FACTORS ASSOCIATED WITH CIRCULATION OF AUJESZKYS-DISEASE VIRUS IN FATTENING HERDS OF AN INTENSIVELY VACCINATED AREA OF NORTHERN GERMANY

In January 1990, a programme to eliminate Aujeszky's disease virus (AD V) infection from the pig farms of an endemically infected, pig-dense area of Northern Germany was initiated. The programme was based on int ensive 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 o f field strains of ADV in fattening pigs of feeder-to-finish and fatte ning units of farrow-to-finish herds during the early phase of the cam paign. The study reports on data collected in the period June 1990 to November 1991. The study population comprised 391 (74.8%) feeder-to-fi nish and 1 32 (25.2%) fattening units of farrow-to-finish herds-a rand om sample of the total 1048 fattening herds in the area. Forty-six (8. 9%) herds had greater-than-or-equal-to 1 gI-seropositive pigs in the s ample (gI + herds). The association between the gI status of the herd and possible risk factors obtained from a questionnaire was assessed b y unconditional maximum-likelihood logistic regression. Herd type (fee der-to-finish or farrow-to-finish pig density of the county in which t he herd was located (number of pigs weighing greater-than-or-equal-to 25 kg km-2), problems with clinical pneumonia, and time since initiati on of vaccination in the area were factors significantly (P < 0.1) ass ociated with ADV infection of the herd. The probability of viral circu lation 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. Fatt ening 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 significantl y higher odds of showing gI-seropositivity than herds in counties with less than 217 pigs km-2 . Also, the respective likelihood of gI-serop ositivity was 2 or 2.2 times higher in herds which had reported modera te or severe problems of clinical pneumonia than in herds with no pneu monia problems.