Trichinosis

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Trichinosis

Trichinosis is a disease caused by the roundworm (nematode) called Trichinella spiralis (T. spiralis). It is readily avoided by proper handling and cooking of certain meats, particularly pork products.

Life cycle of Trichinella spiralis

The life cycle of Trichinella spiralis includes several different stages. The adult worm lives in the intestinal lining of such carnivorous animals as swine, bears , walrus, and rodents. After mating, the male worm dies, while the female worm goes on to develop the offspring.

The embryonic stage, a stage often occurring after birth in many nematode species , occurs within the uterus of the female T. spiralis, so that the offspring which are ultimately discharged from the female are in the larval second stage of life. These larvae (about 1500 from each female worm) travel through the circulatory system , to the heart , then through the blood vessels leading to striated muscle (the muscle of the skeletal system and the heart). Any larvae not arriving in striated muscle will die.

Those larvae that reach striated muscle will grow to a length of about one millimeter, coil, and enclose themselves within a protective wall called a cyst. These cysts continue to live for up to 10 years in this form.

Trichinella spiralis in humans

A pig that has been infected with T. spiralis may have thousands of cysts waiting dormant within its muscles—the very muscles that humans look forward to dining on in the form of pork chops, ham, barbecued ribs, etc. When humans sit down to a meal of undercooked, T. spiralis-infected pig dinner, they are ingesting viable T. spiralis cysts. The cyst walls are broken down by the usual process of food digestion in the stomach, allowing the larvae to continue on to the new host's intestine, where the larvae mature to become adult worms, capable of reproducing a new crop of larvae. When these new larvae are born, they begin their migration throughout the human host's bloodstream to the human host's muscles, where they live for a short while before encysting.

Symptoms of trichinosis

A human host who eats T. spiralis infected meat may experience symptoms to a varying degree. If the meat ingested has only a few cysts, then the human host's "worm burden" is said to be relatively small, so that symptomatology will be relatively moderate. In fact, many infections are subclinical (exhibiting such mild symptoms as to remain undiagnosed).

In a host with a greater worm burden, the initial symptoms will be caused by the presence of the adult worms in the intestine, and usually include fever, diarrhea, abdominal pain , and perhaps vomiting. These symptoms begin about one to two days after eating the offending meat, and may last for a week or so.

When larvae begin their migration through the blood vessels, the host will begin to experience systemic symptoms (affecting the whole body), such as fever, swelling of the face and the area around the eyes, rash, bleeding into the nail beds, retina, and whites of the eyes, and cough. In very severe cases, inflammation of the heart muscle (myocarditis), lungs (pneumonitis), or brain (encephalitis ) may occur, which can lead to the few deaths attributable to trichinosis.

The larvae begin to burrow into the host's muscles and form cysts within about two to three weeks of the initial infection . This produces signs of muscle inflammation (myositis), including swelling of the affected muscle groups, pain, and weakness.

The most frequently affected muscles are the muscles outside of the eye , which control eye movements (extraocular muscles), as well as the muscles of the jaw, neck, upper arm (biceps muscle), lower back (lumbar region), and diaphragm (the muscle which separates the abdominal and chest cavities and aids in inspiration).

Symptoms are at their most severe at about three weeks after infection and decrease very slowly in their severity. Recovery is extremely gradual, and symptoms may be present for as long as three months. Fatigue and myalgia (muscle pain) may take several more months to subside.

Diagnosis

An initial diagnosis of trichinosis relies heavily on the presence of the most classic symptoms of trichinosis (including swelling around the eyes, muscle inflammation, fever, and high levels of the blood cells called eosinophils), coupled with the patient's report of having eaten undercooked meat of a species known to potentially carry trichinosis.

The most common food culprit in the United States has been pork sausage, while outbreaks in Europe have been attributed to wild boar and horse meat, outbreaks in Asia and Africa have been due to dog meat, and outbreaks in Northern Canada have occurred due to consumption of walrus and bear meat.

Treatment

Treatment of trichinosis is primarily aimed at decreasing the severity of the symptomatology—bed rest and medications to relieve fever and muscle pain (aspirin, acetaminophen, or ibuprofen). Steroids (such as prednisone) are reserved for the most severe cases of muscle inflammation, or for complicated cases which include myocarditis.

Two related anti-worm medications (mebendazole and thiabendazole) have been reported to work against intestinal larvae, but not against larvae encysted in the muscles. In particular, thiabendazole has been effective when given to patients who knew within 24 hours that they had eaten infected meat.

Prevention

Prevention of trichinosis is relatively simple. Swine should be fed only grain or cooked garbage; uncooked garbage may contain contaminated pork scraps. Meats of animals prone to trichinosis infection should be cooked thoroughly, so that no pink is present. Freezing at an adequately low temperature 5°F (-15°C) for one week or 0°F (-18°C) for three weeks can kill most encysted larvae, except those species which infect arctic mammals such as walrus or bear.

Resources

books

Andreoli, Thomas E., et al. Cecil Essentials of Medicine. Philadelphia: W.B. Saunders Company, 1993.

Berkow, Robert, and Andrew J. Fletcher. The Merck Manual of Diagnosis and Therapy. Rahway, NJ: Merck Research Laboratories, 1992.

Cormican, M.G., and M. A. Pfaller. "Molecular Pathology of Infectious Diseases." In Clinical Diagnosis and Management by Laboratory Methods. 20th ed. Philadelphia: W. B. Saunders, 2001.

Francis, Frederick. Wiley Encyclopedia of Food Science and Technology. New York: Wiley, 1999.

Isselbacher, Kurt J., et al. Harrison's Principles of Internal Medicine. New York: McGraw Hill, 1994.

Kobayashi, G., Patrick R. Murray, Ken Rosenthal, and Michael Pfaller. Medical Microbiology. St. Louis: Mosby, 2003.

Mandell, Douglas et al. Principles and Practice of Infectious Diseases. New York: Churchill Livingstone Inc., 1995.