Trichinosis (Trichinellosis)

What is trichinosis? What are symptoms of trichinosis?

Trichinosis (also termed trichinellosis, trichiniasis, or trichinelliasis) is a disease caused by parasites, called roundworms (Trichinella spp.), that can infect and damage many body tissues. Although the parasites can pass through the intestinal tract and other tissues, muscle tissues are where the majority of them persist. Trichinosis is usually characterized by two phases; the initial phase (intestinal) of abdominal discomfort, diarrhea, and nausea that begins one to two days after ingestion and the second phase (muscle) of muscle aches, itching, fever, chills, and joint pains that begins about two to eight weeks after ingestion.

What causes trichinosis?

Trichinosis is caused by Trichinella species (also termed parasitic nematodes, intestinal worms, and roundworms) that initially enter the body when meat containing the Trichinella cysts (roundworm larvae) is eaten. For humans, undercooked or raw pork and pork products, such as pork sausage, has been the meat most commonly responsible for transmitting the Trichinella parasites. It is a food-borne infection and not contagious from one human to another unless infected human muscle is eaten. However, almost any carnivore (meat eater) or omnivore (eats meat and plants for food) can both become infected and, if eaten, can transmit the disease to other carnivores and omnivores. For example, undercooked or raw bear meat can contain livable Trichinella cysts. Therefore, if humans, dogs, pigs, rats, or mice eat the meat, they can become infected. In rare instances, larvae that inadvertently reaches cattle feed can infect cattle. There are six species that are known to infect humans:

• T. spiralis found in many carnivorous and omnivorous animals worldwide
• T. britovi found in carnivorous animals in Europe and Asia
• T. pseudospiralis found in mammals and birds worldwide
• T. nativa found in arctic mammals (for example, bears, foxes)
• T. nelsoni found in African mammals (for example, lions, hyenas)
• T. murrelli found in wild animals in the U.S.

Two other species, T. papuae (found in pigs in New Guinea) and T. zimbabwensis (found in crocodiles in Tanzania) have not been reported to infect humans to date. There are other strains (antigenic variants related to named species) that are unnamed and can infect humans.

What is the history and life cycle of trichinosis?

The life cycle of this particular parasite is important in the way disease develops in humans. In 1835, J. Paget and R. Owen first discovered larvae of Trichinella in a piece of human muscle during an autopsy. That same year, R. Virchow and F. Zenker discovered the complicated lifecycle of Trichinella by feeding a dog Trichinella-infected meat.
In humans, the larvae are ingested, released from an encapsulated (encysted) cyst by stomach acid (except for the nonencapsulated T. pseudospiralis), and then develop into adult male and female worms in the gastrointestinal tract. The adult worms mate and then shed larvae that penetrate the gastrointestinal tract and reach the bloodstream and lymph drainage system. The larvae are then distributed to body cells (mainly skeletal muscle cells). Some skeletal muscle cells develop into nurse cells that support and protect the larvae from the host's immune system. For a more complete description of the life cycle that requires at least two hosts, please see http://www.dpd.cdc.gov/dpdx/HTML/Trichinosis.htm with further details and photographs.

How is trichinosis diagnosed?

There are no good tests for the early phase of infection of the intestines. The history from the patient telling the doctor that he or she ate raw or undercooked meat could be the first clue. Unfortunately, most patients do not seek physician help during the relatively short intestinal phase. During the muscle phase of the disease, a presumptive clinical diagnosis can be made in patients that have a history of eyelids swelling, pain, tenderness, and swelling in muscles, especially skeletal muscles and small hemorrhages (bleeding into tissues) under the fingernails and the conjunctivae of the eye that resemble splinters that occur a few weeks after eating raw or undercooked meat from pork or wild animals. Other laboratory findings that are elevated, but not specific for trichinosis, are creatine kinase, and lactate dehydrogenase, two enzymes that increase in the blood when muscle cells are damaged or destroyed. Also, a particular type of white blood cell, eosinophils, are usually increased several times their normal concentration after the muscle phase starts, but eosinophil increases can also occur in other parasitic infections.
More specific tests (indirect immunofluoresence, latex agglutination, enzyme-linked immunosorbent assays) are available that detect antibodies developed by the infected person's immune response to the parasites. However, these tests may not be positive until three or more weeks after infection and may be falsely positive in patients with infections with other parasites or autoimmune diseases. The best test for trichinosis is a biopsy of muscle that shows larvae in the muscle tissue. In general, biopsies are done infrequently and the diagnosis is based on presumptive clinical findings, patient history, and associated blood tests outlined above. In addition, trichinosis often occurs in outbreaks (a number of infections occurring at about the same time). For trichinosis, outbreaks occur when a number of people eat infected meat from the same source. For example, in 2007, over 200 patients were diagnosed with trichinosis in Poland when a meat-processing plant sold Trichinella-contaminated pork to customers. Knowledge of the source of an outbreak can help identify and diagnose individuals that may be exposed to the parasites; it can also allow the source of the infections to be eliminated.

