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

Step RT-PCR Kit (Thermo Fisher Scientific) were used in this study. PEDV rRT-PCRs using other primers, probe, and master mix buffer reagents may generate results somewhat different from what we obtained in this study. Second, correlations of the infectious titers, Ct values and genomic copies/ml in this study were obtained using a PEDV cell culture isolate diluted in cell culture medium. For PEDV present in other sample matrices, this correlation could have some variations. Another objective of this study was to determine the minimum infectious dose of PEDV in 5-day-old and 3-week-old naïve conventional pigs. In 5-day-old pigs, 100% of pigs (4/4 pigs) became infected with 10ml of inoculum having titers 560-0.056 TCID 50 /ml (Ct 24.2-37.6); 25% of pigs (1/4 pigs) became infected with 10ml of inoculum having titer 0.0056 TCID 50 /ml (Ct>45) ( Table 6 ). In 3-week-old pigs, 10ml of inoculum with titers 560-5.6 TCID 50 /ml (Ct 24.2-31.4) resulted in 100% (6/6 pigs) infection while 10ml of inoculum with titers 0.56-0.0056 TCID 50 /ml (Ct values 35.3 ->45) could not establish infection in any pigs under these study conditions ( Table 6 ). The smallest virus magnitude that can cause infection in at least one pig in a group is considered the minimum infectious dose. In the 5-day-old pig study, 4 pigs in each group were housed separately without direct contact between pigs. The minimum infectious dose of PEDV was 0.056 TCID 50 or lower in 5-day-old pigs. In the 3-week-old pig study, due to space limitation, it was not possible to house all pigs individually; however, this did not preclude determination of the minimum infectious dose in the current study because the weaned pig groups 4-6 (dilutions 10 −6 to 10 −8 ) had no single pig positive during 28 days study periods. However, if conditions allowed, for minimum infectious dose studies, pigs should ideally be housed separately during long-term study periods to avoid possible pig-to-pig transmission. The MID of PEDV was 56 TCID 50 in 3-week-old pigs under our study conditions. This suggests that the MID for neonatal pigs is at least 1000-fold lower than it is in weaned pigs.