Introduction - the parasite Trichinella spiralis


I have created this website for the class "Parasites and Pestilence" at Stanford University to provide information on the important parasite Trichinella.

Trichinosis (also known as trichinellosis or trichinelliasis) is caused by ingesting the larvae of the nematode Trichinella in undercooked, infected tissue. The roundworm is found in many different carnivorous animals and scavengers, and can cause serious illness and even death in humans. Trichinella has a nearly worldwide distribitution, making it an incredibly successful parasite.

A single worm is called a trichnia, which is Greek for "hairlike," and larvae are characterized by their coiled shape in muscles.

Nurse cell-parasite complex. Phase interference. Photo by E. Gravè


Skeletal muscle with Trichinella spiralis ( Lab/biology_lab.html)


The parasite's taxonomy is:
  class: adenophorea
  subclass: enoplia
  order: stichosomida (trichurata)
  superfamily: trichocephaloidea
  family: trichinellidae


Important terms:
  • Cyst: A protective wall that forms around the larva. (Note: many other parasites have cysts, and not all are specific to the larval stage).
  • Host : An organism that harbors a parasite (or a commensal partner), providing food and shelter.
  • Reservoir: an animal host that maintains a parasite life cycle in the wild, and functions as a source of the parasite for humans.
  • Larva: The young and immature form of the worm.
  • Nematode: A type of roundworm with a long, unsegmented body.
  • Striated muscle: muscles of the skeletal system and of the heart.


One of the best resources for further information on trichinosis, and for emergency contact, is the CDC:

(404) 332-4559



History of Discovery

Richard Owen and James Paget (pictured on the left, courtesy of pm.roy/paget.jpg) were the first to describe the larva of Trichinella in a human diaphragm in 1835. The parasite was first reported in the United States in 1846 and, by 1860, Virchow and Leukart had described the complete life cycle of Trichinella spirals, though not in great detail. In 1865, it was shown that Trichinella is a significant human pathogen capable of causing severe disease.

William C. Campbell wrote a very comprehensive history of the parasite called "Trichinella and trichinellosis." History buffs may want to check it out.

Long before its discovery in the 1800's, Trichinella was infecting humans. Trichinosis was highly prevalent in ancient Egypt and has been found in many mummies!


Trichinella in Humans

An infected person can experience nausea, diarrhea, vomiting, fatigue, fever, and abdominal discomfort in the first few days after infection from contaminated meat. The incubation period can last between 5 and 45 days, at which time headaches, fevers, chills, cough, eye swelling, aching joints, muscle pain, itchy skin, diarrhea or constipation can set in. Splinter hemorages of the fingernails, swelling and muscle pain are caused by larvae moving through tissue.

Weakness and soreness may last months after other symptoms have subsided. Many people may never be diagnosed since mild or moderate trichinosis is frequently mistaken for the flu or another common illness. A heavily infected patient may have problems related to the central nervous system, in addition to possible heart and respiratory problems. Very rarely do severe cases cause death.

Swollen eyes (periorbital edema)


splinter hemorages prob%20of%20month/Prob%


Are you at risk for trichinosis?

If you eat raw or undercooked meats, especially pork, bear, wild cats, fox, horse, seal or walrus, you are at risk. However, you CANNOT spread trichinosis to others (unless they eat you). See your health care provider and inform them that you eat undercooked meats if you suspect you may have trichinosis. Acting quickly is important.


Reservoir and Transmission

Reservoir hosts include bears, pigs, rats, wild cats, foxes, dogs, wolves, horses, seals and walruses. Domestic pigs are considered the dominant reservoir host, but there are many different typical hosts throughout the world:

  • Temperate regions: pig, wild boar, horse, dog, bear, fox
  • Tropical regions : wharthog, hyena
  • Arctic regions : polar bear, walrus, wolf, fox (

Transmission occurs by ingestion of infected tissue. Domestic pigs frequently become infected by ingestion of uncooked garbage or infected rat carcasses. When humans eat undercooked or raw meat from pigs and other reservoir hosts, they too become infected. Trichinella has no vector.

Wildlife picture courtesy of ../guide/muscle2.cfm ADFandG division of Wildlife Conservation


Trichinella is not just found in mammals:

Recently, Trichinella has been found in saltwater crocodiles in Zimbabwe and Papau New Guinea. Crocodiles are frequently fed uncooked pig meat and kept for human consumption. Since the parasite is so incredibly successful and capable of infecting such a wide range of animals, it is important for there to be effective screening processes and regulation within any meat industry. Measures need to be taken to freeze meat and ensure that carcasses are properly disposed of.




The posterior portion of a gravid female contains the uterus full of eggs.

Adult female

photos courtesy of

Adult male

Both males and females taper at the anterior end. Males are 1.4 to 1.6 mm long and females are about twice that length. Infective larvae are coiled into a spiral. T. pseudospiralis (a species a Trichinella) does not form as distinct of a coil as the other Trichinella species.



Villella (1970)


Life Cycle

  • Larvae are ingested in raw or undercooked meat (ie. pork) and develop into adults in the small intestine.
  • adults are attached to or buried in the intestinal mucosa, where they mate. A female produces ~1,500 living young over a lifespan of 4 to 16 weeks.
  • Larvae enter the lymphatics and mesenteric veins, travel throughout the body, and eventually encyst in striated muscle. A cyst is a kind of protective wall, in this case created in reaction to the host's immune system. Larvae may remain viable for up to 25 years in humans!
  • When viable cysts are ingested by a new host, the cycle begins again.

There are basically two pathogenic phases in the life cycle. Infection of the intestine is called the enteral phase and includes early larval stages in addition to immature and reproductive adult stages. Larvae encyst in nurse cells in the parenteral phase.


Diagnosis and Treatment


Any patient with periorbital edema, fever, myositis, and eosinophilia (a high level of a certain kind of white blood cells) should be tested for trichinosis, especially if there is a history of eating uncooked meat. A muscle biopsy 2-3 weeks after the onset of symptoms can be used for diagnosis through microscopy, but one should be aware that sampling errors can lead to a false negative. A common blood test called the bentonite flocculation (BF) is another diagnostic tool. ELISA or EIA are frequently used, followed by a BF test to confirm the diagnosis.



Early Treatment - Treatment during the accute stage is essential since drugs can do very little once larvae have become established in the muscle. Mebendazole (Vermox) and thiabendazole (Mintezol) are effective against intestinal larvae. Thiabendazole appears to be highly effective when taken within twenty four hours of ingestion of infected meat and, when given in a high dose over time, mebendzole may be the best option for dealing with encysted larvae.





Supportive care - If the diagnosis is relatively late in the infection, supportive care can help relieve some of the symptoms. Aspirin and anti-inflammatory drugs are effective, but corticosteroids may be necessary in more severe cases and when myocarditis is involved.




Trichinosis was once widespread throughout the United States, but, due to increased regulations for the meat industry, most cases in the are now limited to people who consume wild game. An average of 12 cases per year were reported in the US between 1997 and 2001. However, most infections are never identified.

Trichinella infects humans and other mammals throughout North America, parts of South America, central America, parts of Africa, Asia, New Zealand, and Tasmania. The parasite is highly prevalent in Europe, Asia, and Southeast Asia and is endemic in Japan and China. The possibility of Trichinella in Australia is currrently under investigation. Periodically, there are minor epidepics of trichinosis. One of the most recent ones was in Thailand.

There are nine different genotypes described to date and seven of those are considered species: T. spiralis, T. nativa, T. britovi, T. pseudosprialis, T. murelli, T. nelsoni and T. papuae. Strains vary in their temperature sensitivity, such that arctic strains have greater resistance against the cold. The maps below show the distribution of various strains of Trichinella throughout the world (courtesy of 625tutorials/Trichinella03.html).  
Trichinella spiralis (green) is the most common member of the genus and is the primary cause of domestic animal trichinosis. Trichinella nativa (yellow) is common in carnivores and has a low infectivity for swine and rats. Trichinella nelsoni (red) is found throughout subsaharan Africa and primarily infects Hyaenidae and Felidae. Trichinella murrelli (green) is found in portions of North America and infects carnivores.
A Trichinella isolate, termed T-6 (red), is similar to T. nativa. It primarily infects carnivores and is capable of interbreeding with T. nativa. Trichinella britovi (green) occurs in Eurasia. Isolates capable of interbreeding with T. britovi include T-9 (purple) in Japan and T-8 (yellow) from Southern Africa. Two non-encapsulating Trichinella species exist. Trichinella pseudospiralis (red) infects a wide range of mammals and birds. It has moderate infectivity for swine, rats, and mice. Trichinella papuae (yellow) was originally isolated from a wild pig.



Public Health and Prevention Strategies

Picture courtesy of BBQ/ptloin1.28.02.html


There are a number of effective and easy ways to prevent trichinosis:

  • Cook meat, especially pork, bear, walrus, and wild pig thoroughly (to an internal temperature of 170 degrees farenheit).
  • Freeze pork less than 6 inches thick for 20 days at 5 degrees F. Freezing is not very effective for the meat of arctic animals (eg. bears and walrus)
  • Cook all meat fed to pigs and other animals and do not allow pigs to eat carcasses of rats.
  • Clean all tools used to prepare meat.
  • You CANNOT rely on salting, drying, smoking or microwaving to kill the encysted worms.

Better regulation of the meat industry has greatly reduced the incidence of trichinosis in the United States and legislation prohibits feeding uncooked garbage and meat products to pigs. Public awareness has played a large role in encouraging people to freeze and cook pork thoroughly. Feral pig and wild game hunters should be particularly cautious since Trichinella is common among many scavengers and carnivores.


Research on Trichinella

Current research on Trichinella spans a wide variety of issues, from immune response to geographical distribution to vaccine development. Even with a wealth of knowledge on the topic, there is still a lot that we do not know about this parasite and the details of its survival. How does it survive intracellularly in the small intestine? How do mature adults find each other in the intestines (I'm talking about parasite sex)? Why are the sex ratios such that there two females for every male? How does the larva invade the muscle cell? Once inside, how does the larva alter the host cell for survival? These are all questions waiting to be answered... perhaps by you.

An excellent article summarizing current knowledge on the body's response to Trichinella and prospects of a vaccine is "The Immune Response to the Parasitic Nematode Trichinella and the Ways to Escape it. From Experimental Studies to Implications for Human Infection" by F. Bruschi. The article can be found in Current Drug Targets - Immune, Endocrine & Metabolic Disorders, October 2002, vol. 2, no. 3, pp. 269-280(13), published by Bentham Science Publishers. contains a list of world literature on Trichinella, in addition to some great movie clips and a beautiful photo gallery.




Useful weblinks



Bruschi, F. 2002. The Immune Response to the Parasitic Nematode Trichinella and the Ways to Escape it. From Experimental Studies to Implications for Human Infection. Current Drug Targets Immune, Endocrine & Metabolic Disorders. Vol. 2, no. 3, pages 269-280(13). Bentham Science Publishers.

Campbell WC. Historical Introduction. Trichinella and trichinellosis. 1983. (Campbell, WC, ed.). Plenum Press, Pubs, New York pages. 1-30.

CDC. Trichinellosis surveillance-United States, 1997-2001. In: CDC Surveillance Summaries. 2003. 52 (No. SS-6) pages 1-8.

Chastel C. 2004. the Egyptian mummies are speaking about the infections that have made them ill. Hist Sci Med. (2):147-55.

John, David T, Wojciech Krotoski, and Edward Markell. 1999. Medical Parasitology. Philidelphia: W.B. Saunders Company.

Pozio, Edoardo, Morales, Maria, Dupouy, Jean and Camet. Clinical aspects, diagnosis and treatment of trichinellosis. Expert Review of Anti-infective Therapy. 2003, Vol. 1, No. 3, Pages 471-482 .

Pozio E, Owen IL, Marucci G, La Rosa G. 2005. Inappropriate feeding practice favors the transmission of Trichinella papuae from wild pigs to saltwater crocodiles in Papua New Guinea.Vet Parasitol.127(3-4):245-51.

Villella, J.B., 1970. Life cycle and morphology, in: Trichinosis in Man and Animals (S.E. Gould, ed.), Charles C. Thomas, Springfield, Illinois, pages 19-60. ../guide/muscle2.cfm (ADFandG division of Wildlife Conservation) 625tutorials/Trichinella03.html