
droplets, and a few erythrocytes. |
Fig.
2. Hematocrit tube after centrifugation from a cow
with acute lymphoblastic leukemia. The sample had a white blood
cell count of 290,000/µl. The buffy coat is easily recognized
in this tube; it constitutes nearly half of the cellular mass. |
 |
| Fig.
3. Cytocentrifugation sample of cerebrospinal fluid
from a cow with lymphoma involving the central nervous system.
Lymphocytic pleocytosis and two mitotic figures are observed
in this field. Cytoplasmic blebbing is attributable to sample
preparation. |
Clinical signs of infection with BLV are only observed when viral
expression results in malignant lymphoma or lymphocytic leukemia. Clinical
signs vary considerably depending on the location of solid tissue tumors.
Lymphadenopathy is most commonly seen and it may involve single or
multiple lymph nodes of internal or external location. Other organs
frequently involved include the heart, abomasum, kidney, uterus, spinal
canal, intestine, and, rarely, the bone marrow. Cattle may present
with anorexia, anemia, weight loss, decreased milk production, chronic
indigestion, melena, diarrhea, heart failure, posterior paralysis,
and exophthalmos depending upon the location of the solid tumors.
Diagnosis
Diagnosis of BLV infection is based primarily on detection of antiviral
antibodies in serum. The agar gel immunodiffusion (AGID) assay is the
most frequently used means of surveillance and is available as a commercial
kit (Leukassay B, Pittman Moore Co.). This test detects antibodies
to the viral gp51 antigen and is a reliable, rapid, and inexpensive
test with high specificity and sensitivity for BLV. The AGID assay
cannot detect low levels of antibodies (may take 12 weeks post infection
for positive seroconversion) or distinguish between colostral and naturally
acquired antibodies (which may persist for 6 months). False negative
results can occur during the periparturient period when circulating
antibody levels are low. An enzyme linked immunosorbent assay (ELISA)
may be used as a screening test in BLV free herds to detect antibodies
in pooled milk or serum. Detection of virally encoded DNA would eliminate
the need to distinguish between naturally acquired and maternal antibodies,
but such a test is not commercially available.
Additionally, finding
atypical or immature lymphocytes in peripheral blood smears is highly
suggestive of malignant lymphoma. Abnormal lymphocytes
can be present without leukocytosis or lymphocytosis. A definitive
diagnosis of bovine lymphosarcoma may be obtained from histopathologic
evaluation of a tumor. Cytology of tumorous lymph node aspirates may
not reliably distinguish reactive nodes responding to infection
from neoplasia since both conditions have increased numbers of large
lymphocytes.
Control
Control is based on reducing the level of infection or viral transmission
in a herd. Because levels of infection and management practices vary
greatly among producers, a single strategy may not be appropriate for
all herds. The following are suggested management practices which can
be adopted fully or in part to reduce or eradicate BLV infection within
a herd.
- Cull cattle that have a persistent lymphocytosis. They are at increased
risk to spread disease. This practice has reduced infection levels
without removal of all seropositive animals from a herd. Culling
cows with the highest persistent lymphocytosis is another alternative.
- Separate seropositive dams from their calves before colostrum can
be consumed.
- Use sterile needles for intravenous, intramuscular, and subcutaneous
injections. Discard needles after individual use.
- Change obstetric sleeves between each cow.
- Wash and disinfect blood-contaminated instruments between uses
on each animal.
- Discontinue gouge dehorning. Replace this technique with a less
invasive method, such as electrocautery.
- Place calving cows in individual pens. Disinfect or change bedding
in between each individual calving.
- Take serum samples from calves of positive cows before colostrum
consumption to determine if in utero infection has occurred.
- Feed colostrum only from BLV negative cows in herds with a low
prevalence of disease.
- Feed thawed frozen colostrum from seropositive cows in herds with
a high prevalence of disease. Freezing will destroy infected lymphocytes
and provide protective BLV antibodies. Calves cannot undergo serologic
testing until maternal antibodies wane unless precolostral serum
samples are taken.
- Separate seropositive positive and seronegative negative cows into
separate groups. Perform any examinations or manipulations of seronegative
cows first if using a common facility.
References
1. Thurmond MC: Bovine Lymphosarcoma. In: Smith
BP (ed): Large Animal Internal Medicine, 3rd ed. St. Louis, Mosby,
Inc., 2002,
pp. 1067 – 1070.
2. Cockerell GL, Reyes RA: Bovine leukemia virus-associated lymphoproliferative
disorders. In: Feldman BF, Zinkl JG, Jain NC (eds): Veterinary
Hematology, 5th ed. Lippincott Williams & Wilkins, Philadelphia,
2000, pp. 614-619.
3. Nagy DW, Tyler JW, Stoker A, Kleiboeker SB: Association between
the strength of serologic recognition of bovine leucosis virus and
lymphocyte count in bovine leucosis virus-infected cows. J Am Vet Med
Assoc 220: 1681-1684, 2002.
4. Jones TC, Hunt
RD, King NW: Veterinary Pathology, 6th ed. Baltimore, Williams & Wilkins,
1997, pp. 330-331.
5. Evermann JF, Jackson MK: Laboratory diagnostic tests for retroviral
infections in dairy and beef cattle. In: Johnson R, Pelzer
KD (eds): Vet Clin N Am Food Anim Pract: Food Animal Retroviruses 13:87-101,
1997.
6. Hopkins SG,
DiGiacomo RF: Natural transmission of Bovine Leukemia Virus in dairy & beef
cattle. In: Johnson R, Pelzer KD
(eds): Vet Clin N Am Food Anim Pract: Food Animal Retroviruses 13:107-122,
1997.
7. Brunner MA,
Lein DH, Dubovi EJ: Experiences with the New York State Bovine Leukosis
Virus Eradication & Certification Program. In: Johnson
R, Pelzer KD (eds): Vet Clin N Am Food Anim Pract: Food Animal Retroviruses
13:143-150, 1997.
Acknowledgment
The copyrighted
image "Person,
Cow", 1998, enamel on panel by Laurie Halbritter is from her Paintings
and Other Works website and is used with permission. |