Trichinellosis: A rare parasitic infection
Trichinellosis is a parasitic infection occurring in people who eat meat-eating mammals. It is reported to cause about 10,000 infections in humans worldwide each year.
It has become a rare disease in the United States because of improvements in commercial animal husbandry involving pigs, the major source of trichinellosis worldwide. About 10 to 20 cases were reported annually in the U.S. in recent years. Many cases occur in clusters of two or more, related to sharing of the infected meat. Cases occur in the U.S. from ingestion of undercooked meat from feral pigs or bear or other meat-eating wild animals or from “backyard” pigs that have been allowed to forage for garbage. These pigs may get infected by ingesting infected rodents during their foraging.
Although there are other species of Trichinella that can infect humans, Trichinella spiralis is the species that by far most commonly causes human infection. Trichinella murrelli is thought to be the predominant species infecting sylvatic hosts in North America and has been found in bears, raccoons, coyotes and a dog. Meats from herbivores have been reported to cause trichinellosis when ground in a meat grinder that had been previously used to grind pork or other infected meats.
Understanding the life cycle of Trichinella is the key to understanding the signs and symptoms of trichinellosis and their timing as the disease evolves in an infected individual. The life cycle is very similar in animals and humans.
Life cycle of T. spiralis
T. spiralis larvae may be found in cysts in the meat from any meat-eating animal (carnivorous or omnivorous), including bears, walruses, dogs, cats, rodents and, most importantly, swine. There are other species of Trichinella (eg, Trichinella pseudospiralis) that rarely cause disease in the U.S., where the larvae do not form cysts but remain viable in muscle. When humans ingest these larvae or cysts in meat that has not been cooked adequately to kill the larvae, the larvae emerge and grow to the worm stage after about 1 week in the small intestine. During the month that these worms live in the small intestine, the females give rise to larvae that invade the wall of the small intestine and are carried to various organs and muscle in the bloodstream. The larvae encyst in striated muscle and degenerate elsewhere. Once encysted, the larvae can remain viable for years.
Rodents are important for maintaining the life cycle in wild animals and in swine that are not raised using good animal husbandry.
The severity of symptoms depends upon the number of cysts ingested. Because the dose of cysts is often small, the great majority of cases of trichinellosis are asymptomatic or only mildly symptomatic. The initial symptoms of trichinellosis are gastrointestinal, related to the ingested larvae burrowing into the mucosa of the small intestine and developing into adult worms. These symptoms develop after an incubation period of 24 to 48 hours. The patient may develop nausea, vomiting and abdominal pain. Diarrhea may follow. Fatigue may be a feature of this stage of the illness.
Days later, usually by 2 weeks after infection, symptoms occur that coincide with larvae circulating in the blood and lodging in various organs of the body and in muscle. There is fever (which may be high), muscle pain and tenderness, weakness, periorbital edema, headache, pruritus and skin rash. These symptoms can last for as long as 8 weeks or even longer. Splinter hemorrhages and conjunctival petechiae may be observed. As a result of larvae degenerating at various sites, the patient may develop myocarditis, encephalitis, pneumonia and other manifestations. Death is rare but may occur.
Cases of trichinellosis often occur in clusters from more than one person eating the infected food. The diagnosis in an individual is often suspected because of the epidemiological circumstances of multiple cases and the food ingested (eg, bear meat). The clinical findings of fever, muscle pain, periorbital edema and high eosinophilia is suggestive of the disease. In fact, trichinellosis is one of the relatively few diseases that can cause extraordinarily high eosinophil counts. Serological tests for antibodies against Trichinella will confirm the diagnosis, as will a biopsy of striated muscle demonstrating larvae. The serological tests do not turn positive until several weeks into the course of the disease. Muscle biopsy can be helpful in diagnosis in questionable cases but is not usually necessary.
Elevated levels of muscle enzymes (eg, creatinine phosphokinase, lactate dehydrogenase and aldolase) in blood, which indicate myositis, are found in 75% to 90% of infected persons between the second and fifth weeks of infection and may persist for up to 4 months.
In mild cases of trichinellosis, symptomatic treatment with antipyretics and analgesics is sufficient. More severe disease should be treated with antiparasitic drugs and corticosteroids. Treatment with antiparasitic drugs seem to be most helpful when used early in the course of the disease and directed against the intestinal worms. The efficacy of antiparasitic drugs aimed at larvae that are invading muscle or are encysted is questionable.
Albendazole or mebendazole are the antiparasitic drugs of choice. Both are given orally. Albendazole is dosed at 400 mg twice a day for 8 to 14 days and mebendazole at 200 to 400 mg three times a day for 3 days, then 400 to 500 mg three times a day for 10 days.
There may be an advantage to albendazole because blood levels are more reliable.
Thanks to the use of proper husbandry and hygiene in raising of pigs (ie, commercial pork produced indoors, restrictions on garbage feeding and effective rodent control), Trichinella has largely been eliminated from pork in the U.S. and many other countries. Proper cooking or freezing of game meat will kill the larvae and prevent trichinellosis.
Cooking to an internal temperature of 160°F is sufficient to kill larvae in most meats. One exception listed by the CDC is poultry, for which a temperature of 165°F is recommended.
Freezing of pork less than 6 inches thick for 20 days at or below 5°F will kill Trichinella. This may not be adequate for wild game meats.
- CDC. Parasites – Trichinellosis (also known as trichinosis). https://www.cdc.gov/parasites/trichinellosis/. Accessed June 25, 2020.
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- For more information:
- Donald Kaye, MD, MACP, is a professor of medicine at Drexel University College of Medicine, associate editor of the International Society of Infectious Diseases’ ProMED-mail, section editor of news for Clinical Infectious Diseases and an Infectious Disease News Editorial Board Member.
Editor’s note: Kaye died July 4. To read Healio’s obituary, please click here.