Trichinosis, a disease caused by the parasitic intestinal roundworm Trichinella spiralis (trichinae), is contracted by warm-blooded mammals through ingestion of raw or undercooked meat infected by trichinae. Pork and bear meat are primary sources of human infection; beaver, opossums, rats, walruses and whales can also carry the parasite. Infected animals remain asymptomatic; however, symptoms in humans--which can begin as soon as five or a late as 45 days after exposure--can range from asymptomatic to death. Symptoms include fluid retention in the upper eyelids, diarrhea, physical weakness, excessive thirst and sweating, chills, fever, muscle pain, anorexia, breathing difficulties, and perhaps even kidney and heart damage. Severity depends upon the number of parasites ingested. Although trichinosis is found in some grain-fed pigs, swine fed on garbage containing infected meat scraps is the primary source of human trichinosis. In 1954, a campaign to cook garbage before feeding it to swine was implemented in the United States. Inspection of meat in packing plants also helps prevent human infection.
Trichinosis was discovered in 1835 by James Paget, a 21-year-old, first-year medical student in London who, during an autopsy, noticed tiny specks in the muscle tissue of an Italian man. Under a microscope, they turned out to be tiny cysts housing worm larvae which were given their name by his professor. The first record of trichinae in meat for human consumption was in 1846 when Philadelphian physician, Joseph Leidy, noticed tiny specks in a slice of pork he was eating and recalled seeing similar specks in human muscle tissue just days earlier. Under the microscope, they were indeed trichinae. In the 1850s, German scientists Rudolf Leuckart and Rudolph Virchow found the parasite was transmitted animal-to-animal through ingestion of infected meat or feces.
Not until 1860, however, was trichinosis found to cause severe illness and death in humans when German physician, Friedrich A. von Zenker, discovered the parasite during an autopsy of a young servant woman. Tracing her illness, he found she had tasted raw pork sausage before cooking it for Christmas dinner. Although her employers also became sick, the effect was less severe because cooking reduced the number of parasites. von Zenker sent samples of the girl's tissue to Leuckart and Virchow, who traced the complete life cycle of the parasites.
Trichinae larvae migrate to muscle tissue and form a housing, or cyst. The larvae in these cysts hatch into adult worms only after coming into contact with digestive juices in the stomach. The hatched larvae pass into the intestines, mate, and reproduce. One female adult worm can produce up to 1,500 larvae, which then penetrate the intestines, enter the blood stream, pass through the heart, and travel throughout the entire body. They invade the voluntary muscle tissue, feed for about three weeks, coil up tightly, develop their protective housing, and lie dormant--often for many years--until their host dies. If the host tissue is ingested by another animal or a human, contact with the digestive juices causes the larvae to hatch and the life cycle begins again. Although antiparasitic medications will kill adult worms and intestinal larvae, once larvae enter the muscle tissue, they are there for the life of the host and treatment can only relieve symptoms. Experimental studies of Thiabendazole in animals do show reduction in both muscle and intestinal infections, however, and the drug has been used in a few human cases. The only sure way to prevent infection is to cook meat--particularly pork--until the internal temperature is at least 171°F (76.4°C), or freeze it at -13 °F (-24.8°C) for 10 to 20 days.
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