8 THE ROCKY MOUNTAIN SPOTTED FEVER TICK. 
eight days this person developed a typical case of spotted fever. The 
experiment was continued by allowing the same tick to bite a second 
person. In this case again a typical case of spotted fever resulted. 
The results of the important experiments of Drs. McCalla and 
Brereton were not published by them. 
In 1906 Dr. H. T. Ricketts, then connected with the University 
of Chicago, began a series of investigations which must always 
be considered classic. Not being aware of the experiments of Drs. 
McCalla and Brereton, Dr. Ricketts started with the hypothesis of 
Drs. Wilson and Chowning. His first work was devoted to determin- 
ing whether guinea pigs and rabbits are susceptible to the disease 
and consequently suitable for inoculation experiments. The original 
experiments with rabbits were somewhat inconclusive, but it was 
found that the injection of blood from a human being suffering with 
spotted fever invariably brought about the disease in guinea pigs. 
In fact in these animals the disease was found to run a course very 
similar to that in human beings. It was thus determined that guinea 
pigs were suitable subjects for experiments to determine whether 
ticks could transmit the disease. On August 4, 1906, Dr. Ricketts 
announced the results of the first experiment in the tick transmission 
of the disease. A small female tick was placed on a guinea pig which 
had been inoculated with the blood of a patient who died of spotted 
fever. The tick was allowed to feed on this inoculated guinea pig 
for two days. It was then removed and placed in a pill box for 
two days. At the end of that time it was allowed to attach to the 
base of the ear of another guinea pig which had not been inoculated 
with spotted fever. After threeand one-half days the temperature of 
this guinea pig rose and remained above normal for more than seven 
days. The pig also showed practically all of the other symptoms of 
spotted fever. In fact, there was no doubt whatever that the guinea 
pig contracted spotted fever from the bite of the single tick. Asa 
control on the experiment Dr. Ricketts placed two other guinea pigs 
in the cage occupied by the animal upon which the tick had been 
placed. They remained there for two weeks. These two pigs showed 
no indications whatever of fever. Thus the possibility of infection 
by contact or by feces was eliminated. The only difference between 
the conditions surrounding the pig which contracted fever and those 
surrounding the others was that the former was bitten bya fever tick. 
During the following year (1907) Dr. Ricketts succeeded in trans- 
mitting the disease by ticks in a number of additional cases. In one 
experiment he found that the male tick as well as the female is 
capable of transmitting the disease. In other experiments it was 
determined that the larval or nymphal tick may acquire the disease 
and retain it through the molting period, and transmit the infection 
in the following stage to another host. The most interesting experi- 
