Abstract
It is known that passage of influenzal virus directly from human patients to mice is extremely difficult, 1 but that passage to ferrets and then to mice is readily accomplished. It is also probably true that passage of smallpox virus directly from patients to calves or rabbits is difficult, while previous passage through monkeys converts the virus to vaccinia which is then more readily infectious for calves and rabbits. 2 , 3 , 4 Reasoning by analogy, it was suspected that susceptibility to St. Louis encephalitis virus might be transmitted to a wider variety of animals if brain-tissue of suitable animal species were used as inoculum. In spite of the fact that previous attempts to infect guinea pigs with human brain material 5 , 6 or mouse-brain virus 7 , 8 have been unsuccessful, we have considered this worthy of another trial with the view of using guinea pig brain virus for the inoculation of other animals.
In the first series of experiments, begun in the fall of 1937, mouse-brain virus suspended in Locke's solution was introduced intracranially into 2 guinea pigs. After incubationary periods of 14 and 17 days, both animals were found dead. The brains of these guinea pigs were in turn inoculated into animals, and in a similar way the virus was carried through 6 passages as shown in the accompanying diagram. As may be seen, some were found dead, while others were sacrificed when showing convulsions. On several occasions the brains from these animals were passed back to mice, the latter ani-developing typical signs of encephalitis which could be prevented by neutralization with convalescent sera. It may be noted particularly that on one occasion all of the guinea pigs purchased on a certain day survived while the same material injected into others at a later time produced encephalitis in 3 out of 4 animals.
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