The Diagnosis of Infectious Bronchitis of Chickens by the Agar Gel Precipitin Test

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Abstract
Birds known to be free of bronchitis antibodies were infected with field or vaccine strains of IBV and held in isolatioa A total of 935 AGP tests and 310 SN tests were performed on serums collected at frequent intervals from 204 experimentally infected and 15 uninfected control birds. The agar gel precipitin (AGP) test was applied to 7 field cases of infectious bronchitis, and results were compared to those obtained by virus isolation and SN tests. Bronchitis preclpitins were first demonstrated on the 8th day, and generally persisted through the 20th to 94th days after experimental infection. More than 80% of experimentally infected birds were positive betweeen days 10 and 17. Precipitins were demonstrated in 93 of 95 birds infected with a field strain of IBV but in some birds the preclpitins were quite transient Precipitins were produced following administration of commercial bronchitis water vaccine in only 7 of 10 vaccinated birds, and none of these birds was positive after 30 days postvaccination. Precipitins were demonstrated in 33 to 100% of serums from consignments of naturally infected birds bled at least 8 days subsequent to initial observation of respiratory signs. A diagnostic procedure utilizing the AGP test and based on detection of an increment of precipitln titer between two successive bleedings was valid when applied to field cases of infectious bronchitis. The precipitin increment test was most useful in detecting an active bronchitis infection between the 3rd and 14th days after the onset of respiratory signs. Results may be obtained 5 to 11 days after birds are first submitted. AGP-positive serums from a single bleeding of unvacclnated birds with a typical history of infectious bronchitis and no previous respiratory disease could constitute a presumptive diagnosis of infectious bronchitis. Although caution must be exercised in its interpretation (particularly in bronchitis-vaccinated birds), the AGP test, if judiciously applied, should be of considerable value in the diagnostic laboratory.