T. lewisi infection in normal adult 3 month old albino rats raised from a single stock and maintained under identical conditions was studied. Daily quantitative estimates of the trypanosomes in the circulating blood were made and the course of the infection was studied. Bilateral suprarenalectomy in rats lowers the resistance to a subsequent infection with T. lewisi. About 70 per cent of these rats die in an average period of 5.8 days after injection. The multiplication of the parasites, in the circulating stream, however, is not more considerable in the suprarenalectomized than in the previously normal rats, nor is the duration of the disease in the surviving rats any longer than in the normal group. The removal of the suprarenal glands does not alter the immune reaction to the parasite, but lowers the natural resistance of the animal to the toxic effects of the protozoan infection. Bilateral suprarenalectomy does not lessen the immunity of rats recovered from T. lewisi infection to subsequent infection. Unilateral nephrectomy does not influence the course of a subsequent infection with T. lewisi infection. The mortality of splenectomized rats from Bartonella muris anemia increases from 30 to 100 per cent following the injection of T. lewisi at the height of the anemia 7 days after splenectomy. T. lewisi infection 48 days after splenectomy that is to say at a time when the Bartonella anemia is no longer present produces a more severe infection than in normal rats. The number of trypanosomes at the height of infection averages 3 times the ordinary and the infection endures twice as long. Both the immune substance that inhibits the reproduction of the parasite and the lytic factor are markedly depressed. Splenic autotransplantation performed 4 weeks prior to splenectomy raises the resistance of rats to a subsequent T. lewisi infection. Thymectomy in 6 week old rats diminishes the severity of a subsequent trypanosome infection and shortens its course. Both the formation of the immune substance which inhibits reproduction of the trypanosomes and formation of trypanolytic antibodies are stimulated by this procedure. In the adult rat thymectomy shortens the course of the infection but the severity is only slightly diminished. Bilateral gonadectomy in the adult increases the severity of the infection. The number of trypanosomes at the height of the infection is almost three times the normal. However, the duration of the infection is the same as in the normal rats. The reproduction-inhibiting factor is depressed by bilateral gonadectomy but not the trypanocidal factor. Unilateral gonadectomy does not influence the infection.

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