Very small Trichinellinæ, the males of which have two conical appendages at the caudal extremity; the vulva is situated at the border of the anterior fifth of the body. There is only one species.
Trichinella spiralis, Owen, 1835.
Syn.: Trichina spiralis, Owen, 1835.
The male measures 1·4 to 1·6 mm. in length and 0·04 mm. in diameter. The anterior part of the body is narrowed, the orifice of the cloaca is terminal and lies between the two caudal appendages; internal to these are two pairs of papillæ, dorsal one behind the other. The cloaca is evertible for copulation. The females measure 3 to 4 mm. in length and 0·06 mm. in diameter; anus terminal.
Trichinella spiralis in its adult stage inhabits the small intestine of man, pig, wild boar, rat. The young do not leave the body of the host but become encysted in the muscles. Experimentally it develops in the black rat (Mus rattus), the sewer rat (M. decumanus), the domestic pig (Sus scrofa dom.), the wild boar (Sus scrofa ferox), the domestic dog (Canis familiaris), the fox (C. vulpes) the badger (Meles taxus), the polecat (Putorius fœtidus), the marten (Mustela foina), the raccoon (Procyon lotor), the hippopotamus and the cat, and many other mammals (rodents and carnivora); Trichinellæ have been artificially introduced, by administering the encysted stage, into the dog, the mole (Talpa europæa), the mouse (Mus musculus), the hare (Lepus timidus), the rabbit (L. cuniculus), the hedgehog (Erinaceus europæus), the marmot (Cricetus vulgaris), the vole, the dormouse, the sheep, the calf, the horse, etc. Human beings and the pig, rat, mouse, guinea-pig and rabbit are most easily infected; less easily the sheep, calf and horse; with difficulty the cat, dog and badger. Trichinella can also be reared in birds (fowl, pigeon and duck), but the young do not encyst in the muscular system, but are expelled with the fæces. By cold-blooded animals as well as by insects (Calliphora vomitaria), encysted Trichinellæ are evacuated without undergoing any change, but they will still develop if subsequently ingested, say, by rabbits. According to Gujon, however, Trichinella can develop in salamanders, because he has found Trichinella of the muscles in these animals after they had been fed on encysted specimens. A high temperature (30°C.) must be provided in which to keep the experimental animals to ensure the success of the infection.
Fig. 302.—Trichinella spiralis. ♀, mature female: E, embryos; V, vulva; Ov, ovary. ♂, mature male: T, testes. c., newly born larva. d., larva in the muscles. e., encapsuled larva in the muscles. Magnified. (After Claus.)
History.—Encapsuled Trichinellæ had been observed in London by Peacock (1828) and by J. Hilton (1833) in the muscular system of man; soon after (1835), Paget found them in London in an Italian who had died of tuberculosis, and recognized them to be encysted entozoa, which R. Owen described as Trichina spiralis. Soon after, some further observations were reported on the occurrence of encysted Trichinellæ, in man, in England, Berlin, Heidelberg, Denmark, North America; they were also found in the pig (Leidy, Philadelphia) and the cat (Herbst, Göttingen, and Gurlt, Berlin). Herbst even succeeded in infecting a badger with encysted Trichinellæ, and subsequently infected two dogs with the flesh of this badger (1850). In 1855 R. Leuckart (Giessen) also commenced feeding experiments, and, like Küchenmeister and Virchow (1859), first went on the wrong track because it was believed at that time that Trichinellæ were the larvæ either of Trichocephalus or Strongylus. Nevertheless, these experiments yielded some important results; they showed that Trichinellæ become adult in the intestine within a few days, and that the females are viviparous (Leuckart). Until that time Trichinellæ had been regarded as fairly harmless guests of man, but opinions soon changed when Zenker in Dresden (January, 1860), in performing the autopsy of a girl, aged 10, who had entered the hospital with typhoid symptoms and there died, found Trichinellæ (not yet encysted) in the muscles; the intestinal lesions characteristic of typhoid were lacking, but numerous adult Trichinellæ were found in the intestine. Inquiries elicited the fact that at about Christmas time the girl had been taken ill after eating pork, and at the same time the butcher from whom the meat was bought as well as several of his customers fell sick: the pickled pieces of the same meat were full of Trichinellæ. In the face of this information it was not difficult to ascertain the cause of the disease and the manner of infection in Zenker’s case, and it was not long before Leuckart, Virchow and Zenker were able by renewed experiments to demonstrate the cycle of development of Trichinella spiralis. Similar investigations followed by Claus in Würzburg, Davaine in Paris, Fuchs and Pagenstecher in Heidelberg, etc.
Hardly had Zenker’s case been published than numerous observations on trichinosis in man appeared, some referring to isolated cases, others to small or great epidemics, and nearly all from North Germany. The worst epidemic was that of Hadersleben (1865), in which place, numbering hardly 2,000 inhabitants, 337 persons were taken ill within a short time, and of these 101 died. The source of infection proved to be a single pig, the flesh of which had been mixed with that of three other pigs; 200 of the badly infected persons had exclusively eaten raw pork.