Summary
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.
Taxonomy
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 |
Morphology
Host range and geographic distribution
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 murrelli, T. 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
Epidemiology
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
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.
Diagnosis
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
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.
References
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.