CHAPTER XVII 

 PILARIS AND THEIR ALLIES 



General Account. — One of the most interesting and puzzling 

 groups of human parasites are the members of the nematode 

 genus Filaria. They are extremely common parasites in all 

 tropical countries, have a unique and extraordinary life history, 

 are associated with many serious pathological conditions and 

 have figured prominently in the history of medical science. 



Sir Patrick Manson first discovered these worms swarming in 

 human blood, while working on tropical diseases in India. They 

 had previously been observed in various bodily excretions but 

 only in rare cases and in small numbers. Manson found them 

 in enormous numbers in the blood, but only at night. The 

 worms were evidently larvae and since they only rarely and ap- 

 parently accidentally escaped from the body with excretions, 

 the thought occurred to Manson that they must be liberated 

 from the blood by some nocturnal blood-sucking insect. Man- 

 son and others later proved this theory to be correct, and thus 

 took the first step toward our present knowledge of the biologi- 

 cal transmission of disease by insects, a step which marked the 

 beginning of a new era in modern medicine. 



Many species of Filaria from human blood have been described, 

 some of which undoubtedly are not valid species. Some species 

 apparently produce no pathological conditions whatever, while 

 others are associated with, and are usually considered to be the 

 direct cause of, a large number of diseased conditions. Some of 

 the species are of limited geographic distribution while others 

 are of world-wide range, probably due to differences in the ex- 

 tent of the distribution of the intermediate host. In some 

 tropical localities 50 per cent or more of the population are para- 

 sitized by these animals. In South China ten per cent of the entire 

 population is said to be infected and in some South Sea Islands 

 over half of the inhabitants are infected. Recently in an exam- 

 ination of 949 natives from the Congo-Cameron country of 



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