Immunization of Rats against Ascaris suum, by Means of Nonpulmonary Larval Infections

Abstract

Reports in the literature indicate that although the prepulmonary stages of Ascaris suum in laboratory animals may contain functional antigens, the antigens which normally induce protective immunity against ascaris are not available to the host until the pulmonary phase. In the present study ascaris infections in rats were eradicated by thiabendazole therapy prior to the lung phase. The rats became highly resistant to reinfection, as shown by the almost complete failure of the challenge parasites to reach the lungs of the immunized rats. It is concluded either that the prepulmonary migration of ascaris larvae induced immunity in the rats, or that the destruction of prepulmonary stages caused the premature release of functional antigens. In either case it has been shown that at least some animal species can be immunized against ascaris without being subjected to the damaging pulmonary phase. Acquired immunity resulting from infection with Ascaris suum Goeze, 1782, has been demonstrated by many investigators using a variety of host species (Taffs, 1964b). In immunized animals, the migration of ascaris larvae to the lungs is stopped or greatly retarded, with the result that the immune animals experience much less lung damage than do nonimmune animals. Acquired resistance to ascaris thus is of potential importance, in that it is capable of minimizing the phase of the disease that is of greatest pathological, and hence economic, importance. Soulsby (1958, 1961) presented evidence that the molting phase of ascaris, in guinea pigs, was strongly immunogenic, and concluded that the migrating second-stage larvae were of little immunogenic significance. On the other hand, Arean and Crandall (1962), using rabbits immunized by the intraperitoneal injection of dead second-stage larvae, demonstrated the ability of the lungs to overcome challenging second-stage larvae inoculated intravenously. They concurred with Kerr (1938) and Fallis (1948) in believing that effective resistance to a normal oral challenge operates primarily in the liver. Although it had been shown that secondstage larvae contain effective antigens, there remained the question of whether their early migration through the mucosa and into the liver is capable of providing protection against subsequent reinfection; that is, can animals be immunized by live larvae without exposing the host to the damaging pulmonary phase? In an Received for publication 2 April 1965. attempt to answer this question, rats were exposed to a massive ascaris infection, which was terminated chemotherapeutically before the larvae reached the lungs; the immunological status of the rats was then challenged. MATERIALS AND METHODS Male albino rats, of the Holtzman strain, were used. They were housed in groups of five, and were fed a commercial laboratory feed (Purina Laboratory Chow) without restriction. Eggs of Ascaris suum were obtained from the terminal portions of the uteri of adult worms recovered from naturally infected swine. The albuminous coating was removed by immersion in a sodium hydroxide and sodium hypochlorite bath, and the eggs were incubated in water at 28 C for about 6 weeks. Approximately 80% of the eggs in the stock suspension were embryonated, and in preparing the inocula only those which were embryonated were counted. In each experiment the eggs used in the immunizing and challenging exposures were drawn from a single batch of eggs, whose infectivity had been established in mice. Inocula were withdrawn from a stirred suspension in water, and were administered by means of a dosing needle inserted in the esophagus. At necropsy the lungs, and in Experiment 2 the livers, of the rats were crushed and digested in a solution of 0.7% pepsin and 1.0% hydrochloric acid. The digest was searched for larvae, in small volumes in a graduated petri dish, until all of the digest was examined. EXPERIMENTAL PROCEDURES