Plaque assay for African swine fever virus on swine macrophages Brief Report

Summary. A plaque assay developed to detect the infection of African Swine Fever Virus on swine macrophages is described. Plaques were generated by all of the virus isolates tested. The method is suitable not only for virus titration but also for the selection of clones in protocols for isolation/purification of recombinant viruses. ∗ Different methods for African swine fever virus (ASFV) titration have been described. The formation of erythrocyte rosettes around infected blood swine monocytes is a characteristic feature of the ASFV-infected cell [7, 8], being the basis for the standard hemadsorption test for many ASFV isolates [4]. This assay has been widely used for diagnostic purposes but it is obviously restricted to hemadsorbing virus strains. The adaptation of some ASFV isolates to grow in different established cell lines, mostly derived from monkey species, allowed the use of plaque formation assays [4, 11] to titrate the infectious virus in ASFV samples with a more simple and quantitative method but, again, limited to cell culture-adapted viruses. Other methods based on the evaluation of cytopathic effect produced by endpoint dilutions of virus samples in sensitive cell cultures or on the use of recombinant virus expressing chromogenic marker genes [6], have been employed in different laboratories to estimate the infectivity present in ASFV samples. A plaque assay suitable to select ASFV recombinants has been reported previously [9, 14]; however, the detailed method and results have not been appropriately described, and the assay has been never reported to be used in titration of infective ASFV. We have considered of interest to develop an efficient method to generate plaques on cultures of swine monocytes/macrophages (the natural target