The increase in concentration of HVT recoveredfrom spleen samples of vaccinated birds over time isconsistent with the results of Sharma and Burmester(1982). In their study, the amount of HVT recoveredfrom birds 10 days post–vaccination was greater thanthe amount recovered seven days post–vaccination.The results of this study are also consistent with thedata of Islam et al. (2004) on the HVT content of thespleens of vaccinated broilers up to day 35 post–vaccination. These studies all suggest ongoingreplication of HVT within the host over time, at leastuntil day 49 of age. In contrast, Witter and Offenbecker (1978) observed a decrease in the amount of HVTrecovered from weeks 4–12. The amount of recoveredHVT plateaued after 12 weeks of age, seven weeks laterthan in the present experiment. These variations maybe attributed to the use of lower doses of HVT vaccine,layer type birds, contamination of vaccine withreticuloendotheliosis virus and the extractionmethods used by Witter and Offenbecker (1978). Therecent discovery that HVT is shed in dander and canbe quantified in dander using qPCR has resulted indata showing that shedding of HVT peaks at day 21post–vaccination and falls away sharply thereafter tobasal levels, at least to day 60 in broiler chickens (Islamet al. 2005).
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