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Two experiments investigated the rate of MDV transmission in unvaccinated and HVT-vaccinated broiler chickens exposed to various levels of environmental contamination with MDV. Expt. 1 was in positive pressure isolators with continuous airflow of approx. 12-23 air changes/hr (14-27 m3/hr) and Expt. 2 used floor pens with no artificial ventilation. Chickens in Expt. 1 had air from infected shedder chickens containing 106.07 to 109.01 MDV copies per m3 of air ducted into their isolators for 21 days. Chickens in Expt. 2 inhaled air containing 106 to 107.5 copies of MDV per m3 generated by in-contact shedder chickens. In Expt. 1, 54 (range 40-70)% of unvaccinated chickens were MDV positive at day 7 whereas in vaccinated chickens only 7 (0-20)% chickens were positive. At day 21, 100% of unvaccinated chickens were positive but only 67 (50-80)% of vaccinated chickens were positive. By day 56 post-exposure, 50 (43-66)% of unvaccinated and 11 (9-14)% of vaccinated chickens showed gross MD lesions depending on group. In Expt. 2, 8, 50, 92 and 100% unvaccinated chickens were positive for MDV at days 5, 10, 15 and 20 post exposure whereas a maximum of 14% of vaccinated chickens were positive by day 20 post-exposure. Daily infection rate (%/day) was calculated in Expt. 2 and ranged from 6-10 in unvaccinated and 0-2.8 in vaccinated groups. Vaccination with HVT reduced the rate of lateral infection markedly in both experiments. Rate of transmission in Expt. 2 was significantly correlated with level of MDV exposure in vaccinated but not unvaccinated chickens. |
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