Trichinosis (cont.)
What is the history and life cycle of trichinosis?
The life cycle of this particular parasite is important in the way disease develops in humans. In 1835, J. Paget and R. Owen first discovered larvae of
Trichinella in a piece of human muscle during an autopsy. That same year, R. Virchow and F. Zenker discovered the complicated lifecycle of
Trichinella by feeding a dog Trichinella-infected meat.
In humans, the larvae are ingested, released from an encapsulated (encysted) cyst by stomach acid (except for the nonencapsulated
T. pseudospiralis), and then develop into adult male and female worms in the gastrointestinal tract. The adult worms mate and then shed larvae that penetrate the gastrointestinal tract and reach the bloodstream and lymph drainage system. The larvae are then distributed to body cells (mainly skeletal muscle cells). Some skeletal muscle cells develