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. Trichinosis is not to be confused with trichomoniasis, a sexually transmitted disease caused by the Trichomonas vaginalis parasite.
Worldwide, an estimated 10,000 cases of trichinellosis occur every year. Several different species of Trichinella can cause human disease; the most common species is Trichinella spiralis, which has a global distribution and is the species most commonly found in pigs. Other Trichinella species are less commonly reported as the cause of human disease and may be found in different parts of the world, usually infecting wild animals.
In the United States, trichinellosis cases are reported to CDC much less commonly now than in the past. During the late 1940s, when the U.S. Public Health Service began counting cases of trichinellosis, 400 cases in the United States were recorded each year on average. During 2008-2010, 20 cases were reported to CDC each year on average. The overall number of cases reported has decreased because of improved pig-raising practices in the pork industry, commercial and home freezing of pork, and public awareness of the danger of eating raw or undercooked meat products. The number of cases associated with raw or undercooked wild game meats has remained relatively constant over time. Over the past 40 years, few cases of trichinellosis have been reported in the United States, and the risk of trichinellosis from commercially raised and properly prepared pork is very low. However, eating undercooked wild game, particularly bear meat, puts one at risk for acquiring this disease.
Life cycle of Trichinella
Risk factors for trichinosis include:
Improper food preparation: Trichinosis infects humans when they eat undercooked infected meat, such as pork, bear or walrus, or other meat contaminated by grinders or other equipment.
Rural areas: Trichinosis is more common in rural areas. In the United States, higher rates of infection are found in hog-raising regions.
Consumption of wild or non-commercial meats: Public health measures have greatly decreased the incidence of trichinosis in commercial meats, but non-commercial, farm-raised animals have higher rates of infection particularly those with access to wild-animal carcasses. Wild animals, such as bears and walruses, are still a common source of infection.
Trichinosis is caused by Trichinella species (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 in 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.
Trichinosis symptoms vary, depending on the stage of infection, number of invading larvae, tissues invaded, and general physical condition of the person. Many people have no symptoms.
Symptoms of trichinosis occur in two stages.
Stage 1: Intestinal infection develops 1 to 2 days after eating contaminated meat. Symptoms include nausea, diarrhea, abdominal cramps, and a slight fever.
Stage 2: Symptoms from the larval invasion of muscles usually start after about 7 to 15 days. Symptoms include muscle pain and tenderness, weakness, fever, headache, and swelling of the face, particularly around the eyes. The pain is often most pronounced in the muscles used to breathe, speak, chew, and swallow. A rash that does not itch may develop. In some people, the whites of the eyes become red, and their eyes hurt and become sensitive to bright light.
If many larvae are present, the heart, brain, and lungs may become inflamed. Heart failure, abnormal heart rhythms, seizures, and severe breathing problems may result. Death can occur but is rare.
Without treatment, most trichinosis symptoms disappear by the third month of infection, although vague muscle pain and fatigue can persist longer.
Diagnosis and test
- Your doctor may be able to diagnose trichinosis by taking your medical history and asking you about your symptoms. They may also perform certain diagnostic tests to determine whether there any larvae present in your system.
- Your doctor may take a sample of your blood and test it for signs of trichinosis. Elevated levels of white blood cells and the presence of antibodies against the parasite may indicate a Trichinella infection.
- Your doctor may also perform a muscle biopsy if the blood test results are inconclusive. During a muscle biopsy, your doctor will remove a small piece of muscle tissue and analyze it for the presence of Trichinella larvae.
Treatment and medications
- 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.
The best way to prevent trichinosis is to prepare food properly. Here are some tips to follow when cooking meat:
- Use a meat thermometer.
- Don’t sample meat until it’s cooked.
- Cook ground meat and wild game to at least 160°F (71°C).
- Cook whole cuts of meat to at least 145°F (63°C).
- Cook poultry to at least 165°F (74°C).
- Freeze pork less than 6 inches thick for 20 days at 5°F (-15°C) to kill any worms.
- Avoid eating walrus, horse, or bear meat.
- Thoroughly clean any utensils that touch meat.
- Clean meat grinders thoroughly.
- Wash hands thoroughly after handling raw meat.
To prevent a Trichinella infection among animals, don’t allow pigs or wild animals to eat the undercooked meat, scraps, or carcasses of animals that may be infected with Trichinella larvae.