Animal experiments.—— It was generally stated that S. recurrentis (S. obermeieri) could be transferred only to man and monkey. Novy and Knapp infected white mice and white rats. The former they found were especially susceptible, the organism appearing in the blood within twenty-four hours of inoculation and persisting to the third day (eighty hours). About this time they disappear for several days from the blood of the infected mice and until the commencement of relapse. The first relapse may be followed by a second, third, or even a fourth, the number varying in individual mice; with each relapse the parasites reappear in the blood. The interval between the relapses, counting from the first appearance of spirochætes in one to their first appearance in the next relapse, is generally about seven days; occasionally it is only two; sometimes it is as long as ten days. The number of spirochætes in the relapses is much smaller than in the first paroxysm, clearly indicating the development of a partial immunity. Recovery in mice, as in other animals, is the rule.
The same observers found rats to be susceptible, but in them the progress of the disease was different. The period of incubation was longer (forty hours), and there were no relapses. As a result of the consecutive passage of the spirochætes through a long series of rats its virulence was augmented, so that the incubation period became reduced to fifteen or eighteen hours, and the persistence of the parasite in the blood prolonged to sixty hours instead of, as originally, forty-eight hours; at the same time the spirochætes became far more abundant.
The parasite, which is present not only in the circulating blood but in all the organs, seems to produce no serious pathological change in the rat beyond great temporary enlargement of the spleen. Recovery is practically invariable. Young rats are more susceptible than old rats. Immunity persists for many months.
Rabbits and guineapigs are refractory.
Immunity.—— Sabritschewsky in 1896 showed
