(1) The common dog tick, Dermacentor variabilis, is capable of being infected with the virus of St. Louis encephalitis, Hubbard strain, by feeding on inoculated animals and, once infected, can transmit the virus to normal susceptible animals by bite. (2) A female can transmit the infection to her offspring, through all stages of metamorphosis of the 2nd generation into the 3rd generation. (3) Ticks infected under laboratory conditions and kept inactive at a temperature of 12.5°C., remained infective for at least a period of 10 months. Eggs laid by an infected female and stored in a refrigerator (12.5°C.) for 10 months retained infective virus, and larvae hatched from such eggs at the end of the 10 months of dormancy were also infective. (4) The present work, a preliminary account of which appeared in December, 1941 (1, 2), is of theoretical significance since in so far as we are aware, it represents the first successful transmission of St. Louis encephalitis to experimental animals by a blood-sucking vector.

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