Trichinella species in people and animals

Trichinella is a genus of zoonotic nematode that occurs in carnivores and omnivores (mammals, including people, reptiles and birds) around the world, including in Canada.


Trichinella is a genus of zoonotic nematode that occurs in carnivores and omnivores (mammals, including people, reptiles and birds) around the world, including in Canada.  The recent application of molecular techniques has led to the identification of eight species (T. spiralisT. nativaT. britoviT. murrelliT.nelsoniT. pseudospiralis, T. papuae and T. zimbabwensis) and four genotypes (T6, T8, T9 and T12) which have not yet been given species status (see taxonomy section, below).  Each species and genotype has distinct, but often overlapping, geographic distributions, host ranges, and biological characteristics.  Also, the zoonotic potential varies with species and genotype.

The life cycle of Trichinella is indirect and unusual.  The very tiny adult parasites live in the small intestine of the host, and the females produce first stage larvae which enter the intestinal wall and blood vessels and are distributed throughout the body.  In striated muscle these larvae encapsulate within the muscle cells (so-called "nurse cells"), where they can survive for years.  The cycle is continued when this muscle tissue is eaten by another suitable host, in which the larvae develop into adult parasites.  Transmission of Trichinella among its hosts depends on the consumption of infected meat or sometimes carrion (note that the larvae of T. nativa and T6 in tissues are freeze-resistant), and the parasite is often maintained in an ecosystem through complex food webs.

Most of the species of Trichinella are known to be zoonotic, and historically the major source of human infections was pigs with T. spiralis, and this host remains an important source in some areas of the world, especially eastern Europe.  In Canada, pigs are believed to be free of Trichinella, except for a small focus around Halifax, Nova Scotia, where T. spiralis is believed to persist.  Other than a few cases of trichinellosis (the disease associated with Trichinella) in Ontario in 1993 associated with the consumption of wild boar imported from the US, the last case of pig-associated human disease associated with Trichinella was in 1980. In this country, wildlife are considered the major source of human infections, particularly polar, grizzly and black bears, and walrus, although in most years there are probably less than 50 cases in people.  In recent years there have been several outbreaks of human tichinellosis in Europe (primarily France and Italy) associated with the consumtion of horse meat, it is believed that at least one of the horses involved came from Canada.  The horses probably became infected by the inadvertent consumption of infected carrion.

In all host species, including people, infection with Trichinella is much more common than is clinical disease.Other than a small number cases in dogs and cats, Trichinella in its non-human hosts is generally considered to be "non-pathogenic".  In people, however, the parasite can cause serious and sometimes fatal disease, and there continue to be outbreaks in Canada associated with the consumption of inadequately prepared but infected meat from wildlife.  Trichinella in people causes two distinct syndromes: intestinal - often associated with prolonged diarrhea; and muscular - often associated with myalgia and fever.  Infection can be avoided if any possibly infected meat is cooked to reach a core temperature of 71C for at least one minute.  In Canada freezing is not a reliable means of prevention because T. nativa and T6, the most common isolates here, are freeze-resistant and can survive for years in frozen tissues.


Phylum: Nematoda
Class: Enoplea
Subclass: Dorylamia
Order: Trichurida
Family Trichurida

Among parasites of domestic animals, wildlife and people the closest relative to Trichinella are Trichuris, the whipworm, and Capillaria. Within the genus Trichinella there are currently (March, 2010) eight species (five encapsulated and three nonencapsulated), and four genotypes (all encapsulated).  This taxonomy could change as additional species and genotypes are identified.  With encapsulated Trichinella species and genotypes, the post-adult larvae develop in striated muscle cells (nurse cells) each of which becomes surrounded by a collagen capsule.  Nurse cells of nonencapsulated Trichinella do not have this capsule.

 Trichinella species - encapsulated Trichinella species - nonencapsulated Trichinella genotypes                 
 T. spiralis (T1) * Z+ Fh  T. pseudospiralis (T4) * Z+  T6 * Z? Fc
 T. nativa (T2) * Z+ Fc  T. papuae (T10) Z+  T8 Z?
 T. britovi  (T3) Z+ Fhc  T. zimbabwensis (T11) Z?  T9 Z?
 T. murrelli (T5) * Z+    T12 Z?
 T. nelsoni (T7) Z+    
 * occurs in Canada  Z+ known to be a zoonosis
 Z? zoonotic status unknown
Fh freeze-resistant in horses
Fc freeze-resistant in carnivores
Fhc freeze-resistant in horses and carnivores


Adult Trichinella in the small intestine are very small.  Males measure approximately 1.5 mm in length and females approximately 3.5 mm.  The infective larvae that develop in nurse cells in striated muscle are 0.6 to 1.5 mm in length and may just be visible to the naked eye.

