144 BEVIEW — TROPICAL MEDICINE, ETC. 



Parasites— Leiper,' in discussing the above, expressed his belief that the wanderings of immature 



continued hehniiiths through the tissues of their host to their final habitat of sexual maturity or 

 developmental arrest miglit well be the means of disseminating pathogenic agents within the 

 body. As regards (E.ti>fhai/oi:toiaiini hnimpti, he noted that an extensive diarrhusa and scouring 

 in many of the lower animals was produced by species of this group, and he thought that in 

 the Tropics the cysts and lesions of the epithelium associated with these worms might 

 be overlooked. 



A very suggestive paper is that by Ward- on the influence of parasitism on the host. 

 After pointing out that during recent years there has been a tendency to exaggerate the 

 unimportance of human parasites, he admits there are some of which it may fairly be said 

 that even careful study has failed to show a,ny manner in which they affect the host. 



Thus Looss (1894-3) records of .a distome (Hdcrnpluif.t) commonly found in the human aliment.iry canal 

 among Egyptian labourers, that, although present in considerable uumbers, most careful scrutiny fails to disclose 

 any influence which it e.^erts upon the host. This is traceable to the fact that it neither burrows into, nor feeds 

 upon, the raucous lining of the canal, but contents itself with taking its food from the partially digested i-ontents 

 of the intestine. Inasmuch as the organism is very small, this is evidently a negligible factor in the economy of 

 the host; but even here, as I shall show later, there" is the possibility that under some circumstances the organism 

 ra.ay become a menace. Again, Filaria Ion, the African eye-worm, lives for many years in the connective tissue of 

 the human body, wandering from point to point, often not far below the skin. In the course of its migrations 

 it does apparently no injury to the host, who is indeed unconscious of its presence until it happens to come into 

 the connective tissue over the surface of the eye-ball. Here it appeals to the sense of sight, and from here it has 

 most frequently been extracted. But in this "case, again, there are swellings which appear from time to time on 

 the surface of the body, and which are believed by some to bo due in one way or other to this parasite. {See 

 "Calabar SwelUnga," page 25). 



Resting forms, such as bladder worms and young trichinae, are also indifferent bodies, and 

 the guinea worm exercises no influence on its host until the female appears at the surface. 

 As factors determining the degree of influence exerted by the parasite on its host, he mentions 

 the following : — 



1. MnlfipIicafioH.—JJsuaMy the single parasite leaves no effect. The multiplication is 

 of course most serious when it takes place within the host and leads directly to a multiple 

 infection. This is the case with some Nematodes, but in most metazoan parasites, including 

 all Trematodes, Cestodes and some Nematodes this is not so, the eggs having to reach the 

 outer world, and possibly an intermediate host, in order that development may proceed. 

 Here the real danger lies in a multiple infection, through the increase in numbers which such 

 a species often experiences in the intermediate host, or within a limited area in the outer 

 world, so that by the taking in of a single external object a large number of parasites may 

 be introduced. 



2. Size. — In a general way the effects of a parasite are related to its mass as compared 

 to that of the host, but from a special point of view this is absolutely untrue and the 

 secondary effects of an individual species may be out of all proportion to its size. 



3. Site. — This requires no comments. Contrast muscle fibre as a site with the brain 

 or the eye. 



The effects of a parasite on the host, Ward classes as (a) mechanical, (6) morphological, 

 and (c) physiological. These may, of course, overlap. 



(a) Merhanircd. — Examples — Occlusion of a canal, say by a mass of round worms 

 rolled into a ball. Arterial obstruction due to the young sclerostomes in the horse. The 

 severe effects produced by Sclddosomum hsematohiuiu on the capillaries. 



Pressure effects, especially when the parasite is in a condition of active growth. There 

 is also the mechanical effect produced by the movement of parasites, which may irritate and 

 inflame the tissues. Migrating parasites have been known to cause death. 



Again, the abrasion and destruction of surfaces and cells and the opening of abnormal 

 communications may result in those secondary infections of a bacterial nature already 

 mentioned. For example, Ascaris may penetrate the intestinal wall, and serious or fatal 

 peritonitis result. Ward deals with the researches of Blanchard and Guiart, saying :— 



Evidently in producing ulcerations of the intestinal ranccsa, parasites facilitate the absorption of toxins from 

 the canal and permit the inoculation of this layer with pathogenic bacteria from the intestinal contents 

 (Blanchard, 1904). They can thus be the agents of inoculation for numerous diseases. Quiart (1905), who 

 defends this view most strongly, believes that intestinal parasites play an important role in the etiology of diseases 



' Leiper, R. (February 1st, 1908), Report of Meeting of the Society of Tropical Medicine and Hygiene. 

 Journal of Tropical Medicine and Hiigiene, Vol. XI. 



« Ward, H. B. (July 1st, 1907), "The Influence of Parasitism on the Host." Proceedings of American 

 Association for the Advanccinent of Science, Vol. LVI. 



