Page 786 - Veterinary Immunology, 10th Edition

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FIG. 24.6 The production of an attenuated virus by removal of a
gene required for virulence. Genes coding for major antigens
detected by serological techniques can also be removed, ensuring
that vaccinated animals can be distinguished from naturally infected
ones.

Genetic manipulation can also be used to make “marker

vaccines.” For example, pseudorabies virus synthesizes two
glycoprotein antigens called gX and gI. These are potent antigens,
yet neither is essential for viral growth or virulence. They are
expressed by all field isolates of this virus, and so infected animals
will make antibodies to both gX and gI. An attenuated

pseudorabies vaccine has been produced that lacks these proteins.
Vaccinated pigs will not make antibodies to gX or gI, but naturally
infected pigs will. The vaccine will not cause positive serological

reactions in assays for anti-gX or -gI, and the presence of antibodies
to gX and gI in a pig is evidence that the animal has been exposed
to field strains of pseudorabies virus. This type of vaccine, called a
DIVA vaccine (Differentiate Infected from Vaccinated Animals),

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