By injecting into guinea pigs the blood of yellow fever cases occurring in Guayaquil a group of symptoms and lesions closely resembling those observed in human yellow fever were induced in a limited number of instances. Of 74 guinea pigs inoculated with specimens of blood from 27 cases of yellow fever, 8, representing 6 cases, came down with the symptoms; namely, a marked rise of temperature after a period of incubation averaging 3 to 6 days, with simultaneous suffusion of the capillaries, particularly of the conjunctivæs and soles, then preliminary hyperleucocytosis followed by progressive leucopenia, the early appearance of albumin and casts in the urine, which gradually diminishes in volume as the disease progresses. The fever lasts only a few days, rapidly dropping first to the normal and then usually to subnormal. At this period jaundice manifests itself in varying degrees of intensity, first in the scleras, then in the skin and the urine. Hemorrhages from the nasal or gingival mucosa or anus have been observed to occur during this period. Autopsies reveal deep jaundice throughout the entire tissue. The liver is fatty and yellow, the kidney hyperemic, and often swollen and hemorrhagic. Hemorrhagic spots were almost always found in the lungs and gastrointestinal mucosa. Guinea pigs are usually rather sensitive to the infection, though many appeared to be somewhat resistant and some even refractory.
The injection of the yellow fever blood into ringtail monkeys, rabbits, cats, guatusas, weasels, and sloths among the mammalians, and pigeons, ground-doves, bluebirds, mantas, blackbirds, parrakeets, reedbirds, blancos, and toucans among the birds, gave negative results.
In the blood, liver, and kidneys of the guinea pigs experimentally infected with the blood of yellow fever patients a minute organism was demonstrated which closely resembles in morphology the causative agent of infectious jaundice (Leptospira icterohamorrhagiæ).
The leptospira transmitted from yellow fever cases to guinea pigs was found to induce similar symptoms and lesions upon further passage into normal guinea pigs.
The leptospira obtained from cases of yellow fever has been givern the provisional name of Leptospira icteroides.