Since its emergence in the U.S., porcine epidemic diarrhea virus (PEDV) has spread rapidly across the country and resulted in the estimated death of over 7 million pigs in the first year, causing substantial economic losses. The rapid spread of PEDV suggests that the virus is highly transmissible. However, no information has been published regarding the minimum infectious dose (MID; the smallest quantity of virus to establish infection) of PEDV in pigs at different stages of production. The objectives of this study were to 1) determine the oral minimum infectious dose of PEDV in naïve neonatal piglets; 2) determine the oral minimum infectious dose of PEDV in naïve weaned pigs and assess the corresponding antibody responses. A U.S. virulent PEDV prototype isolate (USA/IA/2013/19338) with known infectious titer was serially ten-fold diluted in virus-negative cell culture medium. Dilutions with theoretical infectious titers from 560 to 0.0056 TCID50/ml together with a medium control were orogastrically inoculated (10ml/pig) into 7 groups of 5-day-old neonatal pigs (n=4 per group) and 7 groups of 21-day-old weaned pigs (n=6 per group). In 5-day-old pigs, 10ml of inoculum having theoretical titers 560 – 0.056 TCID50/ml, corresponding to polymerase chain reaction (PCR) cycle threshold (Ct) values 24.2 – 37.6, resulted in 100% infection in each group; 10ml of inoculum with theoretical titer 0.0056 TCID50/ml (Ct>45) caused infection in 25% of the inoculated pigs. In 21-day-old pigs, 10ml of inoculum with titers 560 – 5.6 TCID50/ml (Ct 24.2 – 31.4) resulted in 100% infection in each group while 10ml of inoculum with titers 0.56 – 0.0056 TCID50/ml (Ct values 35.3 – >45) did not establish infection in any pigs under study conditions as determined by clinical signs, PCR, histopathology, immunohistochemistry, and antibody response.

In summary, using a U.S. PEDV prototype isolate diluted in culture medium, we determined the correlations of PEDV infectious titers to PCR Ct values and genomic copies per ml. We further demonstrated that the oral minimum infectious dose of PEDV was 0.056 TCID50 or lower in 5-day-old pigs whereas the MID of PEDV was 56 TCID50 in 3-week-old pigs under our study conditions. Therefore, PEDV infectious dose is age-dependent with a significantly lower MID for neonatal pigs than for weaned pigs. This information should be taken into consideration when interpreting clinical relevance of PEDV PCR results. The observation of such a low MID in neonates also emphasizes the importance of strict biosecurity and thorough cleaning/disinfection on sow farms. Data from this study also suggests a neonatal pig bioassay is more sensitive than a weaned pig bioassay to assess PEDV infectivity. Regarding duration of the swine bioassay, our data indicate that 2-day duration of pig bioassay may not be sufficient to evaluate the infection outcome. We propose that 7-day duration would be considered appropriate for PEDV swine bioassay.

In this study, the effect of PEDV infectious doses on infection outcomes was evaluated in naïve neonatal and weaned pigs. It must be noted that infectious dose effects in immune pigs may be different from naïve pigs. In addition, a U.S. PEDV prototype cell culture isolate was used in this study and it remains to be determined whether or not U.S. PEDV S-INDEL-variant isolate has similar minimum infectious dose.

For more information, please contact Dr. Jianqiang Zhang at Department of Veterinary Diagnostic and Production Animal Medicine, College of Veterinary Medicine, Iowa State University. E-mail: [email protected]