Spontaneous Cure in Trichuris muris Infections in Albino Mice and Its Suppression by Cortisone

Abstract

Following inoculation with eggs of the rodent whipworm, Trichuris muris, all individuals of a strain of albino mouse acquired an infection. The vast majority of the worms were apparently expelled from the mice before reaching sexual maturity. In one experiment the loss of worms occurred between 3 and 5 weeks after infection; in another, it occurred between 2 and 3 weeks after infection. The abrupt loss of worms largely could be suppressed by administration of a single dose of cortisone, either during the infection, or 1 day prior to infection. The successful establishment of sexually mature infections of Trichuris muris (Schrank, 1788) in albino mice has been reported in the USSR by Shikhobalova (1941), and in Great Britain by Keeling (1961). In the United States several workers, including Worley et al. (1962) and Campbell and Collette (1962), have found that experimentally exposed albino mice rarely develop a mature T. muris infection. The refractoriness of the albino mice was loosely termed resistance by the latter groups. No distinction was made between resistance and insusceptibility, but was clearly implied. The term insusceptibility is here used to mean the failure of an organism to provide a suitable environment for a parasite, the failure being independent of the previous or concurrent presence of the parasite or its by-products (see Read, 1958). The present paper presents evidence that the strain of albino mouse used by Campbell and Collette (loc. cit.) is, in fact, highly susceptible to infection with T. muris, but that the parasites are invariably expelled from the host just before they reach sexual maturity. MATERIALS AND METHODS The mice used were male albino mice of the Sharp and Dohme strain, which was originally derived from the Institute for Cancer Research strain; this was the strain, and sex, used by Campbell and Collette (loc. cit.). The mice were approximately 5 weeks old at the time of Received for publication 28 March 1963. exposure to T. muris. The mice were housed in groups of seven to ten; they were kept on absorbent bedding and were fed a commercial laboratory feed (Purina Co.) ad libitum. The parasite used was the strain of Trichuris muris used by Campbell and Collette (loc. cit.), Worley et al. (loc. cit.), and Keeling (loc. cit.). Eggs of the parasite were obtained through the generosity of Miss Eileen Pike, Columbia University, and were incubated at room temperature until embryonated. The number of eggs in a suspension was estimated by aliquot counting, and a suitable inoculum was withdrawn while the suspension was being stirred. The mice were inoculated by stomach tube. In Experiment 2, to further insure that the eggs were uniformly dispersed during counting and inoculation, they were suspended in 5% gelatin. Pyrvinium pamoate, where indicated, was administered orally; cortisone, where indicated, was administered as cortisone acetate and was injected subcutaneously. At necropsy the cecum, colon, and rectum of each mouse were slit open in warm saline and incubated at 37 C for 30 to 60 min. The slit bowels and their contents were then washed on a 200-mesh metal screen by means of strong jets of tap water. To avoid splashing and loss of worms the water was delivered through rubber tubing which was manually pinched so as to deliver a strong fanlike spray. This procedure greatly facilitated the counting of immature Trichuris, as it separated them from the gut tissue and left them straightened and immobile. EXPERIMENTAL PROCEDURE Preliminary data were obtained during the routine exposure of a large group of cortisone-treated albino mice. Nine mice which had not received cortisone were exposed to T. muris at the same time as the others. At necropsy 21 days after exposure, all but one of the untreated mice harbored T. muris, and the number of worms in these