A number of Trichinella nematode (roundworm) species infect humans and cause trichinellosis (trichinosis). In addition to the classical agent T. spiralis (found worldwide in many carnivorous and omnivorous animals), several other species of Trichinella are now recognized, including T. pseudospiralis (from mammals and birds worldwide), T. nativa (from Arctic bears and walruses), T. nelsoni (from African predators and scavengers), T. britovi (from carnivores, pigs, and horses of temperate Europe and western Asia and northern and western Africa ), T. murelli (from bears and horses in North America), and T. papuae (from wild and domestic pigs and saltwater crocodiles in Papua New Guinea and Thailand). Trichinella zimbabwensis is found in crocodiles and monitor lizards in Africa but there are no known associations of this species with human disease.
Trichinellosis occurs worldwide, but is most common in parts of Europe and the United States. Adult worms and encysted larvae develop within a single vertebrate host and an infected animal serves as a definitive host and potential intermediate host. A second host is required to perpetuate the life cycle. The domestic cycle most often involved pigs and anthropophilic rodents, but other domestic animals such as horses can be involved. In the sylvatic cycle, the range of infected animals is great, but animals most often associated as sources of human infection are bear, moose, and wild boar.
Trichinellosis is caused by the ingestion of undercooked meat containing encysted larvae (except for T. pseudospiralis and T. papuae, which do not encyst) of Trichinella species. After exposure to gastric acid and pepsin, the larvae are released from the cysts and invade the small bowel mucosa where they develop into adult worms. Females are 2.2 mm in length; males 1.2 mm. The life span in the small bowel is about four weeks. After 1 week, the females release larvae that migrate to striated muscles, where they encyst. Diagnosis is usually made based on clinical symptoms and is confirmed by serology or identification of encysted or non-encysted larvae in biopsy or autopsy specimens.
Gottstein et al. (2009) reviewed the epidemiology, diagnosis, treatment, and control of trichinellosis.
(Centers for Disease Control Parasites and Health website; Gottstein et al. 2009)
Trichinella is the genus of parasitic roundworms of the phylum Nematoda that cause trichinosis (also known as trichinellosis). Members of this genus are often called trichinella or trichina worms. A characteristic of Nematoda is the one-way digestive tract, with a pseudocoelom (body cavity made up of only an ectoderm and endoderm).
The genus was first recognised in a larval form in 1835. The L1 larvae live in a modified skeletal muscle cell. The adult worms occupy a membrane-bound portion of columnar epithelium, living as intramulticellular parasites of animals, including humans. Infections with this genus have been reported from more than 150 different naturally or experimentally infected hosts. It has been shown to have a worldwide distribution in domestic and/or sylvatic animals.
Trichinella is the smallest human nematode parasite, yet it is also the largest of all intracellular parasites.
Oral ingestion of cyst- or larvae-contaminated tissue is the usual route of infection, but congenital and mammary transmission can occur in rats.[1]
Eight species are currently recognized. Four additional genotypes require adequate description before they can be recognized as valid species.
Two main clades are recognized in the genus: one group (T. britovi, T. murrelli, T. nativa, T. nelsoni, T. spiralis) that encapsulates in host muscle tissue and a second (T. papuae, T. pseudospiralis, T. zimbabwensis) that does not.
The nonencapsulated group infects saurians, crocodilians, and other nonavian archosaurs (T. papuae, T. zimbabwensis) and birds (T. pseudospiralis). The encapsulated group infects synapsid and mammalian hosts. T. spiralis and T. nelsoni appear to be basal in the encapsulated group and T. murrelli and T. nativa the most recently evolved.
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Trichinella species can infect swine, wild omnivores (foxes, wolves, bears, skunk, raccoons, rats, and other small mammals), and humans.
In swine, the prevalence varies from country to country, and regionally. Long-standing meat inspection programs in some European countries have drastically lowered prevalence rates among domestic swine. Domestic swine can be exposed to the parasite by:
In wild animals, Trichinella infection rates vary from region to region and seem to increase in colder climates. Foxes, wolves, and bears have the highest infection rates,[2][3] but small mammals, such as skunks, raccoons, and rats, provide the highest risk to infecting the domestic pig. In horses, natural infections are rare; however, horses from Mexico and Romania have been found to be infected. Other herbivores, such as moose may also be a potential host and source of Trichinella, however data is inconclusive.[4]
Human infection caused by the domestic pig varies from country to country. While some countries do not report any human infection, other countries in Eastern Europe and Asia report hundreds or thousands of cases annually.
The United States reported 25 cases per year from 1991 to 1996, with few implicating raw or undercooked pork. Documented sources of human infection have also included game meats, such as wild boar, bear, walrus, fox, and cougar. From 1997 to 2001 meats other than pork were the most common source of infection, with more cases associated with home-raised pork than commercial. The decreased incidence of trichinellosis in the United States has resulted from changes in pork industry management standards and government regulations.[5]
In Finland, meat inspection revealed a small but worrisome number of swine infections in the early 1980s, peaking in 1996. However, due to the swine industry's modernization process, the number of cases decreased, with the last infected swine diagnosed in 2004. As of 2010 only eight known human infections had been reported in Finland since the 1800s, with the last one more than three decades before.[6]
A study of the sera from 197 wild boars from 25 farms slaughtered in Finland between 2007 and 2008 found four (2.0%) of the sera, originating from three (12.0%) farms, to be Trichinella-seropositive.[7]
Trichinosis is often diagnosed in humans once the larvae invade the muscle tissue. Some symptoms include fever, myalgia, malaise, and edema. Trichinosis treatment focuses on reducing inflammation, and corticosteroids are usually administered. This treatment often leads to complete recovery, but muscle pain and weakness may persist.
Knowledge on the epidemiology, host range and transmission of Trichinella species occurring in wildlife in sub-Saharan Africa is limited. However, hypothetical transmission cycles have been proposed for Trichinella zimbabwensis, Trichinella T8 and Trichinella nelsoni; these cycles consider the role of Lions and Spotted Hyenas as apex predators.[8]
Artificial digestion is used to detect the presence of encysted trichinella larvae in suspected muscle tissue. The meat sample is dissolved by a digestive solution and the remains are examined for the presence of larvae.
Treatment is by either thiabendazole or mebendazole.
Trichinella infection can be prevented by cooking pork meat properly, or by freezing pork. However, freezing pork is not an effective method for killing larvae.
One way to prevent trichinellosis is to cook meat to a safe temperature (at least 145 °F, 63 °C internal temperature as measured by a food thermometer, followed by a three-minute rest for fresh pork).[9] The meat is not safe until cooking is completed. To help prevent Trichinella infection in animal populations, pigs or wild animals should be prevented from eating uncooked meat, scraps, or carcasses of any animals, including rats, which may be infected with Trichinella,[10] in order to break the oral ingestion cycle of infection.
{{cite web}}: CS1 maint: archived copy as title (link) Trichinella is the genus of parasitic roundworms of the phylum Nematoda that cause trichinosis (also known as trichinellosis). Members of this genus are often called trichinella or trichina worms. A characteristic of Nematoda is the one-way digestive tract, with a pseudocoelom (body cavity made up of only an ectoderm and endoderm).
The genus was first recognised in a larval form in 1835. The L1 larvae live in a modified skeletal muscle cell. The adult worms occupy a membrane-bound portion of columnar epithelium, living as intramulticellular parasites of animals, including humans. Infections with this genus have been reported from more than 150 different naturally or experimentally infected hosts. It has been shown to have a worldwide distribution in domestic and/or sylvatic animals.
Trichinella is the smallest human nematode parasite, yet it is also the largest of all intracellular parasites.
Oral ingestion of cyst- or larvae-contaminated tissue is the usual route of infection, but congenital and mammary transmission can occur in rats.
