This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Trichinosis is a parasitic disease produced by the embryos of a worm, the Trichina spiralis, which work their way into the voluntary muscles and there become embedded. Among the parasites liable to be eaten with raw meats this one is by far the worst and most fatal in its effects. The habitat of the adult worm is the small intestine. During the emigration of the embryos into the voluntary muscles a group of characteristic symptoms is produced. The parasites have the following appearance: The adult male measures 1.5 millimetre in length; the female is from two to two and a half times as long. The embryo is 0.6 to 1.0 millimetre 26 in length, and when at rest, encapsulated in the muscle, is coiled in a spiral. It has a blunt tail and a sharp head. The adult male has two small projections from the caudal end. The ovoid capsule first appears translucent, but later, from the deposition of salts of lime, it becomes opaque.
Infection in man is caused by the eating of ham and pork from hogs whose bodies contain the parasite. Among these animals it is of quite frequent occurrence. It is also spread through the agency of rats, which are eaten by hogs (Dock).
After eating flesh which contains the embryos the process of digestion dissolves their capsules, and they are liberated, passing into the small intestine, where, with the presence of the favourable conditions of warmth, moisture, and food, they reach their adult growth in from three to six days. Rapid reproduction takes place in the intestine, and the number of young produced by a single female worm varies up to at least a thousand. In a week or more after the ingestion of the parasites the newly developed embryos pass out through the intestinal wall and mesentery, seeking the muscles, which they enter; they then work their way through the connective tissue between the fasciculi or the separate muscle fibres, and finally enter the latter, where a fortnight's further development produces the complete muscle form.
The observations of Askanazy point to the conclusion that the adult female deposits the embryos within the walls of the intestinal villi, not setting them free in the intestine, as originally supposed. From the villi they penetrate the muscles, as above stated.
Thornbury reported at the Buffalo Academy of Medicine the results of analysis of 500 cases of infected swine. In these animals the parasite was found in the diaphragm 400 times, in the loin 290 times, and in the neck 170 times. " The point of predilection therefore appears to be the diaphragm. This is explained by its close proximity to the digestive tract, from which the trichinae primarily bore." In three cases in man which he examined the trichinae were found principally in the extremities, "one microscope slide from the biceps of an arm containing fifty of the parasites." They were also present in the diaphragm, intercostal and abdominal muscles.
The embryos, when lodged in the muscle fibres, act as foreign bodies or irritants and excite an interstitial myositis with the formation of a capsule around each embryo. One embryo may be lodged in a single capsule. Once within the muscle fibres, embryos remain without change until the capsule, the completion of which requires about six weeks, is in some way destroyed. After formation of the capsule a precipitation of lime salts occurs slowly within it for four or five months. The embryos, thus securely protected in a strong envelope, may retain their vitality for many years, and it is claimed that they have survived for at least two decades, but the worms themselves not infrequently become calcified. Other animals besides hogs may harbour this parasite, among which are the rat, cat, mouse, and fox, and it can be artificially inoculated.
The prevention of trichinosis should consist in the more careful feeding of swine upon grain instead of offal, and for man the only absolute prevention is the extremely thorough cooking of all swine flesh, for a temperature of 1400 F. is fatal to the embryos. Smoking and pickling is also preventive.
"In the usual curing solutions trichinae are killed within six weeks in thin pieces of meat, but in thick pieces they can exist as long as four months "(Dock).
The presence of the parasites in countless numbers in the flesh of the hog may give rise to no symptoms of any kind in the animal, and it is this fact which makes the eating of raw ham, pork, and sausages particularly dangerous unless the meat of the animals killed has been subjected to a searching microscopic examination. It is the safest rule never to eat such meats. The difficulty of detecting the parasite in the hog is considerably increased by the fact that the calcification is very much slower than it is in man, so that the worms are more readily overlooked. The parasites fortunately are completely killed by boiling for some time the meat which contains them, but pickling or corning meat or smoking ham and bacon are not necessarily fatal to them.
Cases of trichinosis are occasionally reported in this country chiefly among the Germans, whose fondness for raw ham and a variety of sausages is well known. Osier reports the finding of 456 cases, including 122 deaths recorded in America. Many persons are often simultaneously affected from eating the meat of the same animal, thus giving the disease the false character of an epidemic.
The symptoms vary with the number of parasites which have been eaten. If very few are ingested, the embryos are not reproduced in sufficient number to give rise to any symptoms. Usually, however, they are well marked, and embrace a stage of gastro-intestinal irritation followed by systemic infection. The patient, three or four days after eating raw pork or ham, suffers from more or less severe abdominal cramps, with anorexia, vomiting, and diarrhoea. The latter occasionally becomes severe. General muscular prostration is also present, and there may be chills. In a number of cases the gastro-intestinal symptoms may not be severe enough to attract attention, and the first symptoms are those of general infection, which develop at the commencement of the second week. There is an increase of temperature, amounting to 1030 or 1040 F., of an intermittent or remittent character. As soon as the embryos have extensively penetrated the muscles they give rise to great local pain and tenderness, accompanied by swelling and tension of all the muscles affected.
The patient naturally assumes the position in which there will be the least strain upon the muscles.
These symptoms increase in intensity, and general cedema is apt to follow, which may appear first in the face. When certain muscles are implicated more serious symptoms may result. If the diaphragm is invaded or other muscles of respiration, there may be extreme or even fatal dyspnoea. If the parasites reach the muscles of the face, jaw, and pharynx, mastication and deglutition become difficult or impossible. There is more or less itching and burning of the skin and perspiration. Urticaria has been observed; anaemia and a maras-mic condition eventually develop in a majority of cases, and marked eosinophilia amounting to 30 or 40 per cent is observed in the blood in this type as it is in other varieties of intestinal parasitic disease. There is comparatively little disturbance of the nervous system, and patients are usually conscious until the time of death, but in some instances a typhoid state supervenes with delirium. The other symptoms which have been reported as occasionally present are loss of tendon reflex, bronchitis, pneumonia, pleurisy, albuminuria, and polyuria.
Mild cases may end in recovery within a fortnight. In severe cases in which there has been great prostration and emaciation convalescence is retarded for many weeks.
Children are more apt to recover than adults, but the outlook depends chiefly upon the number of parasites ingested. Cases presenting severe diarrhoea are more likely to end favourably, probably because some of the parasites are eliminated in this way.