Host range and geographic distribution

Defining the host range and geographic distribution for species within the genus Trichinella advanced rapidly following the development in the 1990s of DNA-based techniques for parasite identification to species and genotype.  In records prior to this  identification to these levels was uncertain, particularly for isolates from wildlife.

Larvae of the genus Trichinella have been recovered from a wide variety of carnivore and omnivore mammals around the world.  Natural infections also occur in horses, which are herbivores.  Larvae of T. pseudospiralis have also been found occasionally in birds in North America, Europe, and Asia , T. papuae in saltwater crocodiles in Papua New Guinea, and T. zimbabwensis in Nile crocodiles, monitor lizards and lions in east Africa and South Africa.  In many countries Trichinella cycles mainly in wildlife (the sylvatic cycle), in others in domestic animals, primarily pigs (the domestic cycle), and in some both cycles are present, especially for T. spiralis.  Human infections result from the ingestion of meat from infected wildlife or domestic animals.

In Canada Trichinella species are found almost exclusively in wildlife, and T. nativa and T6 predominate.  Trichinella murrelliT. pseudospiralis and T. spiralis also occur in wildlife in this country, but reports are rare.  Currently (March, 2010) Trichinella spiralis is present in pigs only sporadically in a small area of Nova Scotia, and the Canadian national pig herd is considered Trichinella-free.  The parasite also seems to be very rare in wild boar in Canada.  Although historically some cases of trichinellosis in people in Canada might have been associated with T. spiralis acquired from pigs, in recent cases where the infection was acquired from wildlife DNA-based techniques have identified T. nativa.  To date human infections with T6 have not been reported in Canada.

Life cycle - indirect

The life cycle of Trichinella species is simple and is considered indirect with infected animals acting as both definitive and intermediate hosts.  The adult male and female nematodes live in the small intestine.  Following reproduction the females produce first-stage (newborn) larvae (NBL) which enter blood vessels and lymphatics in the intestine and travel to other organs and tissues, primarily skeletal muscles, but also cardiac muscle and the brain.  In skeletal muscles each larva penetrates a muscle cell, which transforms into a nurse cell, and grows and develops until it is infective for another host.  This process takes about two weeks and the larvae can survive for decades in this location.  The cycle continues when meat containing the infective larvaeis eaten by a suitable host.  the larvae are released and complete their development to adults, which then reproduce.  NBL production begins about one week after infection.  Often after a few weeks the adult parasites are expelled, probably as the result of an immune response.


The transmission of Trichinella among its host depends on the consumption of infected meat or occasionally carrion, especially frozen carrion in northern regions of the world (the first-stage larvae of T. nativa and T6 in the muscles of carnivores are freeze resistant - see Taxonomy section).  People become infected by consuming infected meat containing  viable first-stage larvae.  The methods used for meat storage and preparation influence larval viability, and are often determined by cultural traditions.

Globally among domestic animals pigs, and to a lesser extent farmed wild boar, are the major sources of Trichinella for people.  While cannibalism within these host groups can result in transmission of the parasites, infected small wildlife species, especially those in close proximity to pigs and wild boars (e.g., rats), are probably a more important source.  Horses can be naturally infected with Trichinella, and it is assumed this occurs following accidental ingestion of infected wildlife or, more likely, carrion.

The transmission of Trichinella among wildlife depends primarily on food chains (trophic linkages), or "who eats who?".  Trichinella infection has been found in a vast array of free-ranging species but the exact structure of the parasite web maintaining the parasite in an area depends on the structure and function of the local ecosystem.  The consumption of wild (bush) meat by people has increased over recent decades in many parts of the world, and in the United States, and other countries with effective measures for the control of Trichinella in pigs, carnivore and omnivore wildlife species have become the most important source of human infections. 

Most important as sources of Trichinella in people in Canada are bears (polar, grizzly, and black) and walrus (the parasite appears to be rare in seals).  While transmission among bears depends on eating other infected species, or each other in the case of polar bears, the routes of transmission among marine mammals are less clear.  Walruses could acquire the parasite by feeding on seals and perhaps other animals they have killed, or on each other (cannibalism), or on the carcasses of infected terrestrial or marine mammals in the sea or on the shore.  It has been suggested that seals become infected primarily by ingesting amphipod crustaceans and fish that have fed on these carcasses.

Pathology and clinical signs

Domestic animals infected with Trichinella species rarely show any clinical signs, although there are occasional published case reports of disease in dogs and cats associated with both adult parasites in the intestine and larvae in the skeletal muscles.

Infection of people with Trichinella species is much more common than is clinical disease.  Two disease syndromes have been identified in people.  One is associated with the adult parasites in the intestine - with often prolonged diarrhea as the primary symptom, and the other with the larvae in skeletal muscles - with myalgia and fever as the primary symptoms.  Rarely larvae invade the myocardium or the brain, causing cardiac or neurological signs, and sometimes death.  The pathology and clinical signs of the second syndrome are caused primarily by the immune response of the host.  Globally the second syndrome is the more common, and the first is associated primarily with Trichinella acquired in northern regions.  The severity of clinical trichinellosis in people is linked to the number of first-stage larvae ingested and perhaps to the species/genotype of Trichinella.

In a recent outbreak of trichinellosis in people in northern Saskatchewan linked to the consumption of dried/smoked bear meat, a case-control analysis identified diarrhea, skin rash, fever, abdominal pain, malaise, myalgia, periorbital oedema, leg swelling and vomiting as symptoms of the disease, and 28 of the 31 cases identified had one of the key symptoms - diarrhea, oedema, myalgia, and skin rash.  The incubation period in this outbreak was approximately two weeks.

Outbreaks of clinical trichinellosis continue to occur in people in Canada, most often in the North and associated with the consumption of meat from wildlife, particularly bears and walrus.  The disease probably occurs more commonly than reported cases indicate, particularly as the symptoms are similar to those of influenza.


In terms of Trichinella and public health the major need is for detection of the parasite in domestic animal and wildlife providing meat for human consumption.  In live animals it is possible to use a variety of techniques to detect antibodies to Trichinella, but there are some problems with test sensitivity and specificity, the former particularly in the early stages of infection.  In dead animals antibodies can be sought in meat juice, but detection of larvae in skeletal muscles, and sometimes other tissues, is a more common approach.  Especially where such testing is governed by legislation it is very important that the technique(s) used be properly validated.  Although historically trichinoscopy - low power microscopic examination of squashed muscle tissue - was widely used, this technique has now been almost totally replaced by the recovery of larvae from muscle tissue using artificial digestion applied to so-called "predilection sites" (e.g., tongue, diaphragm), which differ among hosts.  It is also very helpful, where possible, to use DNA-based techniques to identify the larvae to species or genotype.

The diagnosis of trichinellosis in people is also based on serology and sometimes muscle biopsy to detect larvae, but the latter technique can have major problems with sensitivity.  History, particularly meat consumption, clinical signs and various haematological parameters can also be helpful.

Treatment and control

Because of the lack of clinical signs (other than very rarely in dogs and cats), the treatment of Trichinella infections in domestic animals and wildlife is very seldom warranted.  Treatment of clinical trichinellosis in people usually includes anthelmintics and corticosteroids, together with supportive measures to counteract the effects of the parasite.

The control of Trichinella infection is usually focused on its public health significance.  For domestic animals this includes measures to minimize or remove the risk of animal infection, and for both domestic animals and wildlife measures to minimize or remove the risk of people consuming infected meat.  The first goal applies primarily to pigs and involves preventing access to meat from potentially infected animals, cannibalism, and access to infected rodents and other wildlife hosts in the immediate environment of the pigs.  Achieving this is easier in intensive management systems with high standards for housing and nutrition, but it is often difficult or impossible where pigs are not housed and/or where management is sub-optimal.

Prevention of consumption of Trichinella-infected meat from domestic animals (primarily pigs) by people has three major elements: 1) assessment of the infection status in a population of pigs using serology; 2) examination of samples from carcasses at slaughter; and 3) use of appropriate methods for the preparation of meat from the carcasses (e.g., cooking to a core temperature of 71C for at least one minute).  Wildlife carcasses are rarely tested for Trichinella prior to consumption of the meat.  In some communities in the Canadian Arctic, however, a novel program has recently been established for the local testing of walrus carcasses  prior to distribution of the meat.  Positive animals are not consumed by people.


Gottstein B, Pozio E., Nockler K (2009) Epidemiology, diagnosis, treatment and control of trichinellosis. Clinical Microbiology Reviews 22: 127-145.

Schellenberg RS, Tan BJK, Irvine JD, Stockdale DR, Gajadhr AA, Serhir b< Botha J, Armstrong CA, Woods SA, Blondeau JM, McNab TL (2003). An outbreak of trichinellosis due to consumption of bear meat infected with Trichinella nativa, in 2 northern communities in Saskatchewan. Journal of Infectious Diseases 188: 835-843.


A useful source of information on Trichinella and trichinellosis is the International Trichinella Reference Centre at the Instituto Superiore di Sanita in Rome, Italy: . As well has valuable information.
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