A practical guide for strategic and efficient sampling in African swine fever affected pig farms

In the case of ASF outbreaks in pig farms, EU legislation requires a thorough epidemiological investigation to determine, among other tasks, the extent of infection in the affected farm. The main aim of this study was to implement a reliable sampling strategy to quickly obtain an overview of the extent of African swine fever (ASF) virus spread in an affected pig farm. We developed and tested a three-step approach: (i) identification of sub-units within the affected farm, (ii) categorisation of sub-units, and (iii) targeted selection of animals for testing. We used commercially available lateral flow devices (LFDs) to detect ASF antigen and antibodies under field conditions and compared them with routinely performed laboratory tests (qPCR, ELISA, IPT). The study was conducted in three commercial farms in Latvia that were affected by ASF in July 2020. One of the affected farms was relatively small with only 31 pigs, whereas the other two were large with 1800 and 9800 animals respectively. The approach proved to be helpful and practical for efficient and reliably assess the ASF situation on the farm and to identify subunits within a farm where infected animals are present and subunits which might (still) be free of infection. This important epidemiological information helps to better estimate the high-risk period and to track the potential spread of infection outside the farm. It allows also to prioritise culling and, if appropriate, to pursue a partial culling strategy taking into account the absence of clinical signs, implemented biosecurity measures, quarantine and negative test results, among others. This might be of interest for large commercial farms where the infection was identified very early and has not yet spread widely. Due to its limited sensitivity, the antigen LFD test is useful for testing animals showing signs of disease.