Fig. 305.—Various phases of the calcification of Trichinella of the muscles, which starts at the poles of the capsule.
According to experience, Trichinellæ are not evenly distributed in the muscular system of pigs; the diaphragm, the muscles of the larynx, tongue, abdomen and intercostal spaces are their favourite positions; this predilection for the respiratory muscles is explained by their regular contractions, owing to which regular narrowings of the capillaries take place, thus favouring the settling of the circulating Trichinellæ. The same circumstance probably explains the frequency of the parasites in the tongue.
Possibly also the Trichinellæ that bore direct through the intestine may, from the abdominal cavity, penetrate the muscles in the vicinity. Frequently also encysted Trichinellæ are found in remarkable numbers in the vicinity of the points of insertion of the tendons, this proclivity being probably connected with the fact that the Trichinellæ first of all wander into the muscular fibres and find a natural barrier at the points of insertion of the tendons.
The Trichinellæ, 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, Trichinellæ 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 Trichinæ perish on their passage through the stomach.
The black rat (Mus rattus), and more particularly the sewer rat (Mus decumanus[305]), are the normal hosts of Trichinella spiralis. 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 ingestion of fæcal matter from infected animals which contains trichinæ (Höyberg). 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 trichinous 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, Würtemberg and Austria, 8·3 per cent. were infected with Trichinellæ; of the rats caught in the knackers’ yards, 22·1 were diseased; of those taken in slaughterhouses, 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 an important factor; for instance, the custom of eating pork in a condition that does not affect the life of the enclosed trichinella. In places where such customs do not prevail, epidemics do not occur—at the most there are isolated cases of the disease, although there be a great number of infected pigs. The following conditions prevail in North America: In Boston, Billings found that 4 to 5·7 per cent. of the pigs examined were trichinous; Belfield and Atwood found that 8 per cent. were infected in Chicago; Salmon found on an average that 2·7 per cent. were infected (but at various places the percentage fluctuated between 0·28 to 16·3 per cent.), yet epidemics of trichinosis hardly ever occur in North America, and only isolated cases of the disease are met with in German immigrants, who keep to their native customs.
This report, according to the researches of H. U. Williams, must be considerably modified. This author has examined the muscular system of human cadavers according to the method employed by inspectors of meat for pigs. The investigations were conducted in the Pathological Institute of the University of Buffalo, and the observer has examined 505 bodies since 1894, of which 27 (= 5·34 per cent.) were invaded by Trichinella. The cases, according to the nationality, are divided as follows:—
Examined | Trichinella | Percentage | |||
Absent | Present | ||||
| Americans: | |||||
| (a) Whites | 207 | 201 | 6 | 2 | ·89 |
| (b) Negroes | 70 | 65 | 5 | 7 | ·14 |
| British and Irish | 62 | 57 | 5 | 8 | ·06 |
| Canadians | 12 | 10 | 2 | 16 | ·66 |
| Germans | 49 | 43 | 6 | 12 | ·24 |
| Italians | 12 | 10 | 2 | 16 | ·66 |
| Other nationalities | 27 | 27 | 0 | 0 | |
| Nationality unknown | 66 | 65 | 1 | 1 | ·51 |
Total | 505 | 478 | 27 | 5 | ·34 |
It is worthy of remark that half of all the positive cases were mental patients, who were found to be affected with Trichinella to well-nigh 12 per cent. Trichinosis was not, however, the cause of death in any case. Very frequently the Trichinellæ were found calcified and dead.