1 310 THE RELAPSING FEVERS 



eighth day after the transmissive feed. It can also be spread by 

 direct inoculations such as may take place accidentally in labor- 

 atories or at post-mortems. 



The blood is not merely transmissive to the louse during the time 

 when the spirochaete can be found therein, but also during the 

 apyrexial interval. The louse may be infective for a few hours 

 after a transmissive feed, and perhaps a few cases of human infection 

 may be accounted for in this way, but this is not the usual method. 



After a transmissive feed the spirochaetes quickly disappear from 

 the alimentary canal of the louse, and do not reappear in the coelomic 

 fluid until the eighth to ninth day after the feed; but before this the 

 louse becomes infective from the third to fifteenth day, and usually 

 on the sixth day the louse is most infective. 



Infection is not conveyed by the louse bite, but by the parasite 

 escaping from the body of the crushed louse and entering man 

 through excoriations in the skin, generally made by scratching. 

 One crushed louse is sufficient to produce the disease. 



The incubation period under experimental conditions appears to 

 be six to eight days. 



The louse is not pathologically affected by the germ, of which it is 

 the natural reservoir, the spirochsete being passed from one genera- 

 tion to the next succeeding generation, as has been^shown by the 

 discovery that the young of infected lice are themselves infective. 



Pathology. — ^The presence of the spirochaetes in the blood is 

 associated with fever, which must be due to toxins produced by 

 these organisms, though the severity of the symptoms bears no 

 relationship to the number of the organisms found in the peri- 

 pheral blood. The spirochaetes disappear from the circulation 

 when the temperature falls to normal. The disappearance of the 

 parasites from the blood is brought about by the presence of anti- 

 bodies (agglutinins and parasiticidal substances). In those cases, 

 however, in which relapses occur, the blood remains infectious 

 during the intervals. This is due to some parasites resisting the 

 action of the antibodies. These resistant spirochaetes after a time 

 multiply again in large numbers, giving rise to the relapse. Leva- 

 diti and Roche have demonstrated that serum collected after a 

 first attack destroyed the spirochaetes which caused the attack, but 

 had no effect whatever on the spirochaetes which caused the second 

 attack. The relapses, therefore, are caused by the survival of 

 resistant types of spirochaetes, and recovery depends upon the 

 existence of protective substances in the blood. The immunity 

 acquired after one or several attacks may last for some weeks or 

 months. Animals can be hyperimmunized by repeated inocula- 

 tions after recovery from the first attack. The serum of such 

 hyperimmunized animals shows definite protective and curative 

 properties. 



Morbid Anatomy. — ^The principal pathological feature of the post- 

 mortem is the enlargement of the liver and spleen, the latter organ 

 reaching a weight of i to 2 kilogrammes at times. On section, 



