Trichinella spiralis

Trichinella spiralis

A tiny roundworm caught from undercooked pork or game. Its adults live a few weeks in the small intestine; their young settle in skeletal muscle, where each larva remakes a muscle cell into a nurse cell and lives on for years. Fever, swollen eyelids and aching muscles are its mark.

Trichinella spiralis
On the left, the female and the male seen from the side as if cleared, drawn to one scale, the female the longer. Each is a slender ivory worm, thin at the front, where a narrow coral oesophagus runs along a single row of large gland cells, the stichosome, and thicker behind, where the olive intestine runs to the anus at the tip of the tail. The female's vulva opens far forward, beside the stichosome; behind it her single pink uterus holds tiny curled larvae near the vulva and eggs further back, and her ovary loops near the tail. The male has a blue testis, and his tail ends in two small rounded lobes, the copulatory appendages. At the top right, a larva in muscle: three striated coral muscle fibres lie across the drawing, and the middle one is swollen into a pale nurse cell with many nuclei and no striations, its outside a beige collagen capsule shaped like a lemon. Inside it the ivory larva lies coiled, its row of gland cells along its front half. Thin red capillaries hug the capsule above and below it. At the bottom, the female's front end enlarged: the mouth on the left, the oesophagus running along its stichosome, the vulva on her belly side, and behind it the uterus full of curled larvae, one of them leaving through the vulva.

Point at a name or a part to light it.

Trichinella is a tiny roundworm: the female is about 2.2 mm long, the male about 1.2 mm. One host carries both stages, the adults for a few weeks in the small intestine, then their larvae for years in its muscles, each larva coiled inside a muscle cell it has remade into a nurse cell.

Simplified: the female and the male drawn to one scale but far wider for their length than they are; the nurse cell and the front end enlarged, each by a different amount; the stichosome in the whole worms drawn small; the mouth's stylet, the cuticle's bacillary bands and the male's accessory papillae left out; the ovary, uterus and testis drawn as single tubes; a few dozen of the nurse cell's nuclei drawn; the fibres around it drawn as three bands, and its network of capillaries as two vessels.

5 sources
  1. Centers for Disease Control and Prevention. DPDx: Trichinellosis. Last reviewed 7 June 2024.
  2. Marie C, Petri WA Jr. Trichinosis. MSD Manual Professional Edition, January 2025.
  3. Mitic I, Vasilev S, Gruden-Movsesijan A. Trichinellosis: a zoonosis that still requires vigilance. PLOS Neglected Tropical Diseases 2026;20(1):e0013944.
  4. Wikipedia. Trichinella spiralis.
  5. Abdel Salam SA, El-Darier SM, Ashmawy MI, Abdelaziz M, Arafa FM. Rosmarinus officinalis L. essential oil: in silico molecular docking and in vivo nematocidal activity on intestinal and muscular trichinellosis. BMC Complementary Medicine and Therapies 2025;25.

At a glance

  1. Trichinella spiralis and its relatives are caught by eating undercooked meat that holds their larvae: pork, wild boar, bear, horse.
  2. One host carries both stages. The adults live about a month in the lining of the small intestine; from about a week, the females release live young that travel in the blood and lymph to skeletal muscle.
  3. In the muscle each larva coils inside a nurse cell, a muscle cell it has remade, wrapped in a collagen capsule. It lives for years, then dies and calcifies.
  4. Trichinellosis: upset bowels in the first week, then fever, swelling round the eyes, aching muscles and eosinophilia. Myocarditis and encephalitis can kill.
  5. Serology confirms it, but often not until 3 to 5 weeks after the meal. Albendazole or mebendazole works best early, with steroids for severe disease. Cooking meat through prevents it.

Trichinella spiralis

Trichinella spiralis is the classical cause of trichinellosis (trichinosis), found worldwide in many carnivorous and omnivorous animals. Several other species of the genus infect people, most of them in wild animals: T. britovi in the carnivores of Europe and western Asia, T. nativa in Arctic bears, T. pseudospiralis in mammals and birds. Animals carry the larvae without falling ill.

Between 1986 and 2009 about 66,000 cases and 42 deaths were reported worldwide, 86% of them in the WHO European Region, which takes in Russia. In this century the most European cases have come from Bosnia and Herzegovina, Bulgaria, Lithuania, Poland, Romania, Russia, Serbia and Spain, often in family outbreaks after eating uninspected pork or game.

The adults

  • Tiny: the female about 2.2 mm long, the male about 1.2 mm, so she is about twice his size.
  • The front of both is slender. Their oesophagus runs along a single row of 45 to 55 large gland cells, the stichosome, as in the whipworm.
  • The female has her anus at the tip of her tail and her vulva far forward, at the level of the oesophagus. Her single uterus holds developing eggs at the back and fully formed young toward the vulva: she bears live young, 500 to 1,500 of them, each about 0.08 mm long.
  • The male's tail ends in two copulatory appendages, with small papillae beside them.
  • Where they live: embedded in the cells lining the small intestine.

The larva in its nurse cell

A newborn larva that enters a skeletal muscle fibre does not kill it, unlike most parasites that live inside cells. It remakes it:

  • Within days, the fibre loses its contractile apparatus: the myofibrils, with their Z lines and bands, go, and the cross-striations with them.
  • It gains nuclei, up to about 100, each much enlarged: satellite cells, the muscle's own stem cells, fuse with it.
  • A collagen capsule forms round it, and new capillaries grow round the capsule, called up by the larva.

This nurse cell houses the larva, which grows into the infective muscle larva coiled inside it, without moulting. It is fully encysted in 1 to 2 months and stays alive for several years. Dead larvae are resorbed or calcify, which may begin after 6 months. The larvae favour muscles with a rich blood supply, such as the diaphragm and tongue, the psoas, the pectoralis major and the gluteus maximus. T. pseudospiralis and T. papuae form no capsule.

The life cycle

Trichinella: the life cycle
A map of the life cycle of Trichinella. Every host carries both stages, the adult worms in its small intestine for a few weeks and then their larvae in its muscles, so each is a definitive and an intermediate host at once, and the cycle turns only when one animal eats another's muscle. Top left, the rat's panel, and below it the pig's: the domestic cycle. Each has larvae in nurse cells in its muscles. An arrow runs down from the rat to the pig, which eats an infected rat, and another back up, since rats feed on dead pigs; pigs are also infected from raw meat scraps and from eating dead pigs. Top right, the panel of wild boar and bears: the sylvatic cycle. An arrow runs right from the rat: boars and bears eat rats and other animals. From the pig and from the wild animals, arrows run down to the patient's panel along the foot: a person eats undercooked pork or game. In the patient, the larvae become adult worms in the small intestine, which live about 4 weeks. An arrow runs right and down: after about 1 week the females release newborn larvae, which travel in the blood and lymph. An arrow runs left from them to the larvae in nurse cells: they settle in skeletal muscle, are encysted in 1 to 2 months, live for years and then calcify. No arrow leaves the patient.

Point at a name to light its pathway.

Every host carries both the adult worms and the larvae, so each is a definitive and an intermediate host; the cycle turns only when one animal eats another's muscle. People are infected from undercooked pork or game, and pass nothing infective on.

Simplified: a map, not anatomy: the hosts drawn as icons and the parasites drawn small, nothing to scale; drawn for the genus: T. spiralis keeps mostly to pigs and rats, the other species mostly to wild animals; a rat, a pig, a wild boar and a bear drawn for the many hosts; in each animal only the larvae in muscle drawn, though each carries the adults too; the larvae's moults in the gut, and the mating of the adults, left out; the newborn larvae's way through the blood and lymph drawn as one arrow; wild animals also bring the parasite back to free-range pigs, not drawn.

6 sources
  1. Centers for Disease Control and Prevention. DPDx: Trichinellosis. Last reviewed 7 June 2024.
  2. Marie C, Petri WA Jr. Trichinosis. MSD Manual Professional Edition, January 2025.
  3. Dubey JP, Thompson PC, Fournet V, Hill DE, and others. Over a century of progress on Trichinella research in pigs at the United States Department of Agriculture: challenges and solutions. Food and Waterborne Parasitology 2024;36:e00239.
  4. Rodriguez-Morales AJ, Shehata AA, Parvin R, Tasnim S, and others. Rodent-borne parasites and human disease: a growing public health concern. Animals 2025;15:2681.
  5. Mitic I, Vasilev S, Gruden-Movsesijan A. Trichinellosis: a zoonosis that still requires vigilance. PLOS Neglected Tropical Diseases 2026;20(1):e0013944.
  6. Wikipedia. Trichinosis.

One host carries the whole cycle, so every infected animal is at once a definitive host (it carries the adults) and an intermediate host (it carries the larvae). A second host is needed only to carry it on.

  1. Eaten. Meat holding larvae in nurse cells is eaten. Stomach acid and pepsin free the larvae.
  2. Adults. In the small intestine the larvae enter the lining, moult four times into adults, and mate.
  3. Young. After about a week the females begin to release newborn larvae. The adults live about a month in the gut, then die or are expelled.
  4. Spread. The newborn larvae travel in the blood and lymph throughout the body but survive only in skeletal muscle; those that reach the heart do not settle there.
  5. Walled up. Each settles in a muscle fibre, which becomes its nurse cell. Nothing infective leaves the host: the cycle goes on only if another animal eats the muscle.

The cycle turns in two settings that can meet:

  • The domestic cycle: pigs and rats on farms. Pigs are infected from raw meat scraps fed to them, from eating dead pigs, which spreads it through a herd, and from rats; rats feed on dead pigs and on each other. Horses can be infected too, and their meat has caused outbreaks.
  • The sylvatic cycle: wild animals that eat prey and carcasses, such as foxes, wild boar and bears. Wild boar and bears are the commonest source of infection from game.
  • Where they meet: rats and other scavengers carry the parasite between the two, and free-range pigs that eat wild carcasses bring it back to the farm.

A person is infected like any other host, from undercooked meat, but no animal eats a person's muscles: an accidental, dead-end host.

Trichinellosis

How ill a person becomes depends above all on how many larvae were eaten. Many infections cause no symptoms or only mild ones.

  • The gut phase, in the first week: nausea, abdominal cramps and diarrhoea, as the adults settle in the lining.
  • The muscle phase, 1 to 2 weeks after the meal, as the newborn larvae invade the tissues. The classic triad is fever, swelling round the eyes and muscle pain:
    • periorbital and facial oedema, with bleeding under the conjunctiva, eye pain and sometimes double vision;
    • aching, tender muscles, which may mimic polymyositis: the muscles of breathing, speech, chewing and swallowing may hurt to use;
    • a remitting fever, often 39 °C or more, and headache;
    • splinter haemorrhages under the nails, and rashes.
  • Eosinophilia begins as the larvae invade, peaks 2 to 4 weeks after infection and then falls. Muscle enzymes, creatine kinase and LDH, are raised.
  • Complications of heavy infection come from the inflammation the wandering larvae cause: myocarditis, heart failure and arrhythmias; encephalitis, meningitis and seizures; pneumonitis. Death is from myocarditis or encephalitis. Larvae that reach the heart do not encyst there: the myocarditis is inflammation round the wandering larvae.
  • Recovery: most symptoms are gone by about the third month, once the larvae are walled up. Muscle aches and tiredness may last months, sometimes years. Years later, calcified cysts may show up in the muscles on an X-ray.

Diagnosis, treatment and prevention

  • Suspect it in fever, periorbital oedema, myalgia and eosinophilia, above all when several people ate the same meat. Ask about pork, wild boar, bear and home-made sausage.
  • Serology (an enzyme immunoassay) is the usual test, but antibodies often appear only 3 to 5 weeks after infection, so a negative test is repeated weekly. They last for years, so a test that turns from negative to positive means most.
  • A muscle biopsy can show the larvae after the second week, but it is painful and seldom needed. Leftover meat can be tested for larvae.
  • Treatment: albendazole or mebendazole for 1 to 2 weeks, given as early as possible: they kill the adults in the gut and so cut the number of larvae reaching the muscles, but larvae already encysted may survive them. Painkillers for the muscle pain; prednisone, together with the anthelmintic, for severe allergic symptoms or when the heart or brain is involved. Albendazole and mebendazole are not approved for use in pregnancy or under 2 years of age; there the benefit is weighed against the risk.
  • Prevention:
    • cook pork and game until brown throughout, above 71 °C;
    • freezing kills T. spiralis in pork (pieces under 15 cm thick, at −15 °C for 20 days), but not in game: T. nativa, in Arctic bears and foxes, survives freezing;
    • smoking, salting, drying and microwaving do not reliably kill the larvae;
    • no uncooked meat fed to pigs, and rats kept off pig farms;
    • meat inspection: samples of muscle, above all the diaphragm, are digested artificially and examined for larvae. EU law requires it for pigs, wild boar and horses slaughtered for sale, but not for pigs slaughtered for the family's own use, so home-made pork products and game are behind many outbreaks.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1Ten days after a family feast of home-made wild boar sausage, four guests have fever, swollen eyelids, aching muscles and a high eosinophil count. What is the most likely cause?

2Which stage of Trichinella infects a new host?

3Why is a person a dead end for Trichinella?

4Why is albendazole best given early in trichinellosis?

5A hunter asks whether freezing bear meat will make it safe. What do you tell him?