166 THE EFFECTS OF PARASITES ON THEIR HOSTS. 



be annulled by greater negligence in the preparation of the food. This 

 is especially true of trichinosis, which is hardly less frequent in the 

 villages and among the lower classes than it is in towns and among 

 the well-to-do, although the number of tape-worms, especially when 

 compared with that of thread-worms, is considerably less in the former 

 than in the latter. 



It is impossible, without special knowledge of the various life- 

 histories, to bring the local conditions of parasites into relation with 

 the customs and manner of life of the inhabitants. We must, there- 

 fore, leave many of these problems as yet undetermined, e.g., where 

 the 63 per cent, (according to Bilharz and Meckel) of Fellahs and 

 Copts derive the Distomum hcBmatohium from which they suffer, or 

 why Filaria sanguinis is so frequent in tropical countries. 



On the other hand, we can at once understand the similar distri- 

 bution of Dochmius duodenalis when we remember that this worm 

 passes its youth freely in water, and that stagnant or slowly flowing 

 water is oftener used for drinking in tropical than in colder zones. 

 Similarly we are probably justified in referring the frequency of 

 Uchinococcus among the Icelanders, and other pastoral peoples, to their 

 close and constant contact with numerous dogs/ and to the generally 

 associated want of cleanliness. This disease seems to have been in 

 former centuries much more frequent in Germany than it is now, 

 when custom demands that dogs be kept at a greater distance, and 

 when the dog-tax has also very considerably lessened the number of 

 those animals.^ 



We have spoken as yet only of the local occurrence of helmin- 

 thiasis, without special regard to its geographical distribution. The 

 latter is, in many respects, independent of custom and mode of hfe. 

 It is determined, on the whole, less by man than by the distribution 

 of the intermediate hosts, and by the temperature of the region. 



The importance of warmth as a factor in the distribution of worms 

 may be inferred from what we have already seen (p. 73), that a 

 certain temperature is requisite for the development of the embryo. 

 When this temperature is not attained, or does not last long enough 

 to be efficient, the worm cannot continue to exist. Thus, Ascaris lum- 

 hricoides is wanting in those tracts where the temperature does not 



* On this point consult Krabbe (" Eecherohes helminthol," p. 60), who also mentions 

 that in Iceland there is one dog for every eleven inhabitants, while in Germany there is 

 only one for every fifty, and that besides Tcenia echinococcus is much commoner in Icelandic 

 than in German dogs (see p. 152, note). 



' When (1883) the dogtax in the Grand Duchy of Baden was lowered from 3 florins to 

 l-lj fl. yearly, the number of dogs increased to one for every twenty-eight inhabitants, 

 while before, with at a tax of 3 fl., and now with a 4 fl., there was only one to every forty- 

 nine. 



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