Studies on Typhus Fever Vi. Reduction of Resistance by Diet Deficiency

The average guinea pig or rat possesses a resistance against typhus infection which llm;ts the distribution of the virus and leads to almost invariable recovery of the animals. Many of our efforts during the last 2 years have focused upon the production of increased susceptibility , in order that we might convert the milder, experimental disease into a fatal one or at least obtain an increased yield of Rickettsia bodies for immunological studies. As reported in a preceding paper (1), preparatory benzol injections into rats yielded us a sufficient number of successful experiments to furnish peritoneal exudates rich in Rickettsiae and suitable for the etiological demonstration which we reported. Although we have at the same time studied cultural methods by the tissue-plasma technique and by the Tyrode-serum method of the Maitlands, and have obtained some multiplication, we continued to concentrate upon the susceptibility-enhancement experiments because wherever successful-the peritoneal exudates of animals inoculated in the stages of depression yielded not only many intracellular, but plentiful extracel-lular Rickettsiae which were, therefore, easily freed of plasma and cells by simple washing in the centrifuge. The benzol method was successful in only about one out of three times, since individual differences between rats seemed to necessitate adjustment of time factors and the number and amounts of the benzol injections which could not be approximated without a very large series of experiments. Although we employed the benzol method, nevertheless, in the previously re-333