Figure 2.

Differentiating Infected from Vaccinated Animals (DIVA). (A) Horses immunized with the recombinant canarypox-based EI vaccine seroconvert to haemagglutinin (HA) only (green). Horses infected with equine influenza virus (EIV) or vaccinated with a whole inactivated EI vaccine seroconvert to all EIV antigen (red), including the EIV nucleoprotein (NP) forming the viral ribonucleoprotein complex; (B) Importance of DIVA for emergency vaccination in the face of an EI outbreak: in an area initially free of EI (Step 1), a case of EI is detected (red spot, Step 3), following horse movement and importation of EIV (as an example, Step 2). Prevention measures are implemented (Step 4). In absence of EI vaccination, control of the disease relies entirely on movement restriction, surveillance and is influenced by the horse population density. EI is likely to spread. Ring vaccination could be implemented to support those control measures. In absence of DIVA capacity (red), the origin of seropositive horses found outside the vaccination area could not be determined (e.g., vaccinated horses outside the vaccination area or presence of the virus in the few days prior to serological analyses). The use of a DIVA EI vaccine allows determination of EIV progression inside and outside the vaccination area (i.e., ring vaccination efficacy) and evaluation of compliance with prevention measures such as movement restriction.