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The effects of HYT vaccination-challenge interval and external contamination level on Marek's disease (MD) transmission in one shed of broiler chickens were evaluated on the basis of the simulations of a mathematical model. Results of the simulations demonstrated that increases in vaccination-challenge interval are associated with exponential decreases in MD tumour and death cases. An assessment of the effects of HYT vaccination-challenge interval with external contamination level of 107 viral copy numbers per cubic metre air (YCN/m3 air) suggested that an increase in vaccination-challenge interval from 0 to 5 days, decreased tumour cases at day 50 from 43.5% to 15.8% and death cases at day 50 from 20.5% to 5.1%. MD tumour and death cases at day 50, however, were only influenced by external contamination level within a narrow range (1-5x106 VCN/m3 air). These modelbased simulations have showed that the longer the HYT vaccination-challenge interval, the fewer MD tumour and death cases. External contamination level over 5x106 VCN/m3 air had little effect on disease outcomes. |
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