TRYPANOSOMA 



391 



developed for eighty days. Humidity is without effect. Another 

 important matter, as we shall see later, is the effect of the next 

 following clean feed after an infected feed. 



Methods of Infection. — -Tn nature there can be no doubt that infec- 

 tion can take place in more than one way, and Minchin has pointed 

 out that the different methods may be — 



1. CoNTAMiNATiVE. — This is the infection of the host by swallow- 

 ing encysted parasites. At present there is no proof that this exists 

 in trypanosomes. But Minchin has shown that T. grayi becomes 

 encysted while lying in the proctodseum of Glossina palpalis, by a 

 shortening of the flagellum and an excretion of a cyst-wall, which, 

 beginning at the anterior extremity, gradually spreads round the 

 shortening parasite while the flagellum is being absorbed. The 

 cysts which lie quite free from the proctodseum are first pear-shaped 

 and then oval, and the trophonuclei and kinetonuclei break up into 

 chromidia . Minchin believes that these cysts will be found to infect 

 some vertebrate, but there is no proof of this at present. 



2. Inoculative. — The spread of a trypanosome from one verte- 

 brate host to another was demonstrated by Bruce while studying 

 T. hrucei, and also by the same observer, Nabarro, and Kleine, for 

 T.castellanii, both diseases being spread by tsetse-flies, though with 

 regard to the former it had long been known that the tsetse-fly was 

 the carrier of the disease. 



There are, however, two ways in which a blood-sucker can spread 

 an infection: (a) the direct; (6) the indirect. 



{a) The Direct. — In this the blood-sucker simply transmits the 

 parasite unchanged. 



(6) The Indirect. — -In this method the parasite undergoes a cycle 

 of development in the blood-sucker, the nature of which has been 

 fully discussed already. 



It would appear probable that it is only in the natural hosts that 

 forms develop which are infective for the invertebrate. 



Effects upon the Vertebrate Host.— Trypanosomes may produce 

 pathological effects on their vertebrate host, or may apparently be 

 harmless, and certain harmless trypanosomes will only live in some 

 particular vertebrate and die when injected into others, but no 

 pathogenic trypanosome is restricted to one host. 



The best known of the former are T. gambiense, T. castellanii and 

 T. rhodesiense, the causes of sleeping sickness; S. cruzi, of American 

 trypanosomiasis; T. evansi, of surra in horses, etc., in India and 

 elsewhere; T. hrucei, of nagana in cattle and horses, etc.; T. vivax, 

 of disease in cattle, sheep, and goats, in the Cameroons; T. nanum, 

 of cattle sickness in the Anglo-Egyptian Sudan; T. dimorphon, of 

 the Gambia horse sickness; T. cazalboui, of souma, a disease of 

 cattle in the French Sudan, and T.pecaudi, of sheep in the same place ; 

 T. equinum of mal de caderas in South America ; T. equiperdum, of 

 dourine in horses in Europe and Africa. 



Symptoms. — ^The pathogenic effects of the parasites show them- 

 selves in the production of fever, skin eruptions, emaciation, local 



