144 INSECTS AND MAN 



process on the part of the parasites takes place within the 

 tick's body, and, moreover, a tick once infective remains 

 infective for the rest of its life, which lasts for some years, 

 and transmits the infection to its offspring through the 

 eggs. It is interesting to compare the transmission of 

 relapsing fever by Ornithodorus moubata with the trans- 

 mission of plague by the plague flea, and it is some con- 

 solation to know that the tick has deadly enemies in the 

 shape of ants and rats. 



Relapsing fever is not the only tick-borne disease of 

 mankind. For many years, probably even before the 

 settlement of white man, a disease, transmitted by ticks, 

 and known as spotted fever, has been recognised in the 

 Rocky Mountain region of the United States. Indians 

 warned the first white settlers that they ran considerable 

 risk of contracting a very serious disease if they established 

 themselves in certain localities. It is in Idaho and Montana, 

 and especially in the Bitter Root Valley of the latter state, 

 that Rocky Mountain spotted fever is particularly pre- 

 valent at the present day. A most remarkable fact is that 

 different degrees of virulence exist in different localities ; in 

 Idaho the fatalities from this disease amount to from five 

 to seven per cent., whilst in the Bitter Root Valley they 

 reach the formidable total of seventy per cent. 



As regards the carriage of infection in the blood of man, 

 the conclusions are merely theoretical ; it is not known how 

 long the blood of an infective person remains infective, 

 though the period probably extends for some time before 

 and after the fever has reached its maximum. In certain 

 diseases, such as splenetic fever of cattle, which is, in many 

 respects, similar to Rocky Mountain spotted fever, the 

 disease organisms may remain in the blood for many years 

 without causing any ill effects, but all ticks drawing blood 

 from these apparently immune animals can and do transmit 

 the disease in an acute form to other animals. There is 

 only one tick in the Bitter Root Valley that attacks man, 



