September 22, 1923] 



NA TURE 



447 



there is need for critical determination of the species, 

 for, in addition to E. histolytica, a closely similar species, 

 E. coli, is a common inhabitant of the intestine. This, 

 however, is a harmless commensal, feeding on bacteria 

 and fragments derived from the host's food. The 

 distinction between the two species rests chiefly upon 

 the characters of the nuclei and of the mature cyst — 

 quadrinucleate in E. histolytica and octonucleate in 

 E. coli — and considerable care and technical skill are 

 requisite in many cases before a diagnosis can be given. 

 Yet this distinction is definitely necessary in practice, 

 for indiscriminate treatment of persons with Entamoeba 

 is indefensible ; treatment is only for those w-ith E. 

 histolytica ; it is useless for those with coli, and subjects 

 them needlessly to an unpleasant experience. 



A notable result of recent work is the proof that the 

 more common intestinal Protozoa, formerly believed 

 to be restricted to warmer countries, occur indigenously 

 in Britain. This was first established by a group of 

 observers in Liverpool, and has been confirmed and 

 extended by subsequent workers. There is good 

 reason for believing that in Great Britain the incidence 

 of infection with E. histolytica is about 7 to 10 per cent., 

 and with E. coli about five times as great (Dobell). 



The discovery (1903) of Leishmania, the organism 

 of kala azar and of oriental sore, added another to 

 the list of important human pathogenic Protozoa, but 

 the mode of transmission of this flagellate has not yet 

 been proved. 



Of the problems presented by the parasitic worms, 

 the most momentous are those associated with Ancylo- 

 stoma and its near relative Necator, which are pre- 

 valent in countries lying between 36° N. and 30° S. 

 — 0. zone which contains more than half the population 

 of the earth. Heavy infection with Ancylostoma or 

 with Necator produces severe anaemia, and reduces the 

 host's physical and mental efficiency to a serious degree. 

 Until 1898 there was no suggestion that infection was 

 acquired in any other way than by the mouth, but in 

 that year Looss published his first communication on 

 the entry of the larvae of Ancylostoma through the skin, 

 and in 1903 gave an account of further experiments 

 which proved that dermal infection resulted in 'the 

 presence of worms in the intestine. At the meeting 

 of the British Association in Cambridge in 1904 Looss 

 demonstrated to a small company his microscopical 

 preparations showing the path of migration of the larvae. 

 His investigations served to establish the importance of 

 the skin as the chief portal of entry of Ancylostoma, 

 and pointed the way to effective methods of prevention 

 against infection. 



Another notable advance in helminthology is the 

 working out of the life-cycle of Schistosoma (Bilharzia) 

 — a genus of trematode worms causing much suffering 

 in Egypt and elsewhere in Africa, as well as in Japan 

 and other parts of the world. These worms when 

 mature live in pairs, a male and female, in the veins 

 of the lower part of the abdomen, especially in the wall 

 of the bladder and of the rectum. The eggs, laid in 

 large numbers by the female worm, provoke inflamma- 

 tory changes, and cause rupture of the veins of the 

 organs invaded. Until about ten years ago the life- 

 history of Schistosoma had been traced only as far as 

 the hatching of the ciliated larva or miracidium, which 

 takesjplace shortly after the egg reaches water, Ijut it 



NO. 2812, VOL. 112] 



was then shown that this larva is not, as had been 

 held by Looss, the stage which infects man. Miyairi 

 and Suzuki (1913) found that the miracidium of 

 Schistosoma japonicum entered a fresh-water snail 

 which acted as the intermediate host, and Leiper and 

 Atkinson (1915) confirmed and extended this observa- 

 tion, and showed that the miracidia develop into 

 sporocysts in which cercariae are formed. We owe 

 chiefly to Leiper's work (1915-16) our knowledge of 

 the life-history and method of entry into man of the 

 Egyptian species of Schistosoma. He demonstrated 

 that two species of this parasite occur in Egypt, and 

 established that the miracidia develop in different 

 intermediate hosts : those of S. mansoni enter Plan- 

 orbis, while those of S. haematobium penetrate into 

 Bullinus — the molluscs being abundant in the irrigation 

 canals. The sporocysts produce cercariae, which escape 

 from the snails and gather near the surface of the water, 

 and experiments with young mice and rats showed 

 that the cercariae attach themselves to the skin, enter, 

 and reach the portal system, from w^hich they travel 

 to the veins of the lower part of the abdomen. In- 

 fection of mafi takes place chiefly through the skin when 

 bathing or washing in water containing the cercariae, 

 though infection may also occur through drinking such 

 water. So, at last, these worms which have troubled 

 Egypt for at least thirty centuries have become known 

 in all their stages, and measures for preventing in- 

 fection — which were of great use during the War — 

 have been devised, and curative treatment intro- 

 duced. 



Other recent helminthological researches deserve 

 consideration did space permit, for there has been 

 much excellent work on the life-history of the liver- 

 flukes and lung-flukes of man, and the life-cycle of the 

 tape-worm, Dibothriocephaliis latus, was worked out 

 in 1916-17. Mention should also be made of Stewart's 

 investigations (1916-19) on the life-history of the large 

 round-worm Ascaris lumbricoides , during which he 

 made the important discovery that the larAas on 

 hatching in the intestine penetrate into the wall and 

 are carried in the blood to the liver, and thence through 

 the heart to the lungs, where they escape from the 

 blood-vessels, causing injury to the lungs. The larvae, 

 now about ten times their original size, migrate by 

 way of the trachea and pharynx to the intestine, 

 where they grow to maturity. During last year Dr. 

 and Mrs. Connal have worked out the life-history of 

 Filaria (Loo) loa in two species of the Tabanid fly, 

 Chrysops, and investigations on other Filarias have 

 thrown light on their structure, but there is still need 

 for further researches on the conditions governing the 

 remarkable periodicity exhibited by the larvae of some 

 species (e.g. F. bancrojii ; in some parts of the world 

 the larvae of this species are, however, non-periodic). 

 The period under review has obviously been one of 

 great activity in research on helminthes, and fertile 

 in measures tending to reduce the risks of infection. 



Insects, protozoa, and helminthes not only inflict 

 direct injury on man ; they also diminish his material 

 welfare by impairing the health or causing the death 

 of his horses, cattle, and sheep, by destroying food 

 crops during growth, and, in the case of insects, by 

 devouring the harvested grain. The measure of 

 control which man can gain over insects, ticks, and 



