12 THE KOCKY MOUNTAIN SPOTTED FEVER TICK. 
which transmits it is to be found. Extensive work conducted by 
the Bureau of Entomology has shown with considerable accuracy 
the area in which spotted fever may be thus propagated. The map 
(fig. 1, p. 16) shows the area in which the necessary agent for trans- 
mission occurs, and consequently the possible geographical distribu- 
tion of the disease. This map is based upon the examination of 850 
lots of spotted-fever ticks received from 230 localities during the 
seasons of 1909, 1910, and 1911. 
One of the most remarkable features of spotted fever is the fact 
that strains of different degrees of virulence exist in different locali- 
ties. In Idaho the death rate is from 5 to 7 per cent. In the 
Bitter Root Valley in Montana, however, the death rate is about 
TO per cent. One consideration which has caused the Bureau of 
Entomology to concentrate its efforts in the Bitter Root Valley is 
the possibility that the virulent form of the disease, now restricted 
to that valley, may eventually be carried into other regions where 
the presence of the tick would make transmission possible. 
There are several ways by which the virulent strain of the disease 
might be carried out of the Bitter Root Valley. It could be taken 
either by ticks or in the blood of human beings. Carriage by ticks 
might occur when these animals are transported on men, horses, or 
cattle. Moreover, tick eggs or other stages of the tick which have 
been shown to contain the disease organism might be transported in 
hay or other commodities. There is also a chance that ticks in various 
stages might be transported on the hides of domestic or wild animals. 
As regards carriage of infection in the blood of human beings, our 
conclusions are largely theoretical. It is not known how long the 
blood of a person who is attacked with spotted fe^ier remains in- 
fective. It is probable, however, that it is infective for some days 
before the height of the fever and for some time thereafter. During 
the period either preceding or following the climax of the disease a 
person might leave the Bitter Root Valley. If in another locality 
he should be bitten by the fever tick and the specimen should escape, 
the establishment of the virulent form of the disease would be accom- 
plished. In certain diseases similar to spotted fever, such as splenetic 
fever of cattle, the organism of the disease remains in the blood 
for many years without causing an acute or noticeable attack. 
Nevertheless, all ticks which feed upon these apparently immune 
animals become infected and can transmit the disease in acute form 
to other animals. Although nothing is known as to the persistency 
of the organism of spotted fever in the blood of persons who have 
apparently recovered, there is a possibility that it may remain for 
some months or even years. In this way there is a probability of 
