Genetic diversity and prevalence of Atypical Porcine Pestivirus in the Midwest of US swine herds during 2016-2018.

Atypical porcine pestivirus (APPV), a highly divergent pestivirus, has a wide geographical distribution around the world. APPV is known to cause type A-II congenital tremors in newborn piglets. The main objective of this study is to access APPV prevalence in the US swine herds utilizing a newly developed quantitative real-time RT-PCR assay. Retrospective analysis of 1,785 samples revealed a 19.0% prevalence in Midwest swine herds over a period of three years (2016-2018). Among all clinical and field samples that were APPV positive, 82 samples (24.19%) were also positive for one or more swine viral pathogens. Two APPV US strains identified in this study demonstrated significant sequence diversity (~12% in full genome) compared to the first reported APPV strain from the United States in 2014. Of the two strains identified in this study, USA/023005/2016 is closer to two strains identified in Germany, and USA/047310/2017 shares more similarities with two US strains including Minnesota-1 and ISDVDL2014016573. Partial NS5B sequences (9127-9836 nt of the polyprotein gene) obtained from 54 APPV-positive samples revealed considerable sequence diversity, ranging from 85.8% to 100% nucleotide identity, within the US strains in samples from different geographic regions. Analysis of all US samples indicates high prevalence of APPV in Minnesota (37.35%), followed by Illinois (32.86%), Iowa (30.60%) and Kansas (21.89%). APPV was detected in 15.48% of samples assayed from 2017, slightly higher than that in 2016 (13.08%), but much lower than 2018 (28.77%). Among the various sample types tested, oral fluid samples had the highest prevalence and lowest average Ct value suggesting their suitability as a reliable diagnostic specimen for APPV detection. Overall, sequence variation among APPV strains and prevalence of the pathogen within the United States provides a basis for understanding the genetic diversity and molecular epidemiology of APPV in the US swine herds.