Pigs on balanced rations were given single inocula of 100, 500 or 5,000 infective eggs of pig ascaris (A. suum) or human ascaris (A. lumbricoides) and examined 9 to 91 days later. Rabbits were similarly inoculated. At 9, 11, and 22 days of infection in rabbits more larvae of A. lumbricoides than of A. suum were recovered, while in pigs more larvae of A. suum usually were recovered. A. suum had a more rapid rate of tracheal migration in both pigs and rabbits and, with few exceptions, a shorter prepatent period in pigs (40 to 53 days) than did A. lumbricoides (54 to 61 days). At 45 to 91 days of infection, 20 of 21 pigs inoculated with A. suum harbored 3 to 669 worms, and 8 of 11 pigs inoculated with A. lumbricoides harbored 1 to 23 worms; among 15 uninoculated controls, two harbored 1 and 4 worms, respectively. The observations indicate that in the pig A. lumbricoides is capable of producing patent infection, though it differs markedly from A. suum in the rate and frequency of development to sexual maturity. Numerous attempts have been made to discover differences between Ascaris lumbricoides, the common roundworm of man, and Ascaris suum, which occurs commonly in the pig. Differences in structure of the dentigerous ridges were described by Sprent (1952) and by Abdulrachman and Joe (1954), but these observations were not confirmed by Lysek (1963). Differences have been noted also in antigenicity (Kagan et al., 1958) and in rate and frequency of development in human volunteers (Takata, 1951; Yajima, 1955; Akamatsu, 1959). The rate and frequency of development in their respective hosts could provide useful criteria for differentiation as in the different strains of Schistosoma japonicum in Taiwan and elsewhere (Hsii and Hsii, 1962), but experimental attempts to infect man with the pig ascaris and pig with the human ascaris have given variable results (de Boer, 1935). The early experimental studies were hampered by the difficulty of producing patent A. suum infections in the pig (Ransom and Foster, 1920; Martin, 1926; Roberts, 1934). The purpose of the present study was to Received for publication 27 May 1968. * From a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Parasitology at Tulane University, New Orleans, Louisiana. Supported in part by grants from the National Institutes of Health, Public Health Service, and Texas A & M University Graduate College. further compare the pig ascaris with the human ascaris with respect to rate of pulmonary migration in the rabbit and pig and the rate of growth and the frequency of development to maturity in the intestine of the pig. MATERIALS AND METHODS Experimental animals Sixty pigs in seven litters (designated litters 1-7 in Table I) were obtained from a recently stocked specific-pathogen-free (SPF) swine research farm where ascaris infections had never been observed. Each litter was divided into three equal or nearly equal groups and given infective eggs of A. suum or A. lumbricoides, or not infected and maintained under similar conditions as a check against extraneous infection. The seven litters were used in four experiments (trials I-IV), outlined in Table I. Male pigs of two litters (nos. 3 and 4) were castrated 1 day before inoculation; the others were not castrated. The pigs of litter no. 2 were given anti-hog cholera serum 55 days after inoculation; the remainder were unvaccinated. Rabbits of a New Zealand White stock, 6 to 12 months of age, were used for observations on larval migration, and for checking the infectivity of cultured eggs. Animals used in individual trials were of approximately the same size and age. Housing and feeding of animals Pigs obtained soon after birth (litters 2 and 5) were placed in individual cages for ease of bottle feeding and care. Older pigs were placed individually or in groups in cages or concrete-floored stalls. All cages and stalls were in a closed building and were steam-cleaned prior to the start of each trial. During the trials the stalls were cleaned daily with individual scrapers and running water. Groups of animals receiving different inocula, or none, were housed separately except in the initial trial in which control and test pigs were caged
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