PROTOZOA 163 



The usual sites of infection of Eimeridae are the cells lining the 

 intestines although occasionally they are found in other organs. Heavy 

 infestations may cause extensive destruction of the epithelium, which, 

 in turn results in inflammation and bleeding and the ultimate death 

 of the host. In small numbers they appear to do little damage 

 and many birds which harbour Coccidia are apparently in perfect 

 health. 



The two most familiar genera from birds are Eimeria and Isospora. 

 The latter is found principally in perching birds and is recorded from 

 various passerines, and also from kingfishers (Coraciiformes), hawks 

 (Falconiformes), woodpeckers (Piciformes), owls (Strigiformes) and 

 cuckoos (Cuculiformes) ; 127 species are known to be hosts in the United 

 States alone. The incidence of infection is also very high — often an 

 entire population is affected. Eimeria, on the other hand, parasitises the 

 more primitive orders such as geese ( Anseriformes) , cranes, coots and 

 moorhens (Gruiformes) , pigeons (Columbiformes), cormorants (Pele- 

 caniformes) and game birds (Galliformes). Both genera are said to 

 infect plovers (Charadriiformes) but this record requires corroboration. 

 Owing to the fact that Eimeria is a pest in farmyards while Isospora does 

 not attack poultry, the latter escapes attention except from the specialist. 

 The various species are said to be strictly host-specific, that is to say, 

 they are peculiar to one sort of bird only. At the same time seven 

 species of J^fmmfl are recorded from the domestic fowl alone and three 

 from geese. Some authorities regard the whole lot as varieties of one 

 species E. avium, which in Britain was first reported from wild birds, in 

 the grouse (Fig. 2ja). The largest number of victims are found among 

 chicks under six weeks of age. Altogether young birds are more suscep- 

 tible than adults. After an infection has been present for some time in 

 an individual bird, schizogony gradually decreases and only male and 

 female gametes are produced by the parasite. This leads to the forma- 

 tion of oocysts which pass out of the body and thus infection gradually 

 ceases. The cause of this change is not really known, but \\e can hazard 

 the guess that it is due partly at any rate, to changes in the blood serum 

 of the host, or acquired immunity. 



Eimeria does not need an intermediate host in order to complete its 

 development and birds can be infected directly by ingesting oocysts. 

 The spread of the parasite may be assisted by flies, which act as transport 

 hosts. These insects, both in the larval and adult stages, ingest the 

 oocysts along with the faeces of the birds, on which they feed. They 



FFC— M 