How is trichinosis treated?

Most trichinosis infections, according to the experts, are subclinical or have minor symptoms and do not require any treatment as they are self-limited (all symptoms resolve without treatment). In patients with more intense symptoms, thiabendazole (Mintezol) can be used to eliminate the adult worms in the gastrointestinal tract. Albendazole (Albenza) is another drug that may be used in some cases. The invasive and encysted larva forms of Trichinella species are treated by mebendazole (Vermox). Inflammation of infected tissues is usually treated with prednisone and is frequently used in combination with mebendazole.

Are there complications associated with trichinosis?

About 90%-95% of trichinosis infections have either minor or no symptoms and no complications. Patients with more serious symptoms, however, may develop complications such as heart muscle inflammation (myocarditis), pulmonary problems such as cough, short of breath or lung hemorrhage (lung bleeding). Also, central nervous system (CNS) problems may develop. The CNS symptoms are diverse; they include confusion, delirium, ataxia, seizures, vertigo, auditory and speech changes along with many other neurological deficits. For some patients, these complications can slowly resolve over six months. In other patients, the complications can persist for years.

What is the prognosis for patients with trichinosis?

For those who have minor or no symptoms, the prognosis is excellent with no complications. For the minority of patients that develop complications, the prognosis is still good as most will recover and the symptoms will decrease and disappear. However, for a few patients, the prognosis is less clear as complications, especially those with the brain disease, which may persist for many years. Less than 0.1%-0.3% patients with trichinosis die from the disease.

What are the risk factors for getting trichinosis?

The major risk factor for contracting trichinosis is eating raw or undercooked pork or wild game such as bear meat, wild boar meat, or other carnivore or omnivore meat. The risk of developing infection is related to the amount of larvae ingested. About 10 larvae per gram of meat (muscle) ingested result in a mild infection, about 50-500 result in a moderate infection while over 1,000 result in a severe infection. Consequently, reducing or eliminating the number of larvae in infected meat reduces both the risk and severity of infection. This is accomplished by cooking meat to a uniform temperature of 70 degrees Celsius (158 degrees Fahrenheit) or higher for at least a few minutes to kill encysted larvae. Freezing of pork meat to a temperature of -30 degrees Celsius (-22 degrees Fahrenheit) for one week reportedly kills the encysted larvae. However, this freezing technique for wild game meat may not be effective, as Trichinella in wild animals is usually not as susceptible to freeze killing. Other cooking methods such as microwaving, smoking, or salting meat often fail to kill the encysted larvae and thus the risk of getting trichinosis from meat treated by these methods is increased.

Is trichinosis a common disease?

Trichinosis is a common disease in many wild carnivorous and omnivorous animals worldwide, except for Australia. Because of strict measures by the meat (pork) industry and public-health authorities, especially in developed countries, the incidence of disease in humans has dropped dramatically in the last few decades. For example, the United States has averaged only about 12-20 cases per year in the last two decades. However, there are outbreaks that occur sporadically in the world when meat (especially pork) is improperly cooked or is contaminated with undercooked or raw wild-game meat. If the source of the outbreak is a meat supplier, hundreds of people can be infected as was the situation that occurred in Poland in 2007.

Where can I get more information on trichinosis?

http://www.dpd.cdc.gov/dpdx/HTML/Image_Library.htm
http://www.dpd.cdc.gov/dpdx/HTML/Trichinellosis.htm
http://www.trichinella.org/index.htm
http://www.eurosurveillance.org/ew/2007/070719.asp#2
http://www.trichinella.org/bio_nursecell.htm

Trichinosis At A Glance

• Trichinosis is caused by eating raw or undercooked pork and wild game.
• The contaminated meat is infected with the larvae of a worm called Trichinella spiralis.
• The initial symptoms of trichinosis are abdominal discomfort, nausea, diarrhea, vomiting, fatigue, and fever.
• The severity of symptoms depends on the number of infectious worms consumed in the meat.
• Never eat raw or undercooked pork or wild game.
• If you think you may have trichinosis, seek medical attention.

Medical Author:

Charles Patrick Davis, MD, PhD

Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

Source:Medicinenet.com.