Trichinellosis

• Trichinellosis, also called trichinosis, is a disease that people can get by eating raw or undercooked meat from animals infected with the microscopic parasite Trichinella.

• Trichinellosis, also called trichinosis, is caused by eating raw or undercooked meat of animals infected with the larvae of a species of worm called Trichinella. Infection occurs commonly in certain wild carnivorous (meat-eating) animals such as bear or cougar, or omnivorous (meat and plant-eating) animals such as domestic pigs or wild boar.

Trichinella larvae

Clinical features

• The signs, symptoms, severity and duration of trichinellosis vary. Nausea, diarrhea, vomiting, fatigue, fever, and abdominal discomfort are often the first symptoms of trichinellosis. Headaches, fevers, chills, cough, swelling of the face and eyes, aching joints and muscle pains, itchy skin, diarrhea, or constipation may follow the first symptoms. If the infection is heavy, patients may experience difficulty coordinating movements, and have heart and breathing problems. In severe cases, death can occur.

• For mild to moderate infections, most symptoms subside within a few months. Fatigue, weakness, muscle pain, and diarrhea may last for months.

Duration of symptoms

• Abdominal symptoms can occur 1–2 days after infection. Further symptoms usually start 2–8 weeks after eating contaminated meat. Symptoms may range from very mild to severe and relate to the number of infectious worms consumed in meat. Often, mild cases of trichinellosis are never specifically diagnosed and are assumed to be the flu or other common illnesses.

Transmission

• When a human or animal eats meat that contains infective Trichinella larvae, the acid in the stomach dissolves the hard covering of the cyst around the larvae and releases the worms. The worms pass into the small intestine and, in 1–2 days, become mature. After mating, adult females lay eggs. Eggs develop into immature worms, travel through the arteries, and are transported to muscles. Within the muscles, the worms curl into a ball and encyst (become enclosed in a capsule). The life cycle repeats when meat containing these encysted worms is consumed by another human or animal.

• If you eat raw or undercooked meats, particularly bear, pork, wild feline (such as a cougar), fox, dog, wolf, horse, seal, or walrus, you are at risk for trichinellosis.

Trichinella Life Cycle

USA incidence

• Trichinellosis used to be more common and was usually caused by ingestion of undercooked pork. However, infection is now relatively rare. During 2011–2015, 16 cases were reported per year on average. The number of cases decreased beginning in the mid-20th century because of legislation prohibiting the feeding of raw-meat garbage to hogs, commercial and home freezing of pork, and the public awareness of the danger of eating raw or undercooked pork products. Cases are less commonly associated with pork products and more often associated with eating raw or undercooked wild game meats.

Epidemiology & Risk Factors

• People acquire trichinellosis by consuming raw or undercooked meat infected with the Trichinella parasite, particularly wild game meat or pork. Even tasting very small amounts of undercooked meat during preparation or cooking puts you at risk for infection. Outbreaks occur in settings where multiple people consume the same Trichinella-infected meat.

• 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.

Causal Agent

• Trichinellosis (trichinosis) is caused by nematodes (roundworms) of the genus Trichinella. In addition to the classical agent T. spiralis (found worldwide in many carnivorous and omnivorous animals), several other species of Trichinella are now recognized, including T. pseudospiralis (mammals and birds worldwide), T. nativa (Arctic bears), T. nelsoni (African predators and scavengers), T. britovi (carnivores of Europe and western Asia), and T. papuae (wild and domestic pigs of Papua New Guinea and Thailand). Trichinella zimbabwensis is found in crocodiles in Africa but to date there are no known associations of this species with human disease.

Encysted larvae of Trichinella sp. in muscle tissue

Life Cycle

• Depending on the classification used, there are several species of Trichinella: T. spiralis, T. pseudospiralis, T. nativa, T. murelli, T. nelsoni, T. britovi, T. papuae, and T. zimbabwensis, all but the last of which have been implicated in human disease. Adult worms and encysted larvae develop within a single vertebrate host, and an infected animal serves as a definitive host and potential intermediate host. A second host is required to perpetuate the life cycle of Trichinella. The domestic cycle most often involved pigs and anthropophilic rodents, but other domestic animals such as horses can be involved. In the sylvatic cycle, the range of infected animals is great, but animals most often associated as sources of human infection are bear, moose and wild boar.

• Trichinellosis is caused by the ingestion of undercooked meat containing encysted larvae (except for T. pseudospiralis and T. papuae, which do not encyst) of Trichinella species The number 1. After exposure to gastric acid and pepsin, the larvae are released from the cysts The number 2 and invade the small bowel mucosa where they develop into adult worms The number 3. Females are 2.2 mm in length; males 1.2 mm. The life span in the small bowel is about four weeks. After 1 week, the females release larvae The number 4 that migrate to striated muscles where they encyst The number 5. Diagnosis is usually made based on clinical symptoms, and is confirmed by serology or identification of encysted or non-encysted larvae in biopsy or autopsy specimens.

Disease

Sample of bear muscle tissue, digested. Image photographed at 100x magnification.

Trichinella spiralis larvae

The first symptoms of trichinellosis are gastrointestinal, usually occurring 1–2 days after a person consumes raw or undercooked meat from a Trichinella-infected animal. These symptoms include the following:

• Nausea
• Diarrhea
• Vomiting
• Abdominal pain
• The following classic symptoms of trichinellosis often occur within 2 weeks after eating contaminated meat, and can last up to 8 weeks:

• Muscle pain
• Fever
• Swelling of the face, particularly the eyes
• Weakness or fatigue
• Headache
• Chills
• Itchy skin or rash
• Cough
• Diarrhea
• Constipation

Symptoms may range from very mild to severe and relate to the number of infectious worms consumed in the meat. Many mild cases of trichinellosis are never specifically diagnosed because they are assumed to be the flu or other common illnesses. Furthermore, many people with trichinellosis do not experience any symptoms at all.

If the infection is heavy, persons may have trouble coordinating movements and have heart and breathing problems. Although rare, death can occur in severe cases. For mild to moderate infections, most symptoms go away within a few months.

Trichinella larvae in pressed bear meat

Diagnosis

• partially_digested_tspiralis
• Bear muscle showing the encysted larvae at 200x magnification.

• A diagnosis of trichinellosis is made in patients whose signs and symptoms are compatible with trichinellosis, have a positive laboratory test for Trichinella, and who can recall eating raw or undercooked pork or wild game meat.

• Laboratory diagnosis of trichinellosis is most often made by a Trichinella antibody test. In some cases a muscle biopsy may be performed.

Treatment

• Safe and effective prescription drugs are available to treat both trichinellosis and the symptoms that occur as a result of Trichinella infection. Treatment should begin as soon as possible; a doctor will make the decision to treat based upon symptoms, exposure to raw or undercooked meat, and laboratory test results.

Prevention & Control

• The best way to prevent trichinellosis is to cook meat to safe temperatures. A food thermometer should be used to measure the internal temperature of cooked meat. Do not sample meat until it is cooked.

In addition:

• Wash your hands with warm water and soap after handling raw meat.
• Curing (salting), drying, smoking, or microwaving meat alone does not consistently kill infective worms; homemade jerky and sausage were the cause of many cases of trichinellosis reported to CDC in recent years.
• Freeze pork less than 6 inches thick for 20 days at 5°F (-15°C) to kill any worms.
• Freezing wild game meats, unlike freezing pork products, may not effectively kill all worms because some worm species that infect wild game animals are freeze-resistant.
• Clean meat grinders thoroughly after each use.