Certain aspects of the host-parasite relationship ofNematospiroides dubius(Baylis). II. The effect of sex on experimental infections in the rat (an abnormal host)

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Abstract
The male rat is more susceptible to infections of Nematospiroides dubius than the female. As the rat grows older the resistance of the female rat to infection increases at a greater rate than that of the male. More larvae penetrated the intestinal mucosa to encyst in the male than in the female. More larvae, however, formed cysts in the female than in the male rat by the fifth day. The male harbored more adult worms than the female rat, although this difference was not significant in the immature animals. The sex resistance of the rat to N. dubius infections was removed by bilateral gonadectomy. Castration decreased the susceptibility of the male rat, while spaying increased it in the female compared with the susceptibility in the respective normal hosts. Subsequent replacement of the homologous sex hormone in the gonadecto-mized rat restores the sex resistance, and may even increase it (particularly in the immature animals). Oestradiol increased the resistance of the spayed female rat, while testosterone increased the susceptibility of the castrate male rat to infection. Oestradiol implanted in castrate male rats increased the resistance of these hosts to a greater level than was shown in the normal male rat. The rat shows a marked age resistance over which the sex resistance is superimposed.