Trichinella spiralis

Trichinella spiralis is an viviparous nematode parasite, occurring in rodents, pigs, bears, hyenas and humans, and is responsible for the disease trichinosis. It is sometimes referred to as the 'pork worm' due to it being typically encountered in undercooked pork products. It should not be confused with the distantly related pork tapeworm. Trichinella species, the smallest nematode parasite of humans, have an unusual lifecycle, and are one of the most widespread and clinically important parasites in the world. The small adult worms mature in the small intestine of a definitive host, such as a pig. Each adult female produces batches of live larvae, which bore through the intestinal wall, enter the blood (to feed on it) and lymphatic system, and are carried to striated muscle. Once in the muscle, they encyst, or become enclosed in a capsule. Humans can become infected by eating infected pork, horsemeat, or wild carnivores such as fox, cat, or bear. Males of T. spiralis measure between 1.4 and 1.6 mm long, and are more flat anteriorly than posteriorly. The anus can be found in the terminal end, and they have a large copulatory pseudobursa on each side. The females of T. spiralis are about twice the size of the males, and have an anus found terminally. The vulva is located near the esophagus. The single uterus of the female is filled with developing eggs in the posterior portion, while the anterior portion contains the fully developed juveniles. Trichinella spiralis can live the majority of its adult life in the intestines of humans. To begin its life cycle, Trichinella spiralis adults will invade the intestinal wall of a pig, and produce larvae that invade the pig's muscles. The larval forms are encapsulated as a small cystic structure within a muscle cell of the infected host. When another animal (perhaps a human) eats the infected meat, the larvae are released from the nurse cells in the meat (due to stomach pH), and migrate to the intestine, where they burrow into the intestinal mucosa, mature, and reproduce. Juveniles within nurse cells have an anaerobic or facultative anaerobic metabolism, but when they become activated, they adopt the aerobic metabolism characteristics of the adult. Female Trichinella worms live for about six weeks, and in that time can produce up to 1,500 larvae; when a spent female dies, she passes out of the host. The larvae gain access to the circulation and migrate around the body of the host, in search of a muscle cell in which to encyst. The migration and encystment of larvae can cause fever and pain, brought on by the host inflammatory response. In some cases, accidental migration to specific organ tissues can cause myocarditis and encephalitis that can result in death. This nematode is a multicellular parasite that lives within a single muscle cell, which it extensively modifies according to its own requirements. Nurse cell formation in skeletal muscle tissue is mediated by the hypoxic environment surrounding the new vessel formation. The hypoxic environment stimulates muscle cells in the surrounding tissue to upregulate and secrete angiogenic cytokines, such as vascular endothelial growth factor (VEGF). This allows the migrating T. spiralis larva to enter the myocyte and induce the transformation into the nurse cell. VEGF expression is detected surrounding the nurse cell immediately after nurse cell formation, and the continued secretion of VEGF can maintain the constant state of hypoxia. The first symptoms may appear between 12 hours and two days after ingestion of infected meat. The migration of adult worms in the intestinal epithelium can cause traumatic damage to the host tissue, and the waste products they excrete can provoke an immunological reaction. The resulting inflammation can cause symptoms such as nausea, vomiting, sweating, and diarrhea. Five to seven days after the appearance of these symptoms, facial edema and fever may occur. Ten days following ingestion, intense muscular pain, difficulty breathing, weakening of pulse and blood pressure, heart damage, and various nervous disorders may occur, eventually leading to death due to heart failure, respiratory complications, or kidney malfunction, all due to larval migration.