EXPERIMENTAL QUANTIFICATION OF TRANSMISSION OF GENETICALLY-ENGINEEREDPSEUDORABIES VIRUS

There is concern that live pseudorabies virus (PRV) vaccine or PRV vec tor vaccine strains may spread from vaccinated to unvaccinated pigs. M oreover, if is feared that recombining PRV vaccine strains with relate d vaccine or wild-type strains may lead to spread and survival of reco mbinant PRV. To learn more about to what extent different PRV vaccine strains could spread we used a previously described experimental model to study the transmission of intranasally inoculated PRV mutant strai ns under experimental conditions. We used PRV strains that lacked glyc oprotein E (gE) or thymidine kinase (TK), and a PRV vector vaccine (gE (-), TK-, gG(-)) that expresses the glycoprotein El (El) of hog choler a virus. In addition, we investigated whether intranasally co-inoculat ed gE-negative and gE-positive PRV strains competed in transmission am ong pigs. The extent of transmission was estimated using the reproduct ion ratio R. This ratio has a threshold property; when R1, the infecti on can spread when R<1, the infection will disappear. We found that R for a gE-negative strain was 10.1, and R for a TK-negative strain was 5. Furthermore, the R for the vector vaccine (gE(-), TK-, gG(-)) expre ssing El was 0.18, and did not differ significantly from the R for the control strain without El. The R of gE-negative strain was significan tly 1 (P=0.0005). Co-inoculation with a gE-positive field strain did M ot prevent the transmission of a gE-negative strain. This study shows that a small-scale experiment can be used to estimate the transmission of genetically engineered organisms in their host species. The result s of this study indicate that the deletion of gE alone or TK alone is not enough to prevent spread of PRV among susceptible pigs, and that t ransmission of gE-negative PRV is not firmly limited by co-presence of a gE-positive strain.