Pathogenicity and Viral Distribution of Wild Type Porcine Epidemic Diarrhea Virus and Its Cell Culture Adapted Porcine Epidemic Diarrhea Virus

Abstract

Swine enteritis in all ages is caused by porcine epidemic diarrhea virus (PEDV), transmissible gastroenteritis virus (TGE), rotavirus, Eimeria spp . etc., and is often fatal among neonatal piglets. This study aimed to compare the pathogenicity and nucleotide sequence of ORF3 between wild-type porcine epidemic diarrhea virus (wt-PEDV) and cell culture-adapted PEDV (ca-PEDV). A total of 30 colostrum-deprived piglets that were 1 day old were inoculated with either wt-PEDV or ca-PEDV the wt-PEDV-infected piglets at 24, 36, and 60 h post-inoculation. The nucleotide sequences of wt-PEDV and ca-PEDV were nearly identical (98.7% homology); nucleotide substitutions were noted in ORF3 that caused some amino acid changes. Statistically significant differences were observed in the pathogenicity of ca-PEDV compared with its parental wt-PEDV; ORF3 nucleotide changes were identified in ca-PEDV that possibly influenced PEDV pathogenicity. Statistical analysis of the mean positive scores of the jejunal tissues revealed significant differences in the amount of PEDV nucleic acid between wt-PEDV and ca-PEDV. ISH positive results in piglets orally infected with the Korean strain of PEDV jejunal villus Villus atrophy and fusion were characteristic lesions induced by PEDV in this study. The degree of morphologic changes observed in the small intestines varied depending on the time after inoculation. There was a corresponding increase in the severity of diarrhea and dehydration of the infected piglets, which began as the villus height gradually decreased. Morphometry confirmed a significant reduction in villus height in the jejunum at 60 hpi. Both wt-PEDV- and ca-PEDV-infected enterocytes replicated within them, causing necrosis and sloughing. There were differences between the two viruses in the severity of damage to the small intestines and the amount of infection. The mean VH/CD ratios were more significantly reduced in the jejunum of wt-PEDV-inoculated piglets than in that of ca-PEDV-inoculated piglets at 36 hpi. The low rate of enterocyte loss in ca-PEDV-inoculated piglets could be the result of the inability of the virus to sufficiently infect enterocytes, or it could be because the process of virus replication did not rapidly lead to sloughing of the infected enterocytes. There is evidence that both mechanisms are contributory. The amount of PEDV nucleic acid indicated that ca-PEDV-infected fewer enterocytes and replicated slower than wt-PEDV in the early stage of infection. These results suggested that ORF3 gene alteration may cause a slower ca-PEDV replication in the enterocytes compared with wt-PEDV. Statistically significant differences were observed in the ca-PEDV pathogenicity compared with its parental wt-PEDV. The mechanisms responsible for the different degrees of pathogenicity between wt-PEDV and ca-PEDV are not understood. Nucleotide changes in ORF3 were identified in ca-PEDV, which possibly influence PEDV pathogenicity. These results indicate that in vitro