The initial symptoms of the disease are abdominal discomfort, nausea, diarrhea, vomiting, fatigue and fever. Next usually come headaches, fevers, chills, cough, eye swelling, aching joint muscle pains, itchy skin, diarrhea, or constipation. With heavy infection, patients may experience difficulty coordinating movements and have heart and breathing problems. In severe cases, death can occur. The severity of symptoms depends on the number of infectious worms consumed in meat.
The disease is caused by nematodes (roundworms) of the genus Trichinella. In addition to the classical agent Trichinella 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), and T. britovi (carnivores of Europe and western Asia). Humans ingest 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: 4 weeks). After 1 week, the females release larvae that migrate to the striated muscles where they encyst. Trichinella pseudospiralis, however, does not encyst. Encystment is completed in 4 to 5 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 processed meat of these carnivorous animals (or eating food contaminated with such meat).
As a result of improvements in swine production, trichinellosis has declined steadily in the US. However, infection can also result from eating the meat of wild animals. During 1997-2001, a total of 72 cases of trichinellosis (median: 12 cases annually; range: 11-23 cases) were reported to the CDC (US Centers for Disease Control and Prevention). The majority of these infections came from eating wild game, predominately bear. Wild game meat is becoming progressively more popular and those who prefer to consume not fully cooked meat are at increased risk of infection with trichinella. The CDC advises everyone to cook all meats, particularly wild game, to an internal temperature of 160º F (71º C).
To avoid trichinellosis:
- Cook meat until the juices run clear or to an internal temperature of 170 degrees F (77 degrees C).
- Freeze pork less than 6 inches (15 cm) thick for 20 days at 5 degrees F (-15 degrees C) to kill any worms.
- Cook wild game meat thoroughly. (Freezing wild game meats, unlike freezing pork products, even for long periods of time, may not effectively kill all worms.)
- Cook all meat fed to pigs or other wild animals and do not allow hogs to eat uncooked carcasses of other animals, including rats (which may be infected with trichinosis).
- Clean meat grinders thoroughly if you prepare your own ground meats.
- Remember that curing (salting), drying, smoking, or microwaving meat does not consistently kill infective worms.
If you think you may have trichinellosis, seek medical attention.