The Trichinella (Trichina) Spiralis is a very important parasite that undergoes development in two hosts.

It occurs in two forms in man and the lower animals, as an intestinal fully developed worm and as an encapsulated embryo in the muscular tissue. Is most common in hogs. The adult female is from 2 to 4 mm., the male 1.5 mm. in length. The eggs develop into embryos while within the mother. When an encysted embryo is taken into the stomach the gastric juices dissolve the shell, and the parasite is set free. It very rapidly matures, and in the course of from five to seven days eggs are discharged. Within a few days a couple of thousand embryos will have been liberated. These young parasites penetrate the walls of the intestine, gain entrance either into the blood or the lymphatics, most probably the blood, and in the course of about ten days become lodged in the voluntary muscles.

In the muscle fibers the embryos become encysted in two to three weeks. There is a deposit of lime salts around the curled-up embryos which may either die and become calcified or remain alive for years. The encysted form appears as a small white point.

Infection in man takes place by the eating of meat (measly pork) that contains the trichinae and that has not been properly cooked. A temperature of 650 C. kills the parasite, but pickling and smoking destroy those only which are in the superficial parts, those deep within being unaffected.

When the embryos escape from the cysts into the intestine severe vomiting and diarrhea may occur. During the emigration to the muscles the symptoms are those of muscular rheumatism. In this disease there has frequently been noticed a very great increase of the eosinophile cells in the blood.

Fig. 125. - Fresh Muscle Trichinae (Mosler and Peiper).

Arachnoidea

The insects are external parasites which may prove dangerous by means of their own metabolic products, by acting the part of intermediate host for some parasite, or by mechanically carrying the source of infection.

The first class is not very important. In the second is the mosquito, which may in some of its varieties transmit malaria, yellow fever, or filaria sanguinis hominis.

Flies may mechanically convey typhoid bacilli from feces to articles of food.