Protection of chickens against very virulent marek's disease virus (MDV) by an Infectious clone of Meq-null MDV vaccination

To evaluate the immune-protective effect of gx0101δmeq bacterial artificial chromosome (bac) which contains an infectious meq-null marek's disease virus genome,1-day-old spf chickens were reared separately in isolators with positive filtered air. on 1 day of age,chickens were immunized with 20 μg of gx0101δmeq bac suspended in pbs,and challenge infection with 500 pfu very virulent rmd5 were performed at day 5 and 12 post-immunization separately. during 90 days after challenge,all chickens were recorded and checked for necropsy. the protective index of the two vaccines used was 80 and 40 for cvi988/rispens and gx0101δmeq bac,respectively,after challenged with the very virulent (vv) virus rmd5 at day 5 post-immunization. when challenged with rmd5 at day 12 postimmunization,the protection index of gx0101δmeq bac increased to 67%. except that gx0101δmeq bac can confer protection against vv mdv,a delay in the development of md could be observed in some chickens vaccinated with gx0101δmeq bac. on the other hand,compared with cvi988/rispens,the rescue of gx0101δmeq bac in the body is a prerequisite for access to protection. therefore,there is a blank period after immunization,which provides a chance for infection with the wild mdv. © 2016 pvj.