364 BIOLOGY OF THE PROTOZOA 



a general immunity. Advantage is said to be taken of this fact 

 by parents in countries bordering the Mediterranean who expose 

 children by inoculation of parasites of Oriental sore on arms or legs 

 and so prevent further infection with possible disfiguring scars on 

 more conspicuously exposed surfaces. Absolute immunity conferred 

 by a single infection of Tkeilcria parva, the cause of East Coast 

 fever of cattle, is another example; another case of relative immu- 

 nity is furnished by a single infection of rats with Trypanosoma 

 lewisi; further infections are harmless, although the parasites may 

 not be killed. In the majority of cases, however, the immunity 

 reactions have no permanent value. Here as with bacterial infec- 

 tions the blood may contain natural substances which are inimical 

 to specific parasites. Such individuals are said to be naturally 

 immune. In other individuals a gradual immunity is built up by 

 repeated infections— as in adult natives of a malarial country who 

 have been subject to repeated infections from childhood. Many 

 efforts also have been made to immunize by use of attenuated 

 strains but with dubious results. Some degree of success with 

 attenuated Trypanosoma brucei has been obtained (Ponselle, 1923) 

 and with Plasmodium praecox of bird malaria (Et. and Ed. Sergent, 

 1921). 



Passive immunity, of transient nature, is established in many 

 types of protozoan disease by inoculation of blood serum from 

 actively or normally immunized individuals. Such serums may 

 act as alexins to stimulate phagocytosis (e. g., Laveran and Mesnil, 

 1901, with Trypanosoma lewisi) or to bring about agglomerations 

 and agglutinations resulting in swelling and disintegration (Trypano- 

 somes and Leishmanias). 



Parasitic Flagellates. —The importance of the parasitic flagellates 

 of man centers mainly in the family Trypanosomidae. There is 

 strong evidence to show that these forms, originally, were parasites 

 of the digestive tract of invertebrates, mainly insects, which by 

 contaminative or inoculative methods transmitted their parasites 

 to vertebrates, especially to mammals, where they became adapted 

 to conditions in organ cells and in the blood. Reinfection of the 

 invertebrates follows from their blood-sucking habits and verte- 

 brate and invertebrate thus become mutual carriers of infection 

 which is often pathogenic to the former, but by mutual adaptation 

 apparently harmless to the latter. 



Invertebrate forms which are known to harbor intestinal flagel- 

 lates and some of which have been proved to be, or suspected of 

 being, transmitting agents of vertebrate parasitic flagellates are 

 insects, arachnids and leeches. Of these the insects are by far the 

 most important, Wenyon listing no less than 25-i species containing 

 intestinal flagellates, while arachnids are limited to 5 species and 

 leeches to 11. Excluding insects which do not feed on vertebrates, 



