TRICHINELLA SPIRALIS 427 



in the vicinity of the points of insertion of the tendons, this pro- 

 clivity being probably connected with the fact that the Trichinellae 

 first of all wander into the muscular fibres and find a natural barrier 

 at the points of insertion of the tendons. 



The Trichinellae, in their encysted condition, may remain alive 

 and capable of development for many years in the pig eleven years 

 and in man as much as twenty-five to thirty-one years. Encystment, 

 however, is not a necessary condition for the development of the 

 brood, that is to say, Trichinellae which reach the gut of suitable 

 animals become sexually mature and multiply provided that they have 

 developed so far as to possess a rudimentary genital spot, which 

 occurs when the body is 0*5 to 0-75 mm. long, but all the same 

 a great part of non-encapsuled Trichinae perish on their passage 

 through the stomach. 



The black rat (Mus rattus), and more particularly the sewer rat 

 {Mus decumaiius 1 ), are the normal hosts of Trichinella spiral is. These 

 animals, especially the last-named species, infect themselves very 

 easily, as they are cannibalistic, and they also transmit trichinosis 

 to other species by which they are devoured, such as pigs, dogs, 

 cats, foxes, bears and martens. Rats are infected also by the in- 

 gestion of faecal matter from infected animals which contains 

 trichinae (Hoyberg). Man becomes infected with Trichinella by 

 eating the flesh, insufficiently cooked, of infected pigs, also, but 

 more rarely, by eating the infected flesh of wild boars, dogs, cats, 

 bears and foxes. 



The infection of pigs may likewise take place by their having access to the offal 

 of tnchinous pigs, or being actually fed on it. These are, however, exceptions, 

 which, as a matter of course, are of great importance in certain places. As 

 a matter of fact, the rats examined for Trichinella were always found to be severely 

 infected. Thus Billings, in the knackers' yard at Boston, found that 76 per cent, 

 of the rats were infected, and in an export slaughterhouse 100 per cent, were found 

 to harbour the parasite ; in the city of Boston 10 per cent, of the rats had trichinosis. 

 Heller found that of 704 rats, from twenty-nine different places in Saxony, Bavaria, 

 Wiirtemberg and Austria, 8-3 per cent, were infected with Trichinelke ; of the rats 

 caught in the knackers'" yards, 22' I were diseased ; of those taken in slaughter- 

 houses, 2-3 were infected, and of rats from other localities only 0*3 per cent, 

 harboured the parasite. Leisering found almost the same figures, but in rats from 

 slaughterhouses 5*3 per cent were infected. 



The geographical distribution of T. spiralis does not correspond 

 with the occurrence of trichinosis in man ; local customs are 



1 It is still a matter of dispute and can hardly be definitely settled whether Trichinellse were 

 brought to Europe by the sewer rats which invaded Europe at the end of the eighteenth 

 century, or whether they were imported with the Chinese pig in 1820 or 1830, when it was 

 introduced into England and Germany to cross with the native breeds, or whether finally 

 Trichinellae are also indigenous to Europe. 



