ETIOLOGY OF YELLOW FEVER : IV. THE ACQUIRED IMMUNITY OF GUINEA PIGS AGAINST LEPTOSPIRA ICTEROIDES AFTER THE INOCULATION OF BLOOD OF YELLOW FEVER PATIENTS.

The majority of guinea pigs inoculated with the blood of yellow fever patients escaped a fatal infection. There were a number of instances in which the inoculation of yellow fever blood induced in these animals a temporary febrile reaction on the 4th or 5th day, followed in some cases by slight jaundice, but with a rapid return to normal. Most of these guinea pigs when later inoculated with an organ emulsion of a passage strain of Leptospira icteroides resisted the infection. On the other hand, the animals which had previously been inoculated with the blood of malaria patients or normal guinea pigs died of the typical experimental infection after being inoculated with the infectious organ emulsion. It appears from the results just described that a number of nonfatal, mild, or abortive infections follow the inoculation of blood of yellow fever patients into guinea pigs. The fact that such animals manifested refractoriness to a subsequent attempt to infect with a highly virulent passage strain of Leptospira icteroides is an indication, judging from the reciprocal immunity reaction, that they actually passed through an infection with the same organism, or a strain closely related to it, as that which was used for the second infection experiment.