THE INFLUENCE OF PARASITES 9 



endanger the life of the latter, as simultaneously its own existence 

 would be threatened. The parasite, of course, robs its host, but 

 usually. in a scanty and sparing manner, and the injuries it inflicts 

 can hardly be taken into account. There are, however, numerous 

 cases in which 1 the situation of the parasites or the nature of their 

 food, added to their number and movements, may cause more or 

 less injury, and even threaten the life of the host. It stands to 

 reason that a Cysticercus cellulosce situated in the skin is of but slight 

 importance, whereas one that has penetrated the eye or the brain 

 must give rise to serious disorders. A cuticular or intestinal parasite 

 is, as a rule, less harmful than a blood parasite. A helminth, such 

 as an Ascaris lumbricoides or a tapeworm, that feeds on the residues 

 of foodstuffs within the intestine, will hardly affect its host by 

 depriving it of this material. The case is different when the parasites 

 are very numerous, especially when the heavily infested host happens 

 to be a young individual needing all it ingests for its own require- 

 ments, and therefore unable to sustain the drain of numerous guests 

 in the intestine. Disturbances also set in more rapidly when the 

 intestinal helminthes are blood-suckers, such as Anchylostoma duo- 

 denale, the injury to the host resulting from the kind of food taken 

 by the parasite. 



Generally, the disorders caused by loss of chyle are insignificant 

 when compared with those induced by the GROWTH and agglomera- 

 tion of the helminthes ; the latter may cause chiefly obstructions of 

 small vessels or symptoms of pressure in affected or contiguous 

 organs, with all those complications which may arise secondarily, 

 or they may even lead to the complete obliteration of the organ 

 attacked. Of course the symptoms will vary according to the nature 

 of the organ attacked. 



In consequence, also, of the MOVEMENTS of the parasites, disorders 

 are set up that may tend to serious pathological changes of the 

 affected organs. The collective migrations, undertaken chiefly by the 

 embryos of certain parasites (trichinosis, acute cestode tuberculosis), / 

 are still more harmful, as are also the unusual migrations of other 

 parasites, which, incidentally, may lead to the formation of so-called 

 worm abscesses or to abnormal communications (fistulae) between 

 organs that are contiguous but possess no direct connection. 



Recently, several authors have called attention to the fact that 

 the helminthes produce MATERIALS that are TOXIC to their host ; and 



1 Liihe, M., " Ueber d. Fix. d. Helm. a. d. Darmivand ihrer Wirthe u. die dadurch 

 verursachten path.-anat. Ver Under itngen d. Wirthsdarmes," Trans, of the IV. Intern. 

 Zool. Congr., Berlin, 1901. 



