The number of worm recovered in the PBS control group was similar to a previous report as the worm recovery of this parasite is only approximately 50–60%. It was demonstrated that prior infection with O. viverrini failed to confer protection against reinfection, regardless of the size of the challenging dose. These observations were not unlikely as O. viverrini does not normally invade tissues so the chance of stimulating the host immune system is minimal in this infection. Better protection than previously reported was found in the current study.