Effect of Porcine Epidemic Diarrhea Virus Infectious Doses on Infection Outcomes in Naïve Conventional Neonatal and Weaned Pigs

Thomas, Joseph T.; Chen, Qi; Gauger, Phillip C.; Giménez-Lirola, Luis G.; Sinha, Avanti; Harmon, Karen M.; Madson, Darin M.; Burrough, Eric R.; Magstadt, Drew R.; Salzbrenner, Holly M.; Welch, Michael W.; Yoon, Kyoung-Jin; Zimmerman, Jeffrey J.; Zhang, Jianqiang

Our study confirms that 5-day-old pigs are more susceptible than 3-week-old pigs to PEDV infection. PEDV progresses faster, the fecal virus shedding level is higher, and disease is more severe, in 5-day-old pigs than in 3-week-old pigs (Fig 2A and 2B) . We also observed that the response among four 5-day-old pigs in each group was comparable as reflected by similar virus shedding levels (Table 3 ) and similar microscopic lesions. In contrast, there was more variation in virus shedding among six 3-week-old pigs in each group (Table 3) . These observations were consistent with previous reports on age-dependent resistance of pigs to TGEV infection [29] . However, in either 5-day-old or 3-week-old pig models, once pigs were infected and virus replication began, the initial dose of virus appears to have little impact at the group level on the average amount of fecal viral shedding, average severity of microscopic lesions/IHC staining, or the average magnitude of antibody titer that is subsequently developed. It should be noted that the 5-day-old piglets in this study were fed a mixture of liquid milk replacer and yogurt, which were possibly devoid of IgG, IgA and other immunologic ingredients found in sow milk. Thus, the piglets raised under the current study conditions could be more susceptible to PEDV infection than piglets at the corresponding ages that were naturally nursed by sows under field conditions. Previous studies also demonstrated that PEDV infection induces greater disease severity in neonatal piglets than in weaned pigs although the minimum infectious dose of PEDV was not determined in those studies [24, [30] [31] [32] . The exact reasons for the greater severity of PED in neonatal piglets are unknown but there may be several contributing factors including: 1) an immature immune system in neonatal piglets [33, 34] ; for example, the level of natural IFN-α production by porcine blood mononuclear cells is lower in neonates and the phagocytic cells present in newborn piglets generally have reduced phagocytic activity as compared with adult animals [35, 36] ; 2) neonatal piglets are more vulnerable to dehydration and the electrolyte and fluid imbalance [33] ; 3) the intestinal villi of neonatal piglets are longer and may have more mature permissive enterocytes than weaned pigs [37] ; 4) slower replacement of villous enterocytes (7-10 days) in neonatal pigs compared to 2-4 days in weaned pigs [38] . The virus mainly infects and destroys mature enterocytes lining the villi of small intestine, resulting in shortening and blunting of villi. But the intestinal crypt epithelial cells basically remain uninfected and serve to replace the destroyed villous enterocytes. Regeneration of villous enterocytes in neonatal piglets is not as rapid as that in weaned pigs; this may explain why villous atrophy is more severe in neonatal than in weaned pigs. Recently Jung et al [31] further demonstrated that neonatal pigs have low numbers of LGR5+ (marker for crypt stem cell) cells and low levels of Ki67 staining (marker for crypt proliferation) in contrast to large numbers of LGR5+ cells in the crypts and high proliferation of intestinal crypt cells in weaned pigs, explaining the rapid turnover or recovery rate of villous enterocytes in weaned pigs.