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

Abstract

In the case of African swine fever (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 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) categorization 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 sub-units within a farm where infected animals are present and sub-units 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 prioritize 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.

Keywords: African swine fever; domestic pigs; lateral flow device; outbreak investigation; sampling strategy.

FIGURE 1

Sampling strategy of pigs in African swine fever (ASF)‐affected farms for the purpose of outbreak investigations

FIGURE 2

Schematic view of farm A with sampling units (A1 and A2) and results of testing

FIGURE 3

Schematic view of farm B with sampling units (B1.1, B1.2, B1.3, and B2) and results of testing

FIGURE 4

Schematic view of farm C with sampling units (C1, C2, C3, C4, C5, C5a, C6, C7, and C8) and results of testing

FIGURE 5

Comparison of Ct‐values for all tested blood samples, samples that were qPCR and Ag‐LFD positive and samples that were qPCR positive but Ag‐LFD negative. The boundaries of the boxes indicate the 25th and 75th percentiles, and the line within the box marks the median

FIGURE 6

Number of pigs tested positive for African swine fever (ASF) (qPCR, Ag‐LFD) depending on clinical score of sampled pigs