590 The Blood Flagellates 



forms, through an intermediate crithidial stage, is followed by rupture 

 of the "cyst" to liberate the flagellates into the body fluids. A minimum 

 of four or five days is required for this phase of the cycle. 



In tissue-culture infections (38, 41, 57), trypanosomes of the type 

 usually seen in the blood also have been observed about the fifth day 

 after inoculation with T. cruzi. Muniz and de Freitas (47), using peri- 

 toneal fluid as a culture medium, have obtained stages similar to those 

 normally seen in the vertebrate. The presence of tissue cells apparently 

 is not necessary for transformation of the trypanosomal into the leishma- 

 nial form. On the other hand, metamorphosis into the trypanosomal 

 stage, observed in whole peritoneal fluid, does not occur in cell-free 

 fluid. Regular intervention of a crithidial stage between the leishmanial 

 and the trypanosomal forms in the vertebrate has been questioned by 

 Elkeles (25) who failed to find crithidial forms in his material. Also 

 interesting is the report that crithidial stages from cultures mostly dis- 

 integrate in normal serum, while leishmanial and trypanosomal forms 

 are not affected (44). However, all stages in the classical vertebrate cycle 

 have been observed in chick embryo tissue cultures (41). 



Trypanosomal stages in vertebrate blood are ingested by an insect 

 vector and apparently change into leishmanial forms in the midgut. 

 Fission of leishmanial forms may occur, but metamorphosis into crithidial 

 forms takes place within a day or two. In the crithidial phase, multiplica- 

 tion occurs for some time before small metacyclic trypanosomes are 

 derived from crithidial stages in the hindgut. Completion of the insect 

 cycle requires about two weeks. In cultures, transformation of trypano- 

 somal into crithidial stages seems to depend upon some substance present 

 in washed erythrocytes, peptone and meat infusion. Hematin apparently 

 is not the significant factor (46). This "metamorphosis-inducing factor" 

 presumably would be required also in the digestive tract of the vector. 



Transfer of the parasites to the vertebrate host involves discharge of 

 metacyclic trypanosomes from the hindgut as the bug ingests another 

 meal of blood. Large numbers of flagellates, sometimes as many as 

 2,500/mm3 (21), are deposited in the fecal material. If the trypanosomes 

 reach a break in the skin or the wound made by the insect, infection may 

 result. Or infection may follow contamination of intact mucous mem- 

 branes (the conjunctiva and the oral, rectal, and vaginal mucosae). In 

 addition to the usual transfer by vectors, rodents are known to eat 

 triatomids and may acquire the infection in this way. Such transfer would 

 be favored by the occasional survival of T. cruzi for several weeks in dead 

 insects (75). Carnivores probably can become infected by eating infected 

 rodents. Other direct methods apparently include lacteal and placental 

 transfer in mammals (23). Furthermore, there is always a possibility of 

 transferring the flagellates by blood transfusion in man. 



Vectors and resenjoir hosts. The reported range of T. cruzi extends 



