PROTOZOA 165 



both birds and mosquitoes are found. After considering all the available 

 data, Hewitt calculated that the mean rate of infection for all birds is 

 about 5 per cent. In Germany and Italy the figures are between 4.4 

 and 4.8. per cent., but in California they rise to 18 and 19 per cent. 

 Passerines are more susceptible than other birds but a wide range of 

 hosts — over 200 species — is recorded. These include about 40 on the 

 British fist such as the great tit, white wagtail, swallow, nightjar and 

 various finches, buntings, thrushes, warblers, larks, shrikes and so 

 forth. About twelve species of bird Plasmodium are now recognised 

 although many have been described several times over in error, so that 

 the literature is cluttered up with invalid names. Probably not more 

 than four or five can be expected to occur in Britain. The insect vectors 

 are mosquitoes, of which by far the most important is the house-gnat 

 {Culex pipiens) , the commonest of all British mosquitoes. Certain other 

 species found in this country such as Aedes geniculatus and Theobaldia 

 annulata are also known to be carriers of the disease. 



The Life-cycle of Plasmodium relictum 

 Various species oi Plasmodium parasitise mammals, birds and reptiles 

 but the sexual stage of the life history is always passed in insects. The 

 cycle of the malaria parasite is extremely compHcated and the organism 

 passes through a constant series of changes of form. P. relictum (formerly 

 known as P. praecox) was the species with which Ross carried out his 

 famous experiments. It is common in birds found in tropical and sub- 

 tropical countries, but to a lesser degree it also occurs in temperate 

 cHmates and has been recorded from North America, and in Europe 

 from France, Germany, Austria, Switzerland, Italy and Russia. Some 

 authorities (see Appendix : Hewitt, Wenyon) assume that the un- 

 named Plasmodium which Coles recorded from three song-thrushes and 

 a blackbird in the Bournemouth district of England refers to this species. 

 The life-cycle (Fig. 3) in the vertebrate host commences when the 

 sporozoites in the saliva of the mosquito are introduced into the bird 

 during the process of blood-sucking. The sporozoites at this stage are 

 minute active worm-like vermicules and, on entering the blood stream, 

 they are taken up by leucocytes or endothelial cells of different organs, 

 in which they assume a spherical form and multiply by schizogony. 

 After several generations of so-called exoerythrocytic schizogony, the 

 merozoites enter the circulatory system and invade the red blood 

 corpuscles in which their subsequent development takes place. Once 



