Acquired Immunity to Migrating Larvae of Ascaris suum Induced in Pigs by Repeated Oral Inoculations of Infective Eggs

Abstract

Active immunity against migrating larvae of Ascaris suum developed in specific pathogenfree and conventional pigs following repeated oral inoculations of infective eggs of A. suum. Older pigs developed better immunity; pigs that were 2 weeks old when receiving the first oral inoculations averaged 26,068 larvae in their lungs after challenge compared with 58,926 in controls. Pigs receiving their first inoculation when 6 weeks old averaged 3,462 larvae in their lungs compared with 49,982 in the controls. Pigs developed better immunity when they received more than three egg inoculations. Four-month-old pigs that were exposed briefly to ascaris eggs on the floor of a contaminated room, then later given three additional inoculations, had no larvae in their lungs after challenge while the controls averaged 6,573. Herd pigs that had been exposed to eggs of A. suum since birth were brought to the laboratory and given six additional inoculations ranging from 20,000 to 200,000 eggs. No larvae were found in their lungs compared with an average of 38,132 in lungs of the controls. Serum gamma globulin increased in the pigs as immunity developed. Eosinophilia developed following the second inoculation but did not increase in pigs that appeared to be completely immune. There is a possibility that Ascaris suum infection in pigs can be controlled by immunizing methods such as are used against other swine diseases. Acquired immunity was investigated by Roberts (1934), who fed ascaris eggs to two pigs every day for 100 days. Typical lesions were present at necropsy but no migrating worms were found. He concluded that an immunity had developed. Controlled studies of immunity to A. suum in pigs were impossible until specific pathogen-free (SPF) pigs were developed (Young et al., 1955) because infection with ascaris could not be prevented in the experimental animals. Taffs (1958) studied active immunity to migrating larvae of A. suum using SPF pigs that had not been exposed to ascaris eggs prior to the experimental inoculations. The following report confirms the findings of Taffs and the conclusions of Roberts (loc. cit.). MATERIALS AND METHODS SPF pigs were used in three of the experiments reported herein. SPF pigs are delivered by hysterectomy, maintained in individual isolation chambers, and fed a diet of cow's milk fortified with eggs, vitamins, and minerals (Young et al., loc. cit.). They are free of ascariasis and other diseases and lack passive maternal immunity. Pigs in the fourth experiment were from a conventional herd and had been previously exposed to ascaris. The principals and controls of each experiment were of the same age, housed in separate concretefloored rooms, fed and cared for in the same