1. The infectious agent of Colorado tick fever filtered through 181 mµ membranes caused infection in hamsters and one volunteer. 2. In five out of seven instances, hamsters were infected initially with serum filtered through 24 mµ membranes or the tick fever appeared on serial transfer. 3. We could not infect two volunteers with 24 mµ filtrates, though these filtrates did cause infection in hamsters as shown by development of the fever on serial transfer. In one instance, the serum of the hamsters rendered ill in this way was injected into the volunteer who had remained well after inoculation with the original material and it caused Colorado tick fever. 4. Normal hamster serum was passaged through 10 groups of animals and then two human volunteers were injected with it. They did not come down with Colorado tick fever, although both of them were susceptible to the disease as shown by the fact that they fell ill of it on later inoculation with serum from a natural instance of the disease. 5. The infectious agent of Colorado tick fever passes through 24 mµ membranes. On this basis, it is classified as a virus.
1. The symptoms, history of tick bite, characteristic fever curve, and white blood cell picture should enable the physician to make a diagnosis of Colorado tick fever in nearly every case. 2. The typical white blood cell picture is a depression of the total leucocytes with a shift to the left of the granulocytes. Basophilic cytoplasmic bodies appear occasionally in lymphocytes 3 to 4 days after clinical recovery. 3. The disease can be transmitted serially in human beings by parenteral injection of blood or serum. Such transfers have not resulted in decreased or increased virulence. 4. The naturally acquired and experimental cases of Colorado tick fever are identical in their manifestations. 5. An attack of Colorado tick fever confers a degree of definite immunity to the disease. 6. Colorado tick fever is not a mild form of Rocky Mountain spotted fever since individuals immunized with ground tick vaccine against Rocky Mountain spotted fever are still susceptible to Colorado tick fever. 7. Adult Dermacentor andersoni ticks allowed to feed on typical cases, then carried through to a new generation and fed on susceptible adults, failed to transmit the disease. 8. Colorado tick fever has been successfully transmitted to an experimental animal, the golden hamster.
Values for hemoglobin, erythrocytes, volume of packed cells, leucocytes, and their distribution, and red blood cell diameter are presented as determined on a group of 114 hamsters of both sexes and of various ages.