Epidemiological studies of the persistence of Aujeszky's disease virus between and within herds in France

Abstract Epidemiological studies were carried out to better understand the conditions of the spread of Aujeszky's disease virus (ADV) in areas where pig herds have been vaccinated for several months. Blood samples were taken from fattening pigs and tested with serological techniques, allowing us to differentiate infected and vaccinated animals in all the breeding and fattening herds of administrative areas with variable densities of pig herds. These studies showed that the ADV infection occurs even in vaccinated areas and also that the virus can persist in successive pig populations in the same fattening unit. Moreover, proportions of infected herds seemed to be higher in vaccinated than in unvaccinated herds. A more detailed study was carried out in two breeding and finishing herds in which animals were infected by ADV. Blood samples were collected regularly from fattening pigs which were identified individually and systematically vaccinated. The epidemiological observation lasted from 5 to 6 months in these two herds. Two epidemiological situations were observed in the herds in which the longer-term serological study was carried out: in one, the virus did not spread in the successive batches of pigs; in the other, ADV persisted for at least 5 months in the fattening unit. The epidemiological inquiry, as well as the longer-term serological study, do not allow a definitive conclusion to be drawn about the factors responsible for the persistence of the virus.