86 



THE ANIMAL PAKASITES OF MAN 



host is increased, and symptoms are produced. On the other hand 

 the first mode of multiplication, seen in the lung capillaries, is 

 considered by Chagas to be a process of gametogony, in which sexual 

 forms are differentiated. He finds that (i) the adult trypanosomes 

 exhibit a dimorphism in human blood rarely seen in artificially 

 infected guinea-pigs. In these guinea-pigs (infected from guinea-pigs) 

 the so-called gametogony in the lungs is seldom seen. (2) The 

 intermediate host, Triatoma (Conorliinus), becomes infective if fed 

 directly on infected human blood, but very rarely so if fed on guinea- 

 pigs. Chagas is led to believe that the occurrence of sexual forms 

 constantly in the blood of man implies a greater resistance to infection 

 on the part of man than on the part of guinea-pigs or other animals, 

 assuming the general hypothesis that the formation of gametes 

 represents a reaction of the Protozoon to unfavourable conditions. 

 In human infection the number of parasites is always less than in 

 laboratory animals, and their presence in the blood is transitory, 

 lasting from fifteen to thirty days in acute cases. In many cases 

 examination of the tissues at death has shown the presence of parasites 

 in patients who did not exhibit them in the general circulation. 



FIG. 35. Trypanosoma crtizi. Development in Trialonia megista. 1-6, forms found in 

 the mid gut of Triatoma ; 7 flagellate forms found in ihe posterior part of the gut of Triatoma. 

 (After Chagas. ) 



Life History in the Invertebrate Host. About six hours after the 

 ingestion of infected blood by the bug (Triatoma megista), the kinetic 

 nucleus of the trypanosome moves towards* the nucleus, and the 

 flagellum is usually lost" (fig. 35, 1-5). The parasite becomes rounded 

 and Leishmania-like (fig. 35, ^-5), and multiplies rapidly by division. 

 After a time, multiplication having ceased, the rounded forms become 

 pear-shaped and develop a flagellum at the more pointed end. 

 Crithidial forms (fig. 35, 7) are thus produced and pass into the intes- 

 tine, where they multiply and may be seen in about twenty-five hours 

 after the ingestion of blood. The crithidial forms may also be 

 found in the rectum and faeces. The last stage in the invertebrate 

 is a small, trypanosome-hke type, long and thin with a band-like 

 nucleus and conspicuous kinetic nucleus. These parasites are found 

 in the hind gut and in the body cavity. They find their way into 

 the salivary glands, and are the forms (fig. 36) which are transmissible 



