Recent research into the Rispens vaccine: Transmission, protection and interaction with wild-type MDV

Internationally there is clear evidence of increased virulence of Marek's disease viruses leading to vaccine failure (Witter 1997, 1998). There is also evidence that vaccination with "imperfect" vaccines (which prevent disease but not infection) has contributed to this (Read et al. 2015). These findings warrant ongoing screening of isolates for virulence and attempts to improve our understanding of interaction between vaccine and wild-type viruses. In Australia we have a good understanding of the level of protection against Marek's disease provided by vaccination with HVT (Meleagrid herpesvirus 1) alone or in combination with MDV serotype 2 (Gallid Herpesvirus 3) vaccine virus (Renz et al. 2012; Walkden-Brown et al. 2013). These studies have also provided good insight into the kinetics and shedding of these viruses in co-infections with serotype 1 wild-type pathogenic MDV (MDV-1, Gallid Herpesvirus 2) (Islam and Walkden- Brown 2007; Islam et al. 2008). Our understanding of these phenomena with regard to the MDV-1 vaccine strain Rispens CVI988 has been held back internationally and in Australia by the apparent good efficacy of the vaccine in the field and the lack of an easy test to differentiate between the Rispens vaccine virus and wild-type MDV1 and provide virus quantification. In 2011 the Australian Egg Corporation Ltd (AECL) funded our research group to develop a qPCR test to differentiate between Rispens and wild-type MDV-1 and to use the test to address a range of issues. This paper summarises our findings over this whole area. Each component has reported separately in detail in journal papers and those papers are cited in this overview.