218 The Philippine Journal of Science 1918 



ceptional difficulties, both to the pathologist and to the proto- 

 zoblogist. 



A review of the situation brings out some points in parasi- 

 tology of great interest from both the theoretical and practical 

 viewpoints and raises some questions that promise to form the 

 basis for an interesting series of studies. Among other ques- 

 tions raised is that that bears on the effects produced upon a 

 parasite by the internal reactions of the host, as expressed 

 through the tissues and body fluids. In this connection I mean 

 reactions of the host that tend to favor the parasite as dis- 

 tinguished from the familiar reactions that work to the disad- 

 vantage of the parasite. In the past the attention of parasito- 

 logists and physicians has been focused largely upon the effects 

 of the parasite upon its host — a most natural point of view. 

 But in reviewing the subject, especially in the light of recent 

 evidence, it is hard to escape the impression that the host, on 

 occasion, may transform an apparently harmless parasite into 

 one that is pathogenic or even lethal to its host. This is entirely 

 apart from conditions of lowered vitality and resistance, which 

 proverbially favor the development of infections of all kinds. 

 It would seem that the term "harmless commensal" has been very 

 much overworked. The case of the intestinal nematode Ascaris 

 lumbricoides is very much in point, and apparently the day 

 is not far distant when the terms "harmless commensal" and 

 "symbiont" will be used in the literature with much greater 

 caution than has been shown in the past. It has long been my 

 belief that time would show that all animal parasites that have 

 been regarded as commensals and symbionts in the alimentary 

 tract, if not actually giving rise to lesions, would yet be shown 

 to affect unfavorably the physiological balance of the host in 

 some way. 



Recently this has been strikingly indicated by Gibson, (17) 

 who has noted the apparent restraining influence of Ascaris 

 infection on the growth of puppies. The phenomena were, to 

 be sure, observed in the course of only one experiment, and 

 certain other factors may have intervened to bring about the 

 results he cites, but they certainly are suggestive. Gibson says : 



The existence of an anti-vitamine or at least of growth inhibiting sub- 

 stances formed by ascarids is suggested by an observation which I made in 

 connection with some milk feeding experiments with puppies. In a series 

 of five young puppies fed on cows' milk growth stopped in four of the 

 animals when 44 days old. Following the administration of an efficient 

 vermifuge, there resulted the passage of many ascarids from the four dogs 

 in which growth had ceased. Growth was immediately reestablished. 



