Figure 2.

The importance of Differentiating Infected from Vaccinated Animals (DIVA). The use of an EI vaccine with DIVA capability is an asset to maintain an effective surveillance during an outbreak while emergency vaccination is implemented [3]. (a) The canarypox-based EI vaccine is a live attenuated canarypox virus with the EIV HA gene (green) inserted in its genome (one EIV HA per canarypox vector). The canarypox-based EI vaccine induces a seroconvertion limited to the EIV HA antigen after immunisation (green horse). Infection with EIV or immunization with whole inactivated or sub-unit EI vaccines induce a seroconvertion to several EIV antigens, including the EIV HA (green) and the nucleoprotein (NP, red) [12]; (b) An equid population naïve for EI (Period 1). Due to horse movement and/or importation of an infected animal (Period 2), an EI outbreak is detected (green + red horse, Period 3). Prevention and control measures are implemented (Period 4). In the absence of emergency EI vaccination, disease control relies primarily on movement restriction, active surveillance, biosecurity measures and is heavily dependent of the horse population density. A virus such as EIV is likely to spread quickly (especially in a naïve population such as in Australia in 2007). Emergency vaccination is implemented to support these measures. If an EI vaccine without DIVA capability is used (green + red vaccine), any seroconvertion (green + red horse) detected outside the vaccination buffer zone should be considered as a potential EI case (i.e., it is not possible to discriminate between a vaccinated horse that moved from the vaccination buffer zone or a new infected horse). The use of an EI vaccine with DIVA capability (green vaccine) allows to follow the spread of EIV infection inside the vaccination buffer zone, to identify real EI outbreak and infected horses (green + red horses) outside the vaccination buffer zone and to control the implementation of specific measures such as movement restriction.