Page 896 - Veterinary Immunology, 10th Edition

antigens. The virus may therefore be disseminated without
VetBooks.ir provoking immunological attack. Maedi-visna is associated with

extensive infiltration of the lungs, mammary gland, and central
nervous system with T cells and macrophages. This virus does not

infect lymphocytes so there is no immunosuppression due to a loss
+
of CD4 T cells. Immunosuppression reduces the severity of the
lesions, whereas immunization against the virus increases their
severity. It is suggested that virus-infected macrophages stimulate

the T cells to release cytokines. These cytokines delay the
maturation of monocytes and restrict virus replication. They also
enhance macrophage MHC class II expression and trigger T cell
proliferation and chronic lymphoid hyperplasia.

Alterations in the T Cell System

Viruses may, of course, use the cells of the immune system as their
hosts. Viruses like HIV, feline immunodeficiency virus (FIV), canine

distemper (CDV), and feline leukemia virus (FeLV) infect
lymphocytes and either kill them or otherwise impair their ability
to function normally (Chapter 40). Glucocorticoids are profoundly
suppressive for T cells and T cell responses. Influenza virus triggers
a generalized stress response leading to a sustained increase in

serum glucocorticoid levels and resulting in immunosuppression.

Viral Evasion Through Latency

If viruses cause a state of reversible nonproductive infection, this is

called latency. It is a consistent feature of the herpesviruses. During
latency, viruses express only the absolute minimum number of
genes. Because they do not express viral antigens, they are not
detected by the immune system and may remain in that state for

many years.
In contrast to the short-lived immune response against bacteria,
antiviral immunity is, in many cases, very long-lasting. The reasons
for this are unclear, but they are often related to virus persistence

within cells, perhaps in a slowly replicating or a non-replicating
form as typified by latent herpesviruses. It is usually difficult to
isolate viruses from an animal that has recovered from a

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