Resistance to Trichinosis in Swine Induced by Administration of Irradiated Larvae

Abstract

Preliminary experiments indicate that feeding hogs with irradiated (10,000 rep cobalt-60) trichina larvae induces in the hogs resistance to reinfection with nonirradiated larvae. Resistance to trichinosis can be induced in white rats by preliminary infection with larvae of Trichinella spiralis exposed to 6,000 to 10,000 rep of cobalt-60 (Gould et al., 1955). These doses of cobalt-60 cause morphologic alterations in the developing adult worms, and sharp decrease in, or loss of, ability to reproduce (Gould et al., 1957). Levin and Evans (1942) and Zaiman et al. (1961) demonstrated development of an immune response to trichinosis in experimental animals by infection with larvae that had been exposed to X-irradiation. This method of vaccination conceivably could be applied to persons as a public health measure in certain parts of the world (e.g., the Arctics), where flesh of animals is frequently or customarily eaten raw. The purpose of the present study was to determine if resistance to trichinosis can be induced in swine by feeding them trichina larvae exposed to cobalt-60 irradiation. MATERIALS AND METHODS Twenty-one pigs, each approximately 6 months of age and weighing 80 lb (36 kg), were placed in four groups: Group A consisted of four pigs that were initially infected with 50,000 nonirradiated larvae; Group B of six pigs that were initially infected with 50,000 larvae irradiated with 10,000 rep cobalt-60 (Gould, Van Dyke, and Gomberg, 1953; Gomberg and Gould, 1953; Gould et al., 1955); Group C of five pigs that were given a primary infection of 100,000 nonirradiated larvae, and Group D of six pigs infected with 100,000 larvae irradiated with 10,000 rep cobalt-60. Prior to the initial infection each animal was placed under general anesthesia and two tests were made for evidence of preexisting trichinous infection: (1) tissue was excised from the inner thigh muscles for microscopic compression and microscopic section for presence of trichina larvae, and (2) a sample of blood was withdrawn from the heart to determine if trichina antibodies were present. In every animal both of these tests were negative. All of the pigs were starved for 1 day prior to feeding them larvae. The larvae were contained in gelatin capsules that were secured with catgut between two pieces of bread. The bread was placed in a white porcelain feeding tray that was fastened to the floor and the animals were watched to make sure that the slices of the bread were entirely consumed and that the capsules were ingested. Simultaneously groups of white rats were infected with counted numbers of nonirradiated larvae from the same lot to make certain that the larvae were infective at the time they were fed to the pigs. Samples of blood were obtained from the pigs 7, 14, 28, 42, and 49 days after infection in order to test for trichina antibodies in the serum. On the 49th day, two pigs from Group A, two from Group B, three from Group C, and two from Group D were killed by means of intracardiac injection of an overdose of Nembutal. Fifty-gram portions of the diaphragm, tongue, masseter muscle, and psoas muscle were examined by the standard pepsin-hydrochloric acid digestion method, and larval counts were made. Microscopic sections from these muscles were stained with hematoxylin and eosin. On the 49th day the remaining pigs were reinfected with nonirradiated larvae. Two pigs of Group A and four of Group B each were fed 50,000 nonirradiated larvae, while two pigs of Group C and four of Group D each were given 100,000 nonirradiated larvae. On the 98th day all of the animals were killed, and specimens of skeletal muscle were processed in the same manner as in the case of animals killed 49 days after the initial infection. The serums obtained prior to infection and 7, 14, 28, 42, and 49 days after initial infection were placed in a deep freeze cabinet; subsequently, they were subjected to electrophoretic analysis of the proteins and tested for the titer of trichina antibodies. In the electrophoretic examination, the Received for publication 10 July 1964, * Present address: Jackson Memorial Hospital, Miami, Florida 33136.