264 BACTERIOLOGY FOR NURSES 



normal by crisis. About a week later a relapse occurs, but on this 

 occasion the fever lasts a shorter time before suddenly disappear- 

 ing; a second and sometimes a third relapse occurs after about 

 the same interval of time. The disease is usually benign, the 

 mortality varying from 2 to 10 per cent. 



Immunity. Active immunity follows recovery from an attack 

 of the disease, and the blood of immunized animals will confer pas- 

 sive immunity. Metchnikoff observed that during the fever 

 the spirochetes were rarely taken up by the leukocytes in the cir- 

 culating blood, but that at the time of the crisis the organisms 

 disappearing from the blood accumulated in the spleen and were 

 there ingested in large numbers by the leukocytes. These observa- 

 tions suggested the theory that the immunity produced during the 

 first period of fever is of short duration and does not last until 

 all the organisms are destroyed, and that with the disappearance of 

 immunity the survivors escape from the internal organs and appear 

 again in the blood stream. The second attack is less severe and 

 is of shorter duration than the first, and with each succeeding attack 

 the period of immunity is lengthened until finally it lasts long 

 enough to permit all the organisms to be killed. 



Varieties. Several distinct diseases are caused by spirochetes 

 similar to S. recurrentis. West African tick fever has been shown 

 to be due to S. duttoni, a spirochete twice as long as S. recur- 

 rentis and possessing a number of flagella. East African tick 

 fever is caused by a third variety, S. kochi, and relapsing fever as 

 it occurs in India is thought to be caused by still another form. 

 The West African type of the disease was shown by Dutton to be 

 transmitted by a species of tick. Infected insects may harbor 

 the parasites for several months, and of equal importance as regards 

 the spread of the disease is the fact that the spirochete is trans- 

 mitted to the offspring of the infected tick and may even appear 

 in the third generation. Strong evidence has been produced for 

 believing that head and body lice are the usual agents of trans- 

 mission of European relapsing fever. 



Miscellaneous Spirochetes. In addition to the spirochetes 

 causing syphilis, frambesia, and the various forms of relapsing 



