1. The effect of the rickettsias of rat typhus, tick-bite fever, fievre boutonneuse, and of two other tick-bite-fever-like diseases
(" Hare " and "Appleton ") on the guinea-pig, rat, mouse, rabbit, dog, sheep, and ox is recorded. Rat typhus could be maintained in the guinea-pig and rat but not in the sheep, dog or mouse. "Hare", alone of the other four strains, could be easily passaged in guinea-pigs. Only by the use of cultures from the chorio-allantoic membrane of the developing chick could consistently readable reactions be got in guinea-pigs with the tick-bite fever and fievre boutonneuse strains.
2. A Weil-Felix reaction was obtained with the sera of rabbits inoculated with rat typhus, "Hare" and tick-bite fever. With rat typhus, OX19 was agglutinated by the sera of ten infected animals, OX2 by none and OXK by one serum only; with "Hare" one of nine sera agglutinated OX2, doubtfully OX19 and not OXK; one other serum agglutinated OX19 only. The serum of one of seven tick-bite-fever rabbits agglutinated, in a doubtful fashion, OX19 only. The serum of rats, infected with rat typhus, agglutinated OX19 only and that of guinea-pigs, infected with rat typhus and carrying a banal proteus in their intestine did not agglutinate any one of the three proteus OX strains.
3. Details are given of the duration of infectivity of the brain, blood, and tunica vaginalis of guinea-pigs infected with rat typhus and "Hare". The brain of rat-typhus-infected guinea-pigs was virulent after 22 days, the blood after 21 days, and the tunica after
12 days. The brain of "Hare" -infected guinea-pigs was virulent after 15 days and the blood after 13 days.
4. One two-thousandth hut not 1/4000 of the brain of a rat-typhus-infected guinea-pig was virulent. Much of the virus could be deposited from brain by centrifugation at 4,000 r.p.m. for half-an-hour, and all but a trace was removed at 14,000 r.p.m. for half-an-hour.
5. Attempts to neutralize the various rickettsias in vitro gave unsatisfactory results.
6. Cross-immunity experiments in guinea-pigs permitted the grouping of the 5 typhus strains in the following manner:-
(a) Rat-typhus. - Immunized against itself and to a great extent against the other four typhus-like diseases.
(b) "Hare", tick-bite fever, "Appleton" and fievre boutonneuse. -Gave almost complete reciprocal cross-immunity, but did not immunize against rat-typhus.
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