VARIETIES 511 



less severe than the first attack. This is repeated till the im- 

 munity lasts long enough to allow all the organisms to be killed. 

 It is possible, however, that the survival of resistant spirochetes, 

 or " mutants," may play a part in the production of the relapses. 



Varieties. — As already stated, relapsing fever has been studied 

 in different parts of the world, and, apart from the African tick 

 fever, European, Asiatic, and American types have been dis- 

 tinguished. Differences have been made out with regard to 

 clinical features, pathogenic effects, and immunity reactions. 

 It has been shown, for example, by the work of Novy, Strong, 

 and Mackie, that the American spirochaete is probably a distinct 

 species, as animals immunised against it are still susceptible to 

 infection by the European and Asiatic organisms, and vice versa. 

 The relationship between the two latter is certainly closer, and 

 no distinct immunity differences have been established. Re- 

 lapsing fever in Asia is evidently a much more severe disease 

 than in Europe ; Mackie gives the mortality in Bombay at the 

 comparatively high figure of 38 per cent. But differences in this 

 respect, as well as in pathogenic effects, may simply depend on 

 variations in virulence. At present no definite statement can be 

 made on this point. Sergent aud Foley have described a type 

 of relapsing fever occurring in Algiers, which they consider to 

 be different from the recognised forms, and have given the 

 name sp. berbera to the organism concerned ; and Balfour has 

 observed cases in Khartoum which he thinks are probably of the 

 same nature. 



The fact that tick fever and other spirilloses are conveyed by 

 the bites of insects makes it extremely probable that relapsing 

 fever is transmitted in this way. At first the bed-bug was 

 believed to be the vehicle of transmission, and the experiments 

 of Karlinski and of Tictin, which showed that the spirochsetes 

 might remain alive and virulent in the body of this insect for 

 some time after it had sucked the blood of a patient, lent some 

 support to this view. Attempts to transmit the disease by 

 means of the bites of bugs were, however, generally unsuccessful ; 

 Mackie produced the disease in only one out of six monkeys used 

 for this purpose, though large numbers of bugs, which had bitten 

 relapsing fever patients, were used. On investigating an 

 epidemic of the disease, however, he obtained a considerable 

 amount of evidence on epidemiological grounds that the disease 

 was carried by the body louse. He also found that the spiro- 

 chaetes in the blood which had been ingested underwent great 

 multiplication about three days afterwards, and formed large 

 tangled masses in the stomach contents. The view that the 



