- Things to Know
- Signs and Symptoms
- Risk Factors
Things to know about trichinosis worm infection
- Trichinosis is caused by eating raw or undercooked pork and wild game infected with the larvae of a parasitic worm.
- The contaminated meat is infected with the larvae of a worm called Trichinella spiralis.
- The initial symptoms of trichinosis are:
- 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.
What is trichinosis worm infection?
Trichinosis is a disease caused by parasitic roundworms (nematodes) that can infect and damage body tissues. Nematodes are a major division of the helminth family of parasitic worms (for example, Trichinella spiralis). When ingested, these parasitic worms can pass through the intestinal tract to invade other tissues, such as muscle, where they persist. Trichinosis is also termed trichinellosis, trichiniasis, or trichinelliasis.
What are the signs and symptoms of trichinosis worms?
Trichinosis is usually characterized by two phases:
- The initial phase (intestinal) of abdominal discomfort, diarrhea, heartburn, and nausea begins one to two days after ingestion.
- The second phase (muscle) of muscle aches, itching, fever, chills, and joint pains begins about two to eight weeks after ingestion.
In addition, there can be "splinter" hemorrhages under the fingernails and eye inflammation (conjunctivitis). People with trichinosis infection may:
- Develop swelling around the eyes
- Low fever
- Abdominal cramps
What causes trichinosis worms? Is it spread by pork?
Trichinosis is caused by Trichinella species (parasitic nematodes, intestinal worms, and roundworms) that initially enter the body when meat contains the Trichinella cysts (roundworm larvae) is eaten. For humans, undercooked or raw pork and pork products, such as pork sausage, have 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 the 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 in cattle feed can infect cattle.
Six species are known to infect humans:
- T. spiralis found in many carnivorous and omnivorous animals worldwide.
- T. britovi is found in carnivorous animals in Europe and Asia.
- T. pseudospiralis found in mammals and birds worldwide.
- T. nativa is found in arctic mammals (for example, bears, and foxes).
- T. nelsoni is found in African mammals (for example, lions, and hyenas).
- T. murrelli is 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. Trichinellosis trichinosis is not spread to a person; infection only occurs when raw undercooked meat containing the parasite is ingested.
In 2017, at least 17 people were diagnosed with the disease after eating smoked brown bear cub meat in Kataiga, Russia.
Trichinosis is not a sexually transmitted disease (STD).
How common is trichinosis worm infection?
Trichinosis is a common disease in many wild carnivorous and omnivorous animals worldwide, except for Australia. Infection is now relatively rare in the United States. From 2015 through 2015, 16 cases were reported per year, on average. For example, the United States has averaged only about 12-20 cases per year in the last two decades. However, some outbreaks 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 become infected as was the situation that occurred in Poland in 2007.
How long can trichinosis worms live?
The life cycle of this particular parasite is key to 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 life cycle 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. The life cycle requires at least two hosts. Humans are usually incidental hosts.
Trichinellosis in humans is acquired by ingesting meat containing cysts (encysted larvae) of Trichinella. 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 (female 2.2 mm in length, males 1.2 mm; life span in the small bowel: four weeks). After one week, the females release larvae that migrate to the striated muscles where they encyst. Trichinella pseudospiralis, however, does not encyst. Encystment is completed in four to five weeks, and the encysted larvae may remain viable for several years. Ingestion of the encysted larvae perpetuates the cycle. Rats and rodents are primarily responsible for maintaining the endemicity of this infection. Carnivorous/omnivorous animals, such as pigs or bears, feed on infected rodents or meat from other animals. Different animal hosts are implicated in the life cycle of the different species of Trichinella. Humans are accidentally infected when eating improperly cooked meat of these carnivorous animals (or eating food contaminated with such meat).
What are the risk factors for getting trichinosis worms?
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 meat. The risk of developing an infection is related to the number 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 results 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. Freeze pork less than 6 inches thick for 20 days at 5 F (-15 C). 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 so the risk of getting trichinosis from meat treated by these methods is increased.
What are the complications associated with trichinosis worms?
- About 90%-95% of trichinosis infections have either minor or no symptoms and no complications.
- The World Health Organization (WHO) allows the use of mebendazole in the second and third trimesters of pregnancy; however, both OB/Gyn and Pediatric specialists should be contacted for utilizing various drugs.
- People with more serious symptoms, however, may develop complications, for example:
- Also, central nervous system (CNS) problems may develop. The CNS symptoms are diverse and include:
For some people with trichinosis, these complications can slowly resolve over six months.
In other patients, the complications can persist for years.
What procedures and tests diagnose trichinosis worms?
There are no accurate tests for the early phase of infection of the intestines. The history of eating raw or undercooked meat could be the first clue. Unfortunately, most people infected 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 with a history of eyelid swelling, pain, tenderness, swelling in muscles, small hemorrhages that resemble small splinters under the fingernails, and the conjunctivitis of the eyes. These symptoms occur a few weeks after eating the raw or undercooked meat from pork or wild animals. Other blood tests that are elevated, but not specific for trichinosis, are creatine kinase, and lactate dehydrogenase (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 count increases can also occur in other parasitic infections and allergies.
More specific tests (indirect immunofluorescence, 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 (several infections occurring at about the same time). For trichinosis, outbreaks occur when several 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.
What is the treatment and prognosis for trichinosis worms? Is there a cure?
- Most trichinosis infections have minor symptoms and do not require any treatment as all symptoms resolve without treatment.
- In those 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.
What prevention steps can you take for trichinosis worms?
Yes, trichinosis can be prevented. The CDC has outlined specific ways to prevent this disease as follows:
- The best way to prevent trichinellosis is to cook meat at safe temperatures. A food thermometer should be used to measure the internal temperature of cooked meat. Do not sample meat until it is cooked. USDA recommends the following for meat preparation.
- For whole cuts of meat (excluding poultry and wild game)
- Cook to at least 145 F (63 C) as measured with a food thermometer placed in the thickest part of the meat, then allow the meat to rest* for three minutes before carving or consuming.
- For ground meat (including wild game, excluding poultry)
- Cook to at least 160 F (71 C); ground meats do not require a rest* time.
- For all wild game (whole cuts and ground)
- Cook to at least 160 F (71 C).
- For all poultry (whole cuts and ground)
- Cook to at least 165 F (74 C), and for whole poultry, allow the meat to rest* for three minutes before carving or consuming.
- For whole cuts of meat (excluding poultry and wild game)
*According to USDA, "A 'rest time' is the amount of time the product remains at the final temperature after it has been removed from a grill, oven, or another heat source. During the three minutes after meat is removed from the heat source, its temperature remains constant or continues to rise, which destroys pathogens."
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"Parasites -- Trichinellosis: Prevention & Control." CDC. Updated: Dec 13, 2013. <http://www.cdc.gov/parasites/trichinellosis/prevent.html>.