38o 



NA TURE 



[March 23, 1922 



in one geographical area with West Africa also occurs 

 in the distribution of Necator americanus, but is due, 

 not to an old Atlantic connection between the two 

 continents, but to the spread of African parasites to 

 America by the slave trade. In Egypt more than 

 one-half of the population are affected by Bilharzia. 

 Clonorchis and Paragonimus are limited to the Far 

 East— Indo-China, the Philippines, China, and Japan. 



The life-histories of all the flukes are similar. The 

 eggs are passed out with the faeces, and if they reach 

 water, the embryos which they contain emerge and 

 swim about actively in search of some particular small 

 mollusc (the intermediate host) into which they must 

 penetrate in order to undergo their first metamorphosis. 

 The intermediate hosts are different in variouscountries, 

 and for- the several worms concerned; for Bilharzia, 

 in the Far East, it is the small water snail, Katayama 

 nosophora. This fact was first established by Miyairi 

 and Suzuki, and was confirmed by Leiper and Atkinson ; 

 later Leiper and his colleagues identified the inter- 

 mediates in Egypt as Bullinus contortus, B. Dybowski, 

 and Planorbis Boissyi. From the snail Bilharzia 

 escapes as a more advanced free-swimming larva, which 

 can bore through the skin of man should he venture 

 into infected waters, and, once within the body, it 

 migrates through the tissues to the veins of the liver 

 and abdomen. 



For Clonorchis and Paragonimus the first inter- 

 mediate host is also a small snail, Melania libertina. 

 They do not transfer themselves directly and actively 

 from this animal to man, but to a second intermediate — 

 'Clonorchis to various species of carp, and Paragonimus 

 to freshwater crabs. In these animals, the second 

 intermediate hosts, they riemain passive until they are 

 swallowed by man in food. 



The filarias are long, threadlike worms which live 

 in the connective tissues of various regions of the body. 

 They are associated with the disease known as elephan- 

 tiasis. Geographically they are spread throughout 

 the whole of the tropics. The larvae circulate in the 

 blood in enormous numbers, and are taken up by blood- 

 sucking insects, in which they grow in size ; after the 

 lapse of several days they wander into the proboscis, 

 from which they are injected into the skin of man when 

 the infected insect again feeds. The pioneer work on 

 this subject was done by Manson, and later extended by 

 Low, James, and Leiper. The most important species 

 are Filaria Bancrofti, the larvae of which are carried by 

 various mosquitoes (Culex, Anopheles, and Stegomya), 

 and Loa loa, carried by the mangrove fly Chrysops. 



The guinea -worm, Dracunculus medinensis, is 

 common in India, Turkestan, Persia, Arabia, and 

 tropical Africa. It lives under the skin, and when 

 mature gives rise to a small ulcer, generally on the 

 leg or foot, from which one end of the worm projects. 

 A stream of larvas is discharged through this ulcer into 

 water when the patient bathes. The next stage of its 

 life is passed in a water flea, Cyclops, and it is by 

 drinking water containing these minute Crustacea that 

 man is infected. 



Apart from local disease such as abscesses, elephan- 

 toid swellings, haematuria, etc., the more important 

 helminths produce generalised disease of a very im- 

 portant nature which is surprisingly uniform, whatever 

 be the causal animal. The primary symptom is always 



NO. 2734, VOL. T09] 



anaemia, and the secondary symptoms are such as 

 accompany this condition, namely, general weakness, 

 inability for work or any exertion, disturbance of the 

 heart and circulation, and finally dropsy and death. 

 In mild cases, which fortunately are the most common, 

 the anaemia is not great, and the patient is merely 

 reduced to a lower level of activity, happiness, and 

 efficiency. But when we consider the enormous 

 prevalence of these pests, we can realise the extent 

 of the harm inflicted on mankind by them. It must 

 also be remembered that in most tropical countries the 

 people live only just above starvation level, and that 

 any additional burden will quickly depress them below 

 it. The means by which this anaemia is produced is not 

 clearly understood, but recent work points to the 

 formation of poisonous secretions, toxins, by the worms, 

 which damage both the blood and the blood-forming 

 organs. Bedson has shown that the injection of worm 

 extracts produces acute inflammation of the thyroid, 

 suprarenal capsules, and spleen. 



Our armament for offence and defence against these 

 enemies is at present incomplete, but it is becoming 

 more effective. Offensive measures consist in attack- 

 ing the parasites directly in the bodies of their human 

 hosts, and the main advances have been in the use of 

 oil of Chenopodium against the intestinal worms, 

 Ascaris and Ancylostoma, and the intravenous injection 

 of tartrate of antimony against Bilharzia. The former 

 drug can be used on an enormous scale with great 

 safety and efficiency, and if the inhabitants of a badly 

 infected country can be educated to the point of under- 

 going treatment en masse once a year, a great reduction 

 of disease should result. The second discovery, which 

 we owe to Christopherson, has, at least in theory, 

 entirely changed the future of whole nations . A disease 

 which was previously incurable, and in Egypt, for 

 instance, affects one-half of the people, can now be 

 cured with certainty in a few months. In deahng 

 with ignorant and suspicious native races, however, 

 the rapid adoption of such strange and terrifying 

 methods cannot be expected. 



For defensive measures rehance is placed on im- 

 provements in sanitation and in the personal cleanli- 

 ness of the people, advances which will necessarily 

 be slow. No practical means of destroying eggs or 

 larvae on a large scale in the outer world have yet been 

 discovered. Where an intermediate host exists a 

 reduction of the disease would follow wholesale destruc- 

 tion of, or protection against, the intermediate. In this 

 connection, mosquito destruction has of course already 

 been carried out on a large scale in anti-malaria work in 

 many regions, and it may be extended with the ad- 

 ditional object of fighting worm disease. For the 

 destruction of the snails associated with fluke disease 

 periodical drying of canals and irrigated fields has been 

 advocated by Leiper. The knowledge that Clonorchis 

 and Paragonimus are introduced in food should also 

 make the avoidance of these parasites easy. 



Three things are above all necessary for the con- 

 quest of these plagues : (i) Continued and intensified 

 research into the many points of the intricate life- 

 histories of these parasites and their intermediate hosts 

 which are still obscure ; into new methods of destruc- 

 tion, chemical and physical, of both these groups of 

 animals, whether as eggs, larvae, or adults ; and into 



