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''Trichinella'' is the genus of parasitic roundworms of the
''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:
* Feeding of animal waste products or other feed contaminated with ''Trichinella''
* Exposure to infected rodents or other infected wildlife
* Cannibalism within an infected herd
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, 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.
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.
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.
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.
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
phylum
In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature ...
Nematoda
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
that cause trichinosis
Trichinosis, also known as trichinellosis, is a parasitic disease caused by roundworms of the '' Trichinella'' type. During the initial infection, invasion of the intestines can result in diarrhea, abdominal pain, and vomiting. Migration of ...
(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
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...
and endoderm
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
).
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 parasite
Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host.
Types of parasites
There are two main types of intracellular parasites: Facultative and Obligate.
Facultative intracellular pa ...
s.
Oral ingestion of larvae-contaminated tissue is the usual route of infection, but congenital and mammary transmission can occur in rats.
Phylogenetics
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.Prevalence in animals and humans
''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:
* Feeding of animal waste products or other feed contaminated with ''Trichinella''
* Exposure to infected rodents or other infected wildlife
* Cannibalism within an infected herd
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, 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.
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.
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.
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.
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
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evi ...
, host range and transmission of ''Trichinella'' species occurring in wildlife in sub-Saharan Africa
Sub-Saharan Africa is, geographically, the area and regions of the continent of Africa that lies south of the Sahara. These include West Africa, East Africa, Central Africa, and Southern Africa. Geopolitically, in addition to the List of sov ...
is limited. However, hypothetical transmission cycles have been proposed for ''Trichinella zimbabwensis
''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 ...
'', ''Trichinella'' T8 and ''Trichinella nelsoni
''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 ...
''; these cycles consider the role of Lions
The lion (''Panthera leo'') is a large cat of the genus '' Panthera'' native to Africa and India. It has a muscular, broad-chested body; short, rounded head; round ears; and a hairy tuft at the end of its tail. It is sexually dimorphic; ad ...
and Spotted Hyenas
The spotted hyena (''Crocuta crocuta''), also known as the laughing hyena, is a hyena species, currently classed as the sole extant member of the genus ''Crocuta'', native to sub-Saharan Africa. It is listed as being of least concern by the IUC ...
as apex predators.
Detection
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
Treatment is by either thiabendazole ormebendazole
Mebendazole (MBZ), sold under the brand name Vermox among others, is a medication used to treat a number of parasitic worm infestations. This includes ascariasis, pinworm infection, hookworm infections, guinea worm infections, hydatid disea ...
.
Prevention
''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). 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'', in order to break the oral ingestion cycle of infection.References
{{Taxonbar, from=Q630285 Parasitic diseases Parasitic nematodes of vertebrates Trichocephalida Veterinary helminthology Enoplea genera