Trichinosis

Trichinosis
[[Image:|190px|center|]]
ICD-10 B75
ICD-O:
ICD-9 124
OMIM [1]
MedlinePlus [2]
eMedicine /
DiseasesDB 13326

Trichinosis, also called trichinellosis or trichiniasis, is a parasitic disease caused by the roundworm Trichinella spiralis,, which humans generally ingest by eating raw or undercooked pork and wild game products infected with larval cysts of the species. The individual organism of the roundworm is known as trichina (plural, trichinae), meaning "hairlike" (Longe 2006).

While infection is most common from consumption of infected pigs, flesh of animals such as bears and horses can also be a source. The few reported cases (there are many considered unreported) in the United States are mostly the result of eating undercooked game or home reared pigs. It is most common in the developing world and where pigs are commonly fed raw garbage.

Trichinosis is a completely preventable disease, based on personal and social responsibility. On a personal basis, one can prevent infestation by the larval cysts by properly preparing the meat that one consumes. Likewise, feeding pigs cooked, rather than raw, meat can lower their infestation. On a social level, it is important to educate people about the manner in which trichinosis spreads and how it can be prevented.

Contents

Trichinella spiralis

Trichinella spiralis is a species of nematode or roundworm (Phylum Nematoda), which are invertebrates that have long and slender bodies that taper at both ends. They are thin and round in cross section, though they are actually bilaterally symmetric. However, most bilaterially symmetrical animals have a true coelom, unlike the nematodes, which have a pseudocoelom ("false cavity"). Nematodes are one of the simplest animal groups to have a complete digestive system, with separate orifices for food intake and waste excretion, a pattern followed by all subsequent, more complex animals. Nematodes have no circulatory or respiratory systems, so they use diffusion to breathe and for circulation of substances around their body. Nematodes have a simple nervous system, with a main nerve cord running along the ventral side.

The nematode epidermis secretes a layered cuticle made of keratin that protects the body from drying out, from digestive juices, or from other harsh environments. Although this cuticle allows movement and shape changes via a hydrostatic skeletonal system, it is very inelastic and does not allow the volume of the worm to increase. Therefore, as the worm grows, it has to molt (ecdysis) and form new cuticles. As cuticles do not allow volume to increase, high hydrostatic pressure is maintained inside the organism, yielding the round form of the worms.

About 50 species of roundworms cause enormous economic damage or physical suffering as parasites of plants and animals, and humans themselves are host to about 50 different roundworm species (Towle 1989).

Life cycle

Life cycle of Trichinella spiralis

The Trichinella spiralis life cycle involves several stages.

The adult trichina lives in the intestinal lining of a diversity of meat-eating vertebrates, such as bears, pigs, rodents, and walruses (Longe 2006), as well as such herbivores as horses. In Asia, outbreaks have been tied to dog meat, and in Europe to wild boar and horse meat, and in Northern Canada to consumption of walrus and bear meat (Longe 2006).

After reproduction, the male dies while the female produces the offspring (Longe 2006). The embryonic stage of trichinae occurs within the uterus of the female, so that when they are discharged into the intestinal lining, the offspring are in the second stage of life, the larva stage (Longe 2006).

The trichinae larva travel through the lymphatic system to the circulatory system to the heart, and then through the blood vessels to striated muscle (the muscle of the skeletal system and heart) (Longe 2006). Larvae can penetrate any cell, but can only survive in skeletal muscle. Within a muscle cell, the worms grow to about 1 millimeter in length, curl up, and direct the cell functioning much as a virus does. The cell is now called a "nurse cell." Soon, a net of blood vessels surround the nurse cell, providing added nutrition for the larva inside. The larva are now within a protective wall called a cyst. They can live up to ten years in this form (Longe 2006). An infected pig may have thousands of cysts dormant within its muscles (Longe 2006).

When humans consume an infected animal, they ingest the cysts of T. spiralis. The worm can infect any species of mammal that consumes its encysted larval stages. When an animal eats meat that contains infective Trichinella cysts, the acid in the stomach dissolves the hard covering of the cyst and releases the worms. The worms pass into the small intestine and, in 1–2 days, become mature, and repeat the life cycle.

Signs and symptoms

Symptoms can vary widely in severity (depending to some extent on the number of cysts ingested) and according to the worm's life cycle.

If only a few cysts are ingested, symptoms can be mild, even to the point that the infection is not diagnosed. If a lot of cysts are ingested, then the presence of many adult worms in the intestines can be strong. These symptoms include nausea, heartburn, indigestion, diarrhea, and abdominal pain. Symptoms can begin one or two days after eating the infected meat and last for a week or so (Longe 2006).

During the larval migration, the host may experience symptoms such as fever, swelling of face and area around the eyes, bleeding in eyes and nails, and cough. In severe cases, one can get inflammation of heart muscle (myocarditis), lungs (pnemonitis), or brain (encephalitis) (Longe 2006).

When the larva burrow into the host's muscles and form cysts, this can produce muscle inflammation (myositis), pain, and weakness, with such muscles affected as those outside the eye that control eye movement, jaw muscles, neck, upper arm, lower back, and diaphragm (Longe 2006). One may experience itchy skin, and heightened numbers of white blood cells.

If worms penetrate nervous tissue, they cannot survive, but patients may experience difficulty coordinating movements and respiratory paralysis. In severe cases, death may occur. Heart infection can also cause death.

Symptoms are most severe about three weeks after infection and decrease slowly, and symptoms may last three months, with fatigue and muscle pain taking several more months to subside (Longe 2006). Often, mild cases of trichinosis are never specifically diagnosed and are assumed to be the flu or other common illnesses.

Diagnosis

Initial diagnosis relies on classic symptoms, such as swelling around the eyes, muscle inflammation, fever, and high levels of eosinophis (a type of white blood cell) (Longe 2006).

A blood test or muscle biopsy can identify trichinosis. Muscle biopsies can show the presence of larva by the third or fourth week of infection (Longe 2006). Stool studies rarely reveal adult worms, but larva can sometimes be found in blood or duodenal washings after the second week of infection (Longe 2006).

Treatment

Treatment usually addresses reduction of the symptoms, such as using aspirin and corticosteroids. Thiabendazole and mebendeazole can kill adult worms and larva in the intestine; however, there is no treatment that kills the encysted larvae.

Prognosis is generally good, and indeed most people are unaware that they have even been infected (Longe 2006). An estimated 150,000 to 300,000 people are infected in the United States annually and at any given time there are 1.5 million people infected, but most are light cases and not even identified as infections (Longe 2006). The mortality rate is about 1 percent (Longe 2006).

Epidemiology

Trichinosis was known as early as 1835 to have been caused by a parasite. Richard Owen is generally attributed with the discovery of Trichinella spiralis, but it was James Paget, a first year medical student in London, who first described Trichinella infection in a paper presented on February 6, 1835 (Arnold 2005). Owen presented Paget's observation to the Zoological Society.

The mechanism of infection was unclear at the time. It was not until a decade later that American scientist Joseph Leidy pinpointed undercooked ham as a primary source for the parasite (ANS 2007), and not until two decades afterward that this hypothesis was fully accepted by the scientific community.

Infection was once very common, but is now quite rare in the developed world. From 1991 to 1996, an annual average of 12 cases per year were reported in the United States. The number of cases has decreased because of legislation prohibiting the feeding of raw meat garbage to hogs, increased commercial and home freezing of pork, and the public awareness of the danger of eating raw or undercooked pork products. Today, one of the primary causes of trichinosis in America is the consumption of raw or undercooked wild game meats.

In the developing world, most infections are associated with undercooked pork. For example, in Thailand, between 200 and 600 cases are reported annually around the Thai New Year, when communal eating of pork is common (Arnold 2005). In parts of Eastern Europe, the World Health Organization reports that some swine herds have trichinosis infection rates above 50 percent, and there are correspondingly large numbers of human infections (Arnold 2005).

It has been suggested that trichinosis may be one of several factors that led to religious prohibitions in Islam and Judaism against eating pork products, such as in the kashrut and dhabiĥa halal dietary laws. The medieval Jewish philosopher Maimonides advocated such a theory in his Guide for the Perplexed.

Prevention

Trichinosis is a preventable disease. Preventative measures include:

  • Cooking meat products until the juices run clear or to an internal temperature of 140 °F (62 °C).
  • Freezing pork less than 6 inches thick for 20 days at 5&°F (-15°C) or three days at -4°F (-20°C) kills larval worms.
  • Cooking wild game meat thoroughly. Freezing wild game meats, unlike freezing pork products, even for long periods of time, may not effectively kill all worms. This is because the species of trichinella that typically infects wild game is more resistant to freezing than the species that infects pigs.
  • Cooking all meat fed to pigs or other wild animals.
  • Not allowing hogs to eat uncooked carcasses of other animals, including rats, which may be infected with trichinosis.
  • Cleaning meat grinders thoroughly when preparing ground meats.
  • Control and destruction of meat containing trichinae, e.g., removal and proper disposal of porcine diaphragma prior to public sale of meat.

The Centers for Disease Control and Prevention (2004) makes the following recommendation: "Curing (salting), drying, smoking, or microwaving meat does not consistently kill infective worms." However, under controlled commercial food processing conditions some of these methods are considered effective by the United States Department of Agriculture (2007).

References

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