April, '13] TOWNSEND: VERRUGA AND TICKS 221 



■susceptible to the clisease, hence the appearance of the eruption in 

 these animals. 



This hypothesis explains why verruga is not produced by inocula- 

 tion of blood taken from patients in the fever phase of the disease; 

 why it is produced by inoculation of blood and serum taken from the 

 eruption; why mules show the eruption; why the native fauna is able 

 to carry the disease without showing any evidences of it; why the 

 fever and eruption take place in man and susceptible animals; why 

 the pathogenic organisms of verruga and Rocky Mountain spotted- 

 fever have not been seen in the blood; why ticks are necessary for the 

 natural transmission of both diseases; why the pathogenic organism 

 exists in the gut, salivary glands and ovaries of the tick, but is never 

 seen in them; why some eggs deposited by a tick may be infected 

 while others in the same lot are uninfected; why the pathogenic organ- 

 ism appears to proliferate or gain strength or number in the tick; why 

 the bite of the adult tick from one filling of blood may produce several 

 severe cases of the disease, while the same quantity of blood that the 

 tick can hold artificially injected produces only a mild case; why the 

 different degrees of virulence occur in both diseases; why the adult 

 ticks are infective, and why the nymphs may have the infective stage 

 of the organism in their salivary glands; why the ultramicroscopic 

 primary stage of the organism is the only infective stage; why the 

 organism does not transform in the tick; why it reproduces in the 

 eruption of susceptible animals; why it enters the ovaries and eggs of 

 the tick when the latter has become fully engorged; why one attack 

 of either disease renders the subject immune; why both diseases are 

 transmissable through the placenta to the second generation; why 

 Rocky Mountain spotted-fever is not inoculable from guinea pig to 

 guinea pig in successive generations, and why it is so by alternation 

 between monkey and guinea pig. The same reasoning seems to apply 

 equally throughout to the pathogenic organisms of verruga and Rocky 

 ]\Iountain spotted-fever. 



As a working hypothesis we may thus infer that the ultramicroscopic 

 infective primary or sporozoite stage of the pathogenic organism of 

 verruga occurs in the ticks, in the verruga eruptions of susceptible 

 animals, and in the blood of immunes. It probably undergoes devel- 

 opment only in the blood of susceptible animals, and not in the ticks, 

 and with equal probability it reproduces only in the eruption of sus- 

 ceptible animals. During its development through the trophozoite 

 to the merozoite stage in susceptible blood, it is non-infective. After 

 conjugation of the sexual elements in the dermatic tissues, the infective 

 sporozoite progeny appears which needs new susceptible blood for 

 its development. When a patient recovers from verruga and the 

 eruption has finally sloughed, he has become immune to the disease. 



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