These symptoms are only appreciable in cases of “massive” infestation. If slight, the disturbance passes unperceived. In severe cases the symptoms consist of diarrhœa, loss of appetite, grinding of the teeth, abdominal pain in the form of dull colic, and sometimes irritation of the peritoneum. The embryos carried by the circulation then escape into the tissues and, like the cysticerci, become encysted, preferably in the muscles, in the interfascicular connective tissue towards the ends of the bundles. Each (asexual) parasite plays the part of a foreign body, causing infiltration of serum and exudation of leucocytes in its neighbourhood, and soon becoming encysted in the interior of a little ovoid space surrounded by a fibro-fatty wall. Fat granules accumulate at each end of the cyst.
Fig. 47.—Free larval trichina. (Colin.)
Fig. 48.—Trichinæ encysted in the muscular tissue. (Colin.)
Fig. 49.—Old (degenerated) trichina cyst. (Colin.)
The parasite, which at first appeared straight, soon assumes a bent form, then that of a figure “6,” then of a figure “3,” and preserves a latent vitality throughout the entire period of encystment. These cysts are of very small dimensions, invisible to the naked eye, and their discovery necessitates the use of the microscope. They are about ¹⁄₆₄th inch in length and ¹⁄₁₂₀th inch in width. Very frequently two or three cysts may be found arranged in line, presenting the appearance of beads on a string: more rarely two parasites may be found in one cyst; exceptionally, as many as six or seven. The appearance of “beads on a string” is due to the fact that the parasites follow the interfascicular capillaries.
In animals which are kept for a long time and fattened the cyst walls undergo fatty infiltration. The change is commonest in pigs. In the same way calcareous infiltration sometimes occurs, but only when the parasites have lost their vitality. This calcareous degeneration consists in the deposit of carbonate and phosphate of lime in the walls of the cyst; it never begins before the seventh or eighth month after infestation, and is sometimes much longer delayed.
No man or animal ever becomes infested except by the ingestion of meat or drink containing larval trichinæ. The pig and small rodents are most frequently attacked. Man contracts trichinosis by eating insufficiently cooked infected pork. The fact that small rodents, particularly rats, eat the bodies of their kind explains the persistence of trichinosis in certain regions. Pigs roaming at large, and thus liable to find and eat the dead bodies of such rodents, may contract trichinosis in this way or from eating ordure.