Longitudinal field studies of infectious bronchitis virus and avian pneumovirus in broilers using type-specific polymerase chain reactions

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Abstract
In longitudinal studies, 13 flocks were swabbed twice each week for the life of the flock (up to 46 days). The swabs were analyzed by type-specific reverse transcriptase polymerase chain reactions. Massachusetts type vaccinal infectious bronchitis virus (IBVs), applied at the hatchery, were usually maximal during the first week, as expected and, notably, remained detectable for 3 to 4 weeks, occasionally longer. IBV of the 793/B type (also known as 4/91 and CR88) was detected in 11/13 flocks (85%). The time of first detection of 793/B varied over several weeks and was sometimes within the first week in low amounts, which increased gradually. In some flocks, detection of 793/B remained intermittent. IBV types D274 and D1466 were each detected once, in the same flock, for short periods, in low amounts, and in the presence of higher amounts of 793/B. In swabs from a further 30 broiler flocks, plus those already mentioned, there was an incidence for 793/B, D274 and D1466 of 79, 10 and 2%, respectively. Avian pneumovirus (APV) (avian or turkey rhinotracheitis virus) vaccines, applied at the hatchery or later, were either not detected or were detected only after a delay of 1 to 3 weeks. In five flocks that received no APV-A vaccine and two flocks that received only APV type A vaccine, field infection by APV type B was detected but only during the last week or so of life. In six flocks that had received an APV-B vaccine, no field APV-B, differentiated from vaccinal APV-B by restriction enzyme analysis, was detected. In swabs from 30 other flocks, the great majority of which had not been vaccinated against APV, the incidence of APV types B and A was 50 and 3%, respectively. The results show (a) that vaccinal IBV can be detected for several weeks, (b) the dominance of the IBV 793/B type and