Page 820 - Veterinary Immunology, 10th Edition

have died as a result of poor storage, the use of antibiotics in
VetBooks.ir conjunction with live bacterial vaccines, the use of chemicals to

sterilize the syringe, or the excessive use of alcohol swabs on the
skin. Sometimes animals given vaccines by unconventional routes

may not be protected. When large flocks of poultry or mink are to
be vaccinated, it is common to administer the vaccine either as an
aerosol or in drinking water. If the aerosol is not evenly distributed
throughout a building, or if some animals do not drink, they may

receive insufficient vaccine. Inappropriate vaccination of young
animals prior to loss of maternal immunity remains a problem.
Animals that subsequently develop disease may be interpreted as
cases of vaccine failure.

Failure to Respond

Occasionally, a vaccine may actually be ineffective. The method of
production may have destroyed the protective epitopes, or there

may simply be insufficient antigen in the vaccine. Problems of this
type are uncommon and can generally be avoided by using only
vaccines from reputable manufacturers.
More commonly, an animal may simply fail to mount an immune
response. The immune response, being a biological process, never

confers absolute protection and is never equal in all members of a
vaccinated population. Since immunity is influenced by many
genetic and environmental factors, the range of immune responses

in a large random population of animals follows a normal
distribution. This means that most animals respond to antigens by
mounting an average immune response, whereas a few will mount
an excellent response, and a few will mount a poor immune
response (Figs. 25.2 and 25.3). These poor responders may not be

protected against infection despite having received an effective
vaccine. It is impossible to protect 100% of a large outbred
population of animals by vaccination. The size of this unreactive

portion of the population will vary between vaccines, and its
significance will depend on the nature of the disease. Thus, for
highly infectious diseases against which herd immunity is poor and
in which infection is rapidly and efficiently transmitted, such as
foot-and-mouth disease, the presence of even a few unprotected

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