Page 1179 - Veterinary Immunology, 10th Edition

proliferation. These include the maedi-visna virus, the Aleutian
VetBooks.ir disease parvovirus, and the herpesvirus responsible for malignant

catarrhal fever (MCF). MCF is a fatal lymphoproliferative disease of
cattle and sheep characterized by lymphadenopathy with

widespread tissue accumulations of lymphocytes. Lymphocytes
from MCF-infected animals show prolonged growth in tissue
culture.
Neoplastic transformation may occur in lymphoid cells of both

branches of the immune system. Providing that the tumor cells
have not dedifferentiated as a result of very rapid growth (as in
acute lymphatic leukemia of calves), it is possible to identify the
origin of the cells present in lymphoid tumors by their surface

antigens. For example, the presence of cell surface immunoglobulin
is characteristic of B cells, whereas the presence of CD3 or CD2 is an
identifying feature of T cells.

Bovine Lymphosarcoma

Bovine lymphosarcoma is one of the most common cancers of
cattle. It occurs in two main forms: an enzootic form and a sporadic
form. The enzootic form is caused by bovine leukemia virus (BLV),
a delta retrovirus. BLV is transmitted by infected lymphocytes and

can be spread by contaminated instruments, by vaccines containing
blood, or by biting flies; or calves may be infected in utero. The
primary target of the virus is the pre-B cell, although a specific BLV

receptor has yet to be identified. Early in infection the proportion of
B cells in peripheral blood increases before there is a significant
increase in the number of blood lymphocytes. Eventually some
infected animals develop a persistent lymphocytosis (PL) with
lymphocyte counts in the range of 20,000 to 80,000/µL. Not all BLV-

infected cattle develop PL, although 95% of cattle with this
condition are infected with BLV. These lymphocytes may be
+
enlarged, are CD5 , express increased levels of IgM, and altered
glycosylation. Cells in PL are not malignant and can occasionally
return to the normal state. BLV becomes stably integrated into these
B cells. Some T cells may also contain the BLV provirus. About 1%
to 5% of BLV-infected cattle develop a multicentric lymphosarcoma
between 1 and 8 years after infection. Susceptibility to cancer

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