Application of DIVA vaccines and their companion diagnostic tests to foreign animal disease eradication

The risk of foreign animal disease introduction continues to exist despite Canada's strict regulations concerning the importation of animals and animal products. Given the rapidity with which these diseases can spread, especially in areas with dense livestock populations, eradication efforts which rely solely on quarantine and stamping-out measures can present a formidable undertaking. This, combined with growing economic and ethical considerations, has led to renewed interest in the use of vaccination as a tool in controlling foreign animal disease outbreaks. Vaccination has effects at the individual and population levels. Efficacious vaccines reduce or prevent clinical signs without necessarily preventing virus replication. They may also increase the dose of virus needed to establish an infection and/or reduce the level and duration of virus shedding following infection. Vaccine effectiveness within a population is a function of its ability to reduce virus transmission. Transmission is best described by the reproductive ratio, R, which is defined as the average number of new infections caused by one infectious individual. By helping to reduce the R-value below 1, vaccination can be an effective adjunct in abbreviating an outbreak. Nevertheless, vaccination can also complicate serological surveillance activities that follow eradication, if the antibody response induced by vaccination is indistinguishable from that which follows infection. This disadvantage can be overcome by the use of DIVA vaccines and their companion diagnostic tests. The term DIVA (differentiating infected from vaccinated individuals) was coined in 1999 by J. T. van Oirschot of the Central Veterinary Institute, in The Netherlands. It is now generally used as an acronym for 'differentiating infected from vaccinated animals'. The term was originally applied to the use of marker vaccines, which are based on deletion mutants of wild-type microbes, in conjunction with a differentiating diagnostic test. The DIVA strategy has been extended to include subunit and killed whole-virus vaccines. This system makes possible the mass vaccination of a susceptible animal population without compromising the serological identification of convalescent individuals. The DIVA approach has been applied successfully to pseudorabies and avian influenza eradication, and has been proposed for use in foot-and-mouth disease and classical swine fever eradication campaigns. This paper will survey current vaccine technology, the host immune response, and companion diagnostic tests that are available for pseudorabies, foot-and-mouth disease, classical swine fever and avian influenza.