The virus can be propagated indefinitely in guinea pigs without loss of virulence by weekly blood inoculations in another animal. The virus seems to be transmitted by the salivary secretion of the tick and a tick once infected remains infective for the remainder of life.
Frick succeeded in obtaining anaerobic cultures from infected blood of a bacillus, somewhat resembling the B. typhi exanthematici of Plotz. These bacteria, however, did not show complement fixation with immune serum and were nonpathogenic to guinea pigs.
Spotted fever is a disease of rural districts and tends to give only one case to a house, thus indicating the negative rôle of bedbugs, lice, etc. It is at the time when ticks are most abundant, in the months of the spring, that the disease makes its appearance. The virus is in the blood during the entire febrile course.
Pathology
The cadaver shows marked jaundice with petechial spots on extremities and trunk.
There is marked venous engorgement and the blood is very dark and fluid.
In the blood vessels we have proliferation of the endothelial cells leading to thrombosis.
Ricketts noted enlargement of the lymph glands. The spleen is three or four times the normal size and is quite firm. Microscopically it shows extensive endothelial cell proliferation. The kidneys are enlarged and congested. Gangrene of the prepuce and scrotum are often noted.
Symptomatology
The period of incubation is from five to ten days when the disease sets in with considerable abruptness, with more or less marked rigors, headache, malaise and severe pains of the larger joints, but without inflammatory changes.