Delayed hypersensitivity reaction to pseudorabies virus as a field diagnostic test in swine

A skin test based on a delayed hypersensitivity reaction to pseudorabies virus antigens was used in swine recovered from natural or experimental infections, to determine their immune status. More than 500 animals were subjected to over 700 test inoculations in the field. Approximately 200 tests were conducted on more than 125 swine in the laboratory. All ages of animals were tested, although most of the tests involved shoats and mature animals. Of 4 anatomic sites tested, the skin of the medial surface of the thigh in shoats and the fold of skin at the base of the tail in mature animals were found to be the most satisfactory. In general, the efficiency of the skin test was directly related to the titer of serum-neutralization (SN) tests. The skin test was about 50% as effective as the SN test in identifying swine that had been infected with pseudorabies virus when SN titers of 1:4 or above were in the sera tested. Differences in efficiency between the antigens were not significant. The test was reproducible; its optimal reactions occurred about 24 hours after inoculation; and it did not result in seroconversion (SN test) among susceptible animals. Inasmuch as animals negative to the skin test may not be negative on an SN test, the test is best used for evaluations of the immune status of herds rather than of individuals.