222 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 6 



It is further almost certain that, like the immune native reservoir fauna, 

 he carries the infective sporozoite stage of the organism in his blood. 

 This would explain what becomes of the pathogenic organism, so far 

 as its continuous life-cycle is concerned, after it has reproduced sexu- 

 all}^ in the subcutaneous tissues of the susceptible animal. Having 

 reproduced by conjugation in the eruption, the greatly multiplied num- 

 ber of resulting sporozoites invades the blood of the subject, which 

 being now immune does not afford them the environment requisite 

 to" their development. The sporozoites must remain quiescent in 

 the same infective stage awaiting the opportunity to transfer to sus- 

 ceptible blood. Thus immune man and mules may constitute a second- 

 ary reservoir of verruga — or immune dogs or other animals— but in any 

 event the particular tick which acts as transmitter of the disease is 

 necessary as the medium through which the sporozoites may, during the 

 bite of the tick or during the bites of its progeny, gain access to the 

 blood of susceptible subjects. 



The difference in degree of virulence of both verruga and Rocky 

 Mountain spotted-fever exhibited by different" cases is probably di- 

 rectly due to the relative numbers of the infective sporozoite-stage 

 organisms which gain access to the patient during the bite of the tick. 

 The tick acts as a collector and reservoir of these sporozoites or infec- 

 tive organisms which become stored in its salivary glands. The number 

 of these that gain access to the blood of a susceptible subject during 

 the bite of the tick is directly dependent on the length of time that 

 the tick remains attached. It was found that the minimum duration 

 of attachment of an infective tick required to produce Rocky Mountain 

 spotted-fever in a guinea pig was one and three-quarters hours. The 

 average was found to be about ten hours, while twenty hours of attach- 

 ment were almost invariably effective. Thus a short duration of the 

 bite will produce a milder case in both diseases than double that dura- 

 tion by the same infective tick. The patient may be bitten by several 

 infective ticks at different intervals, in which case each successive 

 biting is certain to increase the number of the infective organisms in 

 his blood. It is thus seen that variation in the number of infective 

 ticks and in the duration of their attachment may produce all the 

 known classes of verruga cases with their numerous intergrades. 



In the uninhabited verruga districts there are few or no large animals 

 for the adult ticks to engorge upon. Probably these ticks drop from 

 the small native mammals as nymphs and change to adults on the 

 ground, after which they crawl everywhere looking for large hosts. 

 The small uninhabited tributary canyons which exist everywhere 

 throughout the region, like the Agua de Verrugas canyon crossed by 

 the famed Verrugas Bridge of the Central railway, almost certainly 

 keep the disease alive and feed it constantly into the main canyons 



