Factors associated with circulation of Aujeszky's disease virus in fattening herds of an intensively vaccinated area of Northern Germany

Abstract

In January 1990, a programme to eliminate Aujeszky's disease virus (ADV) infection from the pig farms of an endemically infected, pig-dense area of Northern Germany was initiated. The programme was based on intensive compulsory vaccination (with killed and attenuated glycoprotein (gI)-deleted vaccines) of all pigs in the area. The objective of this study was to determine herd factors associated with the circulation of field strains of ADV in fattening pigs of feeder-to-finish and fattening units of farrow-to-finish herds during the early phase of the campaign. The study reports on data collected in the period June 1990 to November 1991. The study population comprised 391 (74.8%) feeder-to-finish and 132 (25.2%) fattening units of farrow-to-finish herds—a random sample of the total 1048 fattening herds in the area. Forty-six (8.9%) herds had >/ 1 gI-seropositive pigs in the sample (gI + herds). The association between the gI status of the herd and possible risk factors obtained from a questionnaire was assessed by unconditional maximum-likelihood logistic regression. Herd type (feeder-to-finish or farrow-to-finish), pig density of the county in which the herd was located (number of pigs weighing >/ 25 kg km −2, problems with clinical pneumonia, and time since initiation of vaccination in the area were factors significantly ( P < 0.1) associated with ADV infection of the herd. The probability of viral circulation in the herd was inversely related, in a curvilinear fashion, to time between initiation of vaccination in the area and sampling date. The risk of viral circulation decreased rapidly in the first year of the vaccination programme and at a lower rate in the second year. Fattening units of farrow-to-finish herds were three times more likely to have viral circulation among the fattening pigs than feeder-to-finish herds. Herds in counties with more than 320 pigs km −2 had significantly higher odds of showing gI-seropositivity than herds in counties with less than 217 pigs km −2. Also, the respective likelihood of gI-seropositivity was 2 or 2.2 times higher in herds which had reported moderate or severe problems of clinical pneumonia than in herds with no pneumonia problems.