Diagnostic Aspects Of Bovine Leukemia Virus Infection

Introduction:

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.

Epidemiology:

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.

Disease:

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.

Hematology:

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.

Necropsy:

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, lymphoid cells

are observed in abundance as they replace normal tissues.

Diagnostic Tests:

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 core polypeptide.

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.

ELISA:

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 transplacentally.

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 vaccinated cattle.

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 parturition.

-Inconclusive test results requiring concentration of the serum.

- by ZuhairIsmail, J.U.S.T. University

- edited by EvanJanovitz, DVM,PhD

- edited by Charles Kanitz, DVM, MS, PhD

References:

Miller, J.M.: Bovine Lymphosarcoma,Bov. Proc 29: 34-36, 1988

Johnson, R., and Kaneene, J.B.: Bovine Leukemia Virus. Part I. Descriptive Epidemiology, Clinical Manifestations, and Diagnostic Tests. Compend.ContEduc.Prac. Vet 13:315-327, 1991.

Evennann, J.F.: A Look at How Bovine Leukemia Virus Infection is Diagnosed. Vet Med. 87:272-278, 1992.

Ferrer, J.F., Marshal, R.,Abt,D.A., and Kenyon,     S.J.:     Relationship     BetweenLymphosarcoma and Persistent Lymphocytosis in Cattle: A review. J. Am. Vet. Med Assoc. 175:705-708, 1979.

Duncan,J.R.,Prasse,K.W.,Mahaffey,E.A., Veterinary Laboratory Medicine, third edition. IowaStateUniversity Press, Ames,IA 1994.

Valli,V.E.O., The Hematopoietic System, In, Jubb, K.V.F., Kennedy, P.C., Palmer, P.C.,eds. Pathology of Domestic Animals, fourth edition, volume 3, Academic Press, Inc., San Diego, 1994, pp. 101-265.