Infection, genetic and virulence characteristics of porcine epidemic diarrhea virus in northwest China

From September 2015 to May 2016, epidemic outbreaks of a diarrheal disease caused severe economic losses to the swine industry in northwest China. Typical clinical symptoms of the disease included severe diarrhea, vomiting, dehydration and death. In order to identify the pathogen, 27 intestinal samples were collected from dead piglets in Shaanxi, Gansu and Qinghai provinces and from Ningxia Hui Autonomous Region. All samples were tested using RT-PCR to detect rotavirus (RV), porcine epidemic diarrhea virus (PEDV) and transmissible gastroenteritis virus (TGEV). Twenty-five fecal samples were PEDV positive and five were TGEV positive; no piglets were infected with RV, indicating PEDV was the major pathogenic agent of viral diarrheal disease in these areas. Six strains were successfully isolated from positive samples and were serially passaged 40 times in Vero cells, with obvious cytopathic effects observed after 24 h post inoculation (hpi) and virus titers reaching 1.0 × 10⁷ to 5.62 × 10⁸. Sequence analysis ruled out that isolated strains were vaccine PEDV strains or strains derived from vaccine strains. Five strains belonged to classical strains, while one strain was a novel variant strain. The virulence of new novel variant strain SX1 and classical strain NX1 were tested in vivo using 10-day-old nursing piglets, revealing that both strains were highly pathogenic for piglets with destruction to small intestinal villi. Hematoxylin and eosin staining demonstrated markedly increased mucosal thickness, reduced villus length and villus/crypt (V/C) ratio in infected piglets. These pathological changes correlated with observed significantly reduced intestinal digestion and absorption functions that led to anorexia, dehydration, diarrhea and emaciation. Collectively, this study first reported the PEDV epidemic and phylogenetic analysis in northwest China and the results were important to understanding the infectivity, genetic characteristics, evolution and pathogenicity of PEDV strains, therefore, this experiment had important public health significance.