March 23, 1922] 



NA TURE 



379 



Parasitic Worms of Man and Methods of Suppressing Them. 



By Major F. H. Stewart, Indian Medical Service (Retired). 



ONE of the most interesting and important groups 

 of the animal kingdom is that of the parasitic 

 worms or helminths ; interesting from the point of 

 view of pure science on account of the intricate and 

 varied nature of their life-histories and biological 

 relationships, and important from the effects which 

 they produce on the health of man, domestic animals, 

 and cultivated and useful plants. In the present 

 article a summary is attempted of the results of modem 

 research on the helminths attacking man alone. 



The more important helminths attacking human 



beings can be grouped as follows : (i) the intestinal 



worms, such as the roundworm and the hookworms, 



'2) the trematodes or flukeworms, and (3) the filarias 



iid their allies, which live in the connective tissues. 



The roundworm (Ascaris lumbricoides) and the 

 hookworms (Ancylostoma duodenale and Necator 

 americanus) live in the small intestine of man. The 

 former is an animal of considerable size, from 20 to 

 35 mm. in length, while the latter two are smaller, 

 ID to 13 mm. long. The sexes are separate in all of 

 them, and the females pour out a stream of eggs which 

 are passed out of the human body in the faeces. The 

 eggs ultimately find their way to the surface of the 

 soil, and if the conditions are favourable — i.e. if the 

 ground be moist and the temperature not less than that 

 of a European summer (for Ascaris) or of an Egyptian 

 summer (for Ancylostoma) — a small embryonic worm 

 appears within the tough shell. The hookworm 

 larvae now hatch and lead a free life in mud or in small 

 puddles or pools. Their attack on man is direct and 

 active, for, should the hands or unsho^ feet come into 

 contact with the mud or water which they inhabit, 

 the little needle-shaped larvae are roused to great 

 activity. They bore their way through the skin into 

 the subcutaneous tissue, and are carried in the lymph- 

 stream and blood through the heart to the lungs ; 

 from the lungs they swim up the air passages and down 

 the oesophagus, so reaching the small intestine. This 

 remarkable life-history was worked out by the Austrian 

 zoologist Looss in Cairo, and, although at first received 

 with some scepticism, it has now been fully confirmed. 



In the roundworm, on the other hand, the egg 

 must be swallowed before it will hatch, and this 

 accident (unfortunate from man's point of view) 

 takes place through the consumption of vegetables 

 rown on infected soil, on which eggs have been 

 .^plashed, or as the result of eating with unwashed 

 hands after working on contaminated land. When 

 the egg arrives thus passively in the small intestine 

 of man, it hatches, and a little larval worm emerges. 

 Until recently it was supposed that this larva remained 

 in the small intestine and simply grew to adult size 

 without further adventure, but the present writer has 

 been able to show that this is not so. The larva bores 

 into the wall of the bowel, enters a vein, and, passing 

 through the liver and heart in the bloodstream, reaches 

 the lungs ; from the lungs it migrates to the intestines 

 by a route similar to that adopted by the hookworm. 



In the hookworm one object of the migration is 

 obvious, since the larva is merely taking the most 

 sure and direct route to its goal — that is, from the 



NO. 2734, VOL. 109] 



first point at which it comes into contact with man, 

 be this the skin of hands or feet, or of any other 

 part of the body, to the small intestine. It is true 

 that from the finger-tips it might be carried to the 

 mouth and so reach the intestine directly, but only 

 a small percentage of those larvae which have succeeded 

 in finding man could count on this fortunate chance. 

 By skin penetration, on the other hand, a high per- 

 centage should succeed, and it must be remembered 

 that only a few of all the larvae which have hatched 

 ever succeed in finding man, while only a few of the 

 eggs reach such favourable surroundings as allow the 

 larvae to form or to hatch. 



There is, however, a second object for the migra- 

 tion of the hookworm, the one which is the only 

 motive in the case of Ascaris — namely, that the 

 young larva is not adapted to survive among the 

 strong digestive juices. The young Ascaris lacks not 

 only a stout cuticle, but also that power of chemical 

 defence by which the older parasite resists digestion 

 by its host. Both cuticle and constitutional resistance 

 are developed during the migration, while the larva 

 is being nursed by the blood and lymph, by the bland 

 and nourishing juices of its host. It seems that direct 

 invasion through the skin was the line of attack by 

 the primitive ancestral parasitic worms, and that the 

 present physiological necessity of the migration is due 

 to inheritance. 



In geographical distribution Ascaris lumbricoides 

 is cosmopolitan, occurring in all lands both temperate 

 and tropical. The hookworms are also very widely 

 distributed, being absent only from the colder parts 

 of the temperate zones. Even there they occur 

 sporadically in artificially warm situations, such as 

 mines and tunnels ; the well-known outbreaks of 

 " miners' anaemia," both in the mines of England 

 and the Continent, e.g., in the St. Gothard tunnel, 

 were due to this cause. The proportion of the popula- 

 tion affected, especially in the tropics, is extraordinarily 

 high, figures of from 40 to 98 per cent, having been 

 recorded in various countries from the examination 

 of large numbers of the populace. The degree of 

 infestation is highest in the Far East — in China, Indo- 

 China, the Dutch East Indies, and particularly in the 

 tropical Pacific islands. The West Indies and tropical 

 South America also return high percentages, while the 

 southern States of the American Union yield figures 

 which prove that it is not only among dark-skinned 

 races that the parasites become very, numerous. 

 Even in E^urope 20 per cent of the adult population 

 of Italy and one-half of the children of Central Europe 

 carry the roundworm. 



The flukes are flattened oval worms which live 

 in the veins of the abdomen (Bilharzia), in the bile- 

 ducts and gall-bladder (Clonorchis), and in the tissues 

 of the lungs (Paragonimus). Bilharzia occurs over large 

 areas of the tropics and sub-tropics. Three species 

 are known from man, one of which occurs in Mesopo- 

 tamia, Egypt, and East Africa, the second in Central 

 and South America, the West Indies, and West Africa, 

 and the third in Japan, China, and the Philippines. 

 The association of the West Indies and South America 



