Prospective Case–Control Study of Determinants for African Swine Fever Introduction in Commercial Pig Farms in Poland, Romania, and Lithuania

Abstract

To investigate potential husbandry-related risk factors for African swine fever (ASF) incursion on commercial pig farms in Lithuania, Poland, and Romania, a prospective, matched case–control design was carried out from August 2021 to September 2023. For each participating commercial pig farm where an ASF outbreak occurred, two control farms were randomly selected and matched by herd size and county. On both case and control farms, questionnaires related to farm management and biosecurity measures were carried out, and stable flies (Stomoxys calcitrans) and biting midges (Culicoides spp.) were collected to identify a possible association between the number of ASF virus (ASFV)- or ASFV DNA-positive vectors and presence of ASF on the farms. After testing for potential multicollinearity, a conditional logistic regression model was performed on one complete and three imputed datasets. To evaluate the best-fit model, the Akaike information criterion (AIC) method was used. This study generated more knowledge on risk and protective factors for ASF introduction on commercial pig farms related to (1) farm location (risk: closer distance to ASF outbreaks); (2) the wild boar (WB)–pig interface (risk: attractive crops for WB cultivated near the farms); (3) biosecurity (protective: carcasses collection by the rendering company without entering the holding and closed containers for carcass disposal, risk: sharing machinery with other farms or organizing unusual events on the farm); (4) insect-mediated mechanic transmission (protective: placement of insect screens on all doors and windows and risk: the number of biting midges collected on the farm). Manure from other holdings applied on the soil near the farm was in two of four models found significant and could be related to possible mechanical transmission by stable flies or to an increased infection pressure. Some of the identified husbandry-related risks and protective factors can have a direct practical value for the farmers.