Influence of vaccine deposition site on post-vaccinal viraemia and vaccineefficacy in broiler chickens following in ovo vaccination against Marek's disease

In ovo vaccination against Marek's disease is a widely used technology in t he broiler industry. A series of experiments was carried out to determine t he site of vaccine deposition in the egg during automated in ovo vaccinatio n, and the effect of vaccine deposition site and dose on vaccine responses following vaccination with cell-associated herpesvirus of turkeys in commer cial broiler chickens. Vaccine deposition site following automated in ovo v accination was principally influenced by the age of embryo, with egg size h aving a smaller effect. The frequency of vaccine deposition inside the embr yo body increased as incubation progressed from day 17.5 to 19.5. In experi ments using manual vaccine deposition intra-embryonically (IE) or extra-emb ryonically (EE) at day 18.5, EE vaccine deposition resulted in a significan tly delayed development of post-vaccinal viraemia relative to both IE vacci nation and subcutaneous vaccination at hatch. There were no effects of vacc ine dose (2000, 4000 or 8000 plaque forming units) on the timing of post-va ccinal viraemia. The timing of post-vaccinal viraemia was found to be a goo d indicator of the level of protection provided by the vaccine against chal lenge with earlier viraemia associated with better protection. IE vaccine d eposition induced significantly greater protection than EE deposition again st challenge with a virulent strain of Marek's disease virus. IE deposition consistently produced a high level of protection (68 to 84%) irrespective of vaccine dose or challenge day, while EE vaccine deposition produced no o r low levels of protection (0 to 27%) depending on the vaccine dose and day of challenge. The growth of challenged chickens was also affected by site of vaccine deposition, with significantly higher live weights at day 56 of age in IE compared with EE vaccinated groups. These data suggest that the s ite of vaccine deposition within the embryo is an important determinant of the success of in ovo vaccination.