Page 897 - Veterinary Immunology, 10th Edition

herpesvirus infection. Some time later, however, especially when
VetBooks.ir the individual is stressed, the herpesvirus may reappear and may

even cause disease. During the latent period, when it is present in
the host but cannot be reisolated, the virus nucleic acid persists in

host cells, but its transcription is blocked, and viral proteins are not
made. The persistent virus may periodically boost the immune
response of the infected animal and in this way generate long-
lasting immunity to superinfection. The immune responses in these

cases, although unable to eliminate viruses, may prevent the
development of clinical disease and therefore serve a protective
role. Immunosuppression or stress may permit disease to occur in
persistently infected animals. The association between stress and

the development of some virus diseases is well recognized. It is
likely that the increased steroid production in stressful situations
may be sufficiently immunosuppressive to permit activation of
latent viruses or infection by exogenous ones.

Sometimes viruses may interact with bacteria to overcome the
immune system. For example, Mannheimia hemolytica and bovine
herpesvirus (BHV-1), acting together, cause severe respiratory
disease in cattle. BHV-1 infection increases expression of the β -
2
integrin LFA-1 on lung neutrophils. The leukotoxin of M. hemolytica
binds to this integrin and then kills the neutrophils, permitting

growth of the invading bacteria.

Inhibition of Apoptosis

Apoptosis may be considered a protective response since viruses
also die when a cell dies. This is especially significant if a cell dies

before viruses are released. It is therefore to a virus's advantage to
delay apoptosis until progeny viruses can be released. Thus cowpox
and some herpesviruses (including equine herpesvirus-2) encode
apoptosis inhibitors in their genomes. Viruses may also benefit by

killing cells of the immune system. Rapidly dividing lymphocytes
are susceptible to death signals. For example, lymphoid cell
apoptosis is a feature of canine distemper, a disease characterized
by severe immunosuppression (Chapter 40).

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