YELLOW FEVER 601 
in great detail by Nogiichi, especially with respect to the peculiarly 
bent or hooked ends, distinctive of the Leptospirata, it appears \'ery 
possil)le that the two are identical. If such prove to be the case,, 
the specific name of the i)robable etiological agent of yellow fever 
would be "interrogans," in place of "icteroides," the name given by 
Xoguchi. 
Noguchi has isolated a spirochete, which he has called Leptospira 
icteroides,^ from the blood of yellow fever patients. The organism 
induces symptoms and lesions closely resembling those of yellow fever 
in guinea-pigs; domestic animals, excepting young dogs, are refractory 
to infection with it. 
Morphology.— The organism is a delicate, tapering spiral, measuring 
about 0.2 micron in diameter, and 4 to 9 microns in length,- with 
very sharply-pointed ends. It multiples by transverse fission, and it 
is actively motile. The end or ends in the free swimming forms are 
bent into a curve resembling a hook. Stained Leptospirata are some- 
what coarser than those observed with the dark-field illuminating 
apparatus in the living condition. The organism is somewhat smaller 
than Leptospira icterohsemorrhagise. 
Isolation and Culture.— The organism grows fairly readily in media 
containing at least 10 per cent of blood serum (human or rabbit), 
and requires oxygen for its development. The reaction must be very 
near neutrality. It will not grow in competition with bacteria which 
induce obvious chemical change in the nutritive environment. While 
the growth is most luxuriant at 37° C, the limits are quite wide, 
10° to 42° C. It induces no obvious change in reaction in cultural 
media, and hemoglobin is unaltered. 
Pathogenicity.— Guinea-pigs are the most satisfactory experimental 
animals for the study of the pathogenicity of the Leptospira. 
The incubation period is from three to six days, at the end of which 
time the organism may be found in the blood stream, the liver, and 
kidneys. It is transmissible from animal to animal and may be cidti- 
vated from the blood of infected pigs. Noguchi also has found that 
infected female mosquitoes (Aedes aegypti^), obtained by allowing 
the insects to feed upon the fresh organs derived from guinea-pigs 
in the earlier stages of the disease, will transmit the infection to normal 
guinea-pigs. This confirms the early studies of the United States 
Army Board upon the transmission of yellow fever by the Aedes. 
Immunity.— Guinea-pigs which survived an infection with the 
organism were found to be refractory to subsequent infection, and 
blood serum from convalescent yellow fever patients was found to 
dissolve the Leptospira when both were introduced into the peritoneal 
cavities of normal guinea-pigs (Pfeiffer phenomenon). 
Leptospira icteroides and Leptospira icteroha^morrhagi;e (the incit- 
ing agent of infectious jaundice) appear to be quite similar. Immune 
1 Jour. Exp. Med., 1919, 29, 585. 2 Ibid., 1919, 30. VA. 
' Formerly called Stegomyia fasciata and Aedes calopus. 
