Five M. rhesus fatally infected with yellow fever virus ran varied and typical courses, death occurring from 82 hours to 10 days after infecting. Batches of A. aegypti were fed daily on each monkey and specimens of blood injected into other animals. By mosquito transfer, the virus was found to be circulating in the peripheral blood 1 or 2 days after the infecting and the same interval before the onset of fever; in one instance, mosquitoes became infectious by feeding on a monkey 12 hours after its inoculation. Mosquitoes continued to acquire infectivity during the febrile period and for 1 day thereafter, except in one instance when death occurred during fever which prevented post-febrile testing. By subinoculation of blood, the disease was transferred before and after, as well as during the same interval as in mosquito transmission. In one of two attempts, the virus was carried by this means as early as 12 hours after the donor animal was infected. Following the first day of the post-febrile period, blood transmissions were irregularly fatal beyond the period infective for mosquitoes.

These results point to a remarkably rapid multiplication of the virus in the animal host, in one case a blood subinoculation (0.5 cc.) being successful at the first test 24 hours after the donor monkey was bitten by only 2 A. aegypti. The regular acquisition of infectivity by mosquitoes fed during the incubation period is of especial interest in indicating the infectivity of human cases for mosquitoes before the appearance of clinical symptoms. This offers one explanation for the insidious propagation of epidemics of yellow fever and should be useful in the institution of control activities during an outbreak of this disease.

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