204 



SCIENCE 



[N. S. Vol. XXV. No. 632 



that the number of parasites is kept down. 

 Yet under a favorable combination of cir- 

 cumstances the numbers of a given parasite 

 may be enormously increased and with this 

 the dangers from the species also enhanced. 



Naturally, immediately related to the 

 factor of numbers is the question of com- 

 parative size. In a general way the effects 

 of a parasite are related to its mass as com- 

 pared with that of the host, and this will 

 be clearly manifest in the subsequent dis- 

 cussion. From a special point of view, 

 however, this is absolutely untrue and the 

 secondary effects of an individual species 

 may be out of all proportion to its size. 

 This will also receive more detailed discus- 

 sion at a later point. 



Evidently, both of the factors which 

 have just been mentioned are largely rela- 

 tive and distinctly such with regard to the 

 seat of the injury, i. e., the organ of the 

 body which is attacked; thus a parasite 

 may be quite harmless if located in con- 

 nective tissue, or between the muscle fibers, 

 while if the same specimen were to be lo- 

 cated in the brain or in the eye its effect 

 would be very serious. A small organism 

 would pass entirely unnoticed in the ali- 

 mentary canal, but in the heart or blood 

 vessel, it might well cause serious disturb- 

 ances, or under certain conditions even the 

 death of the host, so that the pathogenic 

 significance of a parasite depends essen- 

 tially upon its location. 



The effects of a parasite on the host may 

 be roughly classified as mechanical, mor- 

 phological and physiological. To be sure, 

 a sharp separation is not possible at all 

 points and frequently the one influence in- 

 volves concurrently or subsequently such 

 as belong to another group. 



Animal parasites, for the most part, pro- 

 duce only local changes and these are ex- 

 plicable on purely mechanical grounds, or 

 they are structural or functional within 

 limited areas. But physiological influences 



sometimes manifest themselves as symp- 

 toms of a general disease. Such mani- 

 festations ordinarily accompany the multi- 

 plication of the parasite within the host or 

 at least its presence there in large numbers. 



Many of the effects upon the host pro- 

 duced by parasites may be explained in 

 purely mechanical fashion, and under this 

 heading are included those which produce 

 even some of the more complicated and 

 far-reaching results of parasitic infection 

 through the secondary effects which they 

 induce. 



Eegarding the parasite as a passive mass, 

 a purely inert body, it may bring about 

 a stoppage of various passageways and 

 thus induce serious consequences for the 

 host. Such an occlusion of a canal may 

 follow under perfectly normal conditions, 

 but more frequently there is something un- 

 usual that tends to induce the condition; 

 either that the parasite is in an abnormal 

 location or that some factor enters into its 

 normal habitat to aid in purely mechanical 

 fashion in the stoppage of the passageway. 

 Thus, the accumulation of a group of com- 

 mon round worms into a ball will suffice at 

 times to occlude entirely the alimentary 

 canal, and the worms, tangled and twisted 

 together, stop the passage of food materials 

 and waste. Unless the condition is accu- 

 rately diagnosed and promptly alleviated 

 by operative means, this stoppage of the 

 canal results in the death of the host. 

 Such has been the result in man in several 

 cases on record and similar instances are 

 more frequent among other hosts. But in 

 one of the more carefully investigated cases 

 it was found that the mass of seventy-two 

 ascarids were inextricably tangled together 

 by a long hair swallowed by accident and 

 in some way twisted tighter and tighter 

 through the contortions of the parasites. 



The young selerostomes of the horse 

 live in the large arteries of the abdomen. 

 Their presence in these arteries explains 



