Immunity Following Treatment. - In 1904, Ehrlich and Shiga discovered an interesting method for producing immunity. On curing, by one or more injections of trypanred, mice infected with the parasites of caderas, they found that these could no longer be acutely infected if re-inoculated with the same trypanosomes. As a result of the cure an immunity had developed. No signs of immunity, however, were observed among the untreated mice. There was never a spontaneous cure or even a chronic course of the disease.
Protection not Due to the Dye. - That the resistance to infection manifested by the treated mice was not due to unexcreted medicament, Ehrlich and Shiga proved by treating normal mice and subsequently inoculating them with trypanosomes. They found that injections of virus made as early as one and two days after treatment could infect. In these cases, however, the incubation period might last eighteen days or longer. After the second day the protection due to the dye diminished rapidly, and in one of their tables it is seen that mice inoculated on the fourth, fifth, or sixth day after treatment became infected as quickly as their normal controls, and died either at the same time or only one or two days later than these animals.
The Immunity Inefficient. - The immunity which followed cure was not, however, efficient. Even when mice were reinoculated as early as one to seven days after the curative treatment, none of them survived for any length of time. Two died negative for trypanosomes at dates too early to exclude the possibility of relapses, and, after incubation periods of twelve to fifty-three days, all of the others became infected and died.
The Immunity Temporary. - Their experiments also indicated that the immunity was of short duration. The sooner the tests were made after the curative treatment, the longer, as a rule, was the incubation period. Twenty days after the treatment the delay in infection was slight, and by the thirtieth day it was scarcely noticeable....