ABSTRACT: An infection model with enterotoxigenic Escherichia coli (ETEC) harboring the F4 fimbriae can be used to assess the impacts that various challenges associated with weaning (e.g., dietary, psychological, environmental) have on the expression of postweaning diarrhea. The objective of this study was to develop a novel inoculation method for administering an ETEC culture that would induce a higher proportion of ETEC-F4 diarrhea, in pigs that genetically showed ETEC-F4 susceptibility or resistance. The study was designed as a factorial arrangement of treatments with the factors being 1) partially susceptible or resistant to ETEC-F4 based on genetic testing, and 2) 4 challenge treatments, being a) a conventional liquid broth method using a drenching gun [Positive control (PC)], b) a Syringe method, c) a Capsule method, and d) Negative control [pigs not challenged (NC)]. At 21 ± 3 d of age (mean ± SEM), 48 male castrate pigs (Large White × Landrace) weighing approximately 7.0 ± 1.18 kg were allocated to 4 treatment groups in 2 replicate pens (6 pigs per pen). Initial ETEC-F4 susceptibility was based on a DNA marker test and each treatment group had 9 partially susceptible and 3 resistant pigs. On days 7 and 8 after weaning, pigs were challenged with ETEC (serotype O149:K88; toxins LT1, ST1, ST2, and EAST). On each inoculation day the PC pigs were orally dosed with 9 mL 7.12 × 109 colony-forming unit (CFU), the Syringe pigs with 0.8 mL 6.72 × 109 CFU, the Capsule pigs were orally administered 2 capsules containing 0.8 mL 3.28 × 109 CFU, and the NC pigs 1 mL of phosphate-buffered saline (PBS) solution. Approximately 72 h after infection, 44, 22, 78, and 0% of partially susceptible pigs in the PC, the Syringe, the Capsule, and the NC group had developed ETEC-F4 diarrhea (P = 0.007). Partially susceptible pigs had a higher diarrhea index (DI) compared to resistant pigs (31.5 vs. 4.8, P < 0.001). The NC group had a lower DI compared to the PC and Capsule pigs (3.9, 38.1, and 40.3, respectively, P < 0.005). Following infection, genetically resistant pigs in the Capsule group had a DI of zero and the partially susceptible pigs had a DI of 55.6 (P = 0.014). This study showed that genetically screening pigs and using a Capsule to deliver ETEC-F4 can increase cases of diarrhea and the efficiency of the challenge model. Taken together, these methods have the potential to reduce the number of pigs needed in future experimental infection studies.
