'In ovo' vaccination against Marek's disease is a widely used technology in the broiler industry.A series of experiments was carried out to determine the site of vaccine deposition in the egg during automated 'in ovo' vaccination, and the effect of vaccine deposition site and dose on vaccine responses following vaccination with cell-associated herpesvirus of turkeys in commercial broiler chickens. Vaccine deposition site following automated 'in ovo' vaccination was principally influenced by the age of embryo, with egg size having a smaller effect. The frequency of vaccine deposition inside the embryo body increased as incubation progressed from day 17.5 to 19.5. In experiments using manual vaccine deposition intra-embryonically (IE) or extra-embryonically (EE) at day 18.5, EE vaccine deposition resulted in a significantly delayed development of post-vaccinal viraemia relative to both IE vaccination and subcutaneous vaccination at hatch. There were no effects of vaccine dose (2000, 4000 or 8000 plaque forming units) on the timing of post-vaccinal viraemia. The timing of post-vaccinal viraemia was found to be a good indicator of the level of protection provided by the vaccine against challenge with earlier viraemia associated with better protection. IE vaccine deposition induced significantly greater protection than EE deposition against 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 or 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 site of vaccine deposition within the embryo is an important determinant of the success of 'in ovo' vaccination.

The polymerase chain reaction (PCR) has recently emerged as an additional tool for the monitoring and diagnosis of Marek's disease. We investigated a number of factors that may influence the interpretation of PCR results in commercial broiler chickens including the effects of route of infection and herpesvirus of turkeys (HVT)-vaccination status. We also investigated the suitability of peripheral blood lymphocytes (PBL) and spleen as tissues for Marek's disease virus (MDV) detection. HVT-vaccinated and unvaccinated commercial broiler chickens were challenged or not challenged with virulent MDV either by intraperitoneal injection or inhalation of feather dust containing the virus. Blood and spleen samples were collected at weekly intervals to day 35 post-infection for PCR of spleen or PBL. Live weight and lymphoid organ weights were also measured. Spleen and PBL were found to provide similar sensitivity of detection of MDV with a small advantage in favour of spleen. In terms of the timing of detection of MDV, intraperitoneal challenge broadly mimicked natural challenge via inhalation, although infection of birds by inhalation of infective feather dust resulted in slightly later but more complete detection of MDV in challenged birds. Vaccination with HVT delayed the detection of MDV by approximately 10 to 14 days and did not protect against the reduced growth observed in challenged chickens at day 35 post-challenge.