An ultraviolet-attenuated egg vaccine for swine ascariasis: parameters affecting the development of protective immunity.

Pigs inoculated with UV-irradiated Ascaris suum eggs developed immunity against challenge exposure with A suum infective eggs. Trials were conducted to determine the optimal conditions necessary to induce high levels of protective immunity and minimize liver lesions related to larval migration. Protective immunity was defined as the percentage of reduction in the recovery of larvae from the lungs of inoculated pigs, compared with controls, 7 days after challenge exposure with 5 X 10(3) or 10(4) infective eggs. Initial experiments showed that protective immunity was higher in pigs inoculated orally with UV-irradiated eggs than in those inoculated intraperitoneally. The highest level of protection (94%) was obtained by immunization with 5 oral inoculations of 10(4) eggs that had been UV-irradiated at total energy doses (ET) of 150, 100, 75, 75, and 75 mu W-min/cm2, respectively. A level of 83% protection was induced with as few as 2 oral inoculations of eggs irradiated at 150 and 75 ET, respectively. However, protection was reduced when pigs received eggs that were irradiated at and above 400 ET. The minimum number of irradiated eggs required to induce significant (P less than 0.05) protection was determined by feeding pigs 10(4), 10(3), or 10(2) eggs irradiated on 3 consecutive weeks at 150, 100, and 75 ET; the levels of protection induced were 94%, 89%, and 83%, respectively. Pigs inoculated with equal numbers of UV-irradiated eggs or normal eggs developed a similar level of protective immunity after challenge exposure. However, worms developing from the inocula were detected in the intestines of some of those pigs that had received normal eggs, but not in those pigs given UV-irradiated eggs.