Effect of vaccination against Aujeszky's disease compared with test and slaughter programme: epidemiological and economical evaluations.

Abstract

In January 1990, a 6-year program was initiated to eliminate endemic Aujeszky's Disease virus (ADV) infection from the pig herds in an area of Northern Germany, bordering Southern Denmark, with intensive pig farming. In the first 3 years of the campaign, an intensive compulsory vaccination program, with glycoprotein I (gI)-deleted vaccines, of all pigs in the area was employed. Beginning in June 1990 and for the first 3 years of the project, approximately 200 herds randomly selected from all herds in the area, were serologically tested each quarter. In each farrow-to-feeder (FAFE), feeder-to-finish (FEFI) and farrow-to-finish (FAFI) herd, 20 female breeding pigs, 20 finishing pigs (> or = 50 kgs liveweight) and 10 female breeding pigs and 10 finishing pigs, respectively, were blood sampled. The sera were tested by the Herd-Check Anti-PRV(S) ELISA test (IDDEX Inc., ME). Sera positive to this test were examined by the HerdCheck Anti-ADV gI-ELISA test (IDDEX Inc., ME). Data on potentially confounding management factors were collected through a pilot-tested questionnaire, administered to farmers by 2 veterinarians who blood sampled the pigs. For fattening herds (FEFI and fattening sections of FAFI herds), the association between the odds of > or = 1 gI+ finishing pigs and the time between initiation of the program in the area and sampling date (a surrogate for the effect of the program) was modelled using ordinary logistic regression. The association between the odds of gI+ females in seropositive (> or = gI+ females) FAFE and FAFI herds and time since initiation of the program was investigated with logistic-binomial regression models. Results of the study show that the longer the period from the beginning of compulsory vaccination to the date the herd was sampled the lower the odds of gI+ fattening herds and gI+ female breeding pigs in herds of the area. The beneficial effect of mass vaccination on the reduction of ADV spread was accounted for by this relationship. For fattening herds this relationship appeared curvilinear, with the reduction in the log-odds being more rapid in the 1st year of the program. This non-linear pattern indicates that for the elimination of the risk of ADV-infection from fattening herds of the area, the mass vaccination program should be complemented with additional measures such as test-and-slaughter of infected breeding pigs. A computerized economical model to estimate the effects of ADV-infection at the herd and area level has been developed. The analytical structure consists of a basic epidemiological model linked to an economic estimation framework. The economic model predictions allow priorities to be given to alternative control strategies. Mass vaccination of all pigs in regions with endemically infected herds followed by test-and-removal of seropositive animals is the most cost-effective way to control the spread of ADV within the swine population. Other possible control strategies such as intensive vaccination or complete test-and-removal all had higher overall costs, either because of the less efficient production, or because of the high costs of straight test-and-removal.