168 THE EFFECTS OF PARASITES ON THEIR HOSTS. 



hood. 1 Just as for free-living animals, so for parasites, there are 

 within the area of distribution certain more or less restricted haunts, 

 in which the creatures occur either exclusively or at least much 

 more frequently. Where any intestinal worm has once established 

 itself in virtue of a fortunate combination of favourable conditions, 

 there it remains for long, indeed till the conditions change, for the 

 possibilities of infection increase in proportion to the number of the 

 parasites. 2 In this way there arise what have been called " foci 

 of infection," especially in our time in the case of Trichina, but also 

 of tape-worms and other parasites. By such " foci," helminth iasis 

 may be spread over an ever-increasing area. Thus Filaria Medinensis 

 has been carried by slaves from the west coast of Africa to Tropical 

 America, and has thus obtained a wide distribution. Since the 

 intermediate host of this parasite is one of our commonest animals, 

 the worm is therefore in no way restricted to Asia or warmer climates, 

 and its acclimatisation in our own country is by no means an im- 

 possibility. 



When opportunities of infection multiply in any way, then there 

 arise, even among men, decided worm-epidemics. Knox tells of a 

 formidable tape- worm epidemic which broke out in October 1819 

 among the English soldiers engaged in the Kaffir war, after they had 

 been feeding for a lengthy period on the flesh of overdriven and un- 

 healthy oxen. 8 Similarly, in the year 1820, a fourth part of the 

 Egyptian army serving under Mohamed Bey in Kordofan suddenly 

 fell victims to Guinea- worms after they had remained healthy for two 

 years. 4 And according to Bartholin and Kiichenmeister the "fiery 

 serpents " of the Old Testament were most probably Guinea-worms. 



The epidemics of Trichina which occur every year in North 

 Germany are so well known that it seems almost unnecessary to 

 devote special attention to them. An epidemic appearance of Ascaris 

 has also been described in former times. But what they then 

 called worm- epidemics were mostly dysenteric troubles, in the course 



1 [For an instance of this in the case of Distomum, see Thomas, Quart. Journ. Micr. 

 Sci. y N. S., vol. xxiii, p. 99, 1883. W. E. H.] 



2 In the Punjaub, where Tcenia saginata is in certain sections of the population 

 almost as frequent as in Abyssinia, in 1869, according to official reports, not fewer 

 than 5 '55 p.c. (in 1868 even 6 '12 p.c.) of the oxen slaughtered at the military stations 

 were infected, and badly so, with cystic worms. (See Lancet, p. 860, Dec. 1872.) The 

 natural occurrence of Cysticercus in cattle is exceedingly rare with us. Similarly T. 

 marginata, T. coenurus, and T, echinococcus in dogs, are in Iceland 4, 18, and 47 times 

 commoner than in Denmark (Krabbe), and the same proportion holds for the related cystic 

 stages. 



3 Froriep's Notizen, Bd. i., p. 122, 1822. Friedberger observed a disease due to tape- 

 worms among pheasants, Zeitschr. f. Veterinarwiss., p. 97, 1877. 



* Clot, " Aper?u sur le ver dragonneau," Marseilles, p. 30, 1830. 



