j-72 FLEAS, FLUKES AND CUCKOOS 



those which produce sleeping sickness in man, and nagana in cattle, are 

 transmitted by tsetse flies. Only one of the horse trypanosomes, T. 

 equiperdum, is known to have a direct life-cycle and passes from horse 

 to horse during the sexual act. In the invertebrate host these parasites 

 develop in the alimentary canal, finally giving rise to the infective 

 forms. In some trypanosomes the latter are produced in the mouth 

 parts or salivary glands, and are then inoculated into the vertebrate 

 during the blood meal. In others, well illustrated in the rat flea 

 infected with T. lewisi, the infective forms develop in the hind gut, and 

 infection occurs when the host accidentally ingests the flea or its drop- 

 pings. It is beheved that trypanosomes have evolved from a more 

 primitive type of flagellate which is normally parasitic in the gut of 

 insects. During the part of the cycle within the invertebrate host the 

 trypanosomes pass through various stages in which they appear to 

 revert to ancestral forms. 



The carriers of bird trypanosomes are not known for certain. Some 

 development appears to take place in mosquitoes and it has been 

 claimed that T. loxiae and T. noctuae (from the crossbill and little owl) 

 multiplied and produced crithidia-like forms after ingestion by the 

 house-gnat. It has been claimed that the red mite {Dermanyssus gallinae) 

 can transmit one species of bird trypanosome. The fact that nesdings are 

 so frequendy infected in nature suggests that the carrier is an arthropod 

 breeding in their nests. 



Very often the blood of wild birds is infected with various sorts of 

 parasites. One thrush examined by Coles was found to harbour 

 simultaneously all three genera of bird malaria parasites {Plasmodium, 

 Haemoproteus and Leucocytozoon) a Trypanosoma and a filariid worm. This 

 is confusing, and even trained scientists have fallen headlong into the 

 trap and have described, with great enthusiasm, completely different 

 organisms as stages in the life-cycle of the same species. 



Another well known parasite from the same order is Histomonas 

 meleagridis.This is a flagellate with an amoeboid phase, harmless if present 

 in the intesdne of chickens; but in turkeys it invades the liver and 

 intestinal wall, causing a mortal illness commonly known as "black- 

 head." It does not form a cyst but is transmitted directly when a bird 

 accidentally ingests contaminated faeces or the eggs of the caecal 

 worm [Heterakis) which act as transport hosts. 



There are also numbers of Trichomonadidae (Fig. 2,d) found in 

 birds. These are spindle- or pear-shaped flagellates with a sdff rod-like 



