Leptospira icteroides was subsequently isolated from cases of yellow fever in Mexico, Peru and Brazil. It is actively motile, measures 4-9 /2 long and 0.2µ wide and there are two spirals for each micron. The ordinary microscope does not reveal its presence in the living condition, but it is easily seen by means of dark ground illumination. It grows only on special culture media. The isolation of Leptospira icteroides has made it possible to prepare a preventive vaccine, similar in type to that in use for protection against typhoid fever, and a curative serum. While a given locality may be freed from yellow fever by destruction of mosquito lar vae, this type of preventive work requires some months. New comers meantime are in danger, and protection, even of a tem porary nature, is welcome. Injection of a small quantity of killed cultures of Leptospira icteroides, as first shown in experimental animals, confers temporary protection from the yellow fever infection, the inoculation taking effect within about two weeks. The results of prophylactic inoculation of 20,000 or more in dividuals since 1919 indicate that if persons are satisfactorily vaccinated they are protected against yellow fever for about six months. The curative serum has been tried in several hundred cases. When it has been used early in the disease, i.e., before the fourth day of illness, the death-rate has been comparatively small (16% as compared with the usual so% or more in yellow fever). It is without appreciable benefit when given later in the
illness, when the micro-organisms have done irreparable injury to the liver and kidneys.
Since 1925 doubt has been thrown on the causal relationship of Leptospira icteroides to yellow fever by discovery of the fact that the common rhesus monkey (Macacus rhesus) and to a lesser degree the Indian crowned monkey (M. Sinicus) are very suscep tible to the disease which may be transmitted to them by the bites of infected mosquitoes or inoculation with blood of a yellow fever patient. Material from such infected animals or from yellow fever cases is without effect upon guinea pigs which are extremely sus ceptible to L. icteroides. Since L. icteroides is serologically identi cal with L. icterohaemorrhagia it is thought probable that the organism described by Noguchi as the cause of yellow fever had been obtained from cases of Weil's disease or possibly from patients with a double infection. Working with a strain of yellow fever isolated in Senegal and rhesus monkeys A. W. Sellards and E. Hindle have advanced matters further in favour of a filterable virus theory by showing that like some other filterable viruses yellow fever infective material is unaffected by cold and if frozen will maintain its virulence for at least twelve days (Brit. Med. Tr. 1928, i. 713).