Page 908 - Veterinary Immunology, 10th Edition

VetBooks.ir Some Antiviral Vaccines

Because of the lack of antiviral drugs, vaccination is the only
effective method for the control of most viral diseases in domestic

animals. As a result, the development of viral vaccines is, in many
ways, more advanced than the development of their bacterial
counterparts. It has, for example, proved relatively easy to
attenuate many viruses so that effective vaccines containing
modified live virus (MLV) are readily available.

As discussed in Chapter 25, MLV vaccines are usually good
immunogens, but their use may involve certain risks. The most
important problem encountered is residual virulence. One serious

example of this was the development of clinical rabies in some dogs
and cats following administration of older strains of MLV rabies
vaccine. Some strains of infectious bovine rhinotracheitis and
equine herpesvirus-1 vaccines may cause abortion when given to
pregnant cows or mares, respectively, and MLV bluetongue

vaccines may cause disease in fetal lambs if given to pregnant ewes
(Chapter 23). More commonly, the residual virulence in these
vaccines causes a mild disease. Thus intraocular or intranasal

rhinotracheitis or calicivirus vaccines may cause a transient
conjunctivitis or rhinitis in cats. MLV infectious bursal disease
vaccines, some canine parvovirus-2 vaccines, and some bovine viral
diarrhea vaccines can cause a mild immunosuppression (see Fig.
40.1).

Because of problems of this nature, attempts have been made to
minimize residual virulence in vaccines. One method involves the
use of temperature-sensitive (ts) mutants. Ts strains of BHV-1, for

example, will grow only at temperatures a few degrees lower than
normal body temperature. When this organism is administered
intranasally, it is able to colonize the relatively cool nasal mucosa
but is unable to invade the rest of the body. Thus the vaccine can
stimulate local immunity without incurring the risk for a systemic

invasion. (It also has the advantage that its activity is not blocked
by maternal immunity.) Some vaccine viruses may persist in
vaccinated animals and cause a prolonged carrier state. Although

this is a problem largely associated with herpesviruses, concerns

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