Experiments on the Transmission of an Icterogenic Agent in Yellow Fever Vaccine to Horses and Swine

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Abstract
Horses may be slightly susceptible to the icterogenic agent in yellow fever vaccine. Of the 6 horses inoculated with icterogenic vaccine, 4 showed an increase in icteric index and 3 an increase in serum bilirubin 2-4 months after inoculation. The 2 horses that failed to show these changes received the smallest inoculums, 150 human doses of vaccine containing active yellow fever virus as compared with 220 to 400 human doses in the other horses. One of the 2 negative horses had gross and microscopic changes in its liver which were suggestive of hepatic injury. Of the 5 horses receiving human material from cases of jaundice resulting from yellow fever vaccination, 2 showed evidence of subclinical icterus, but no convincing signs of liver damage at autopsy. The observations suggest that a mild disease apparently occurs in horses and has a characteristic incubation period of 2-4 months as does this form of jaundice in human beings. The data obtained by attempts to pass the icterogenic agent from one horse to another are too incomplete and the susceptibility of horses is apparently too low to permit any conclusions as to whether or not this agent is infectious. Further elucidation of the nature of the icterogenic agent would seem to require a more susceptible and readily available exptl. animal; results are not conclusive in regard to the nature of the icterogenic agent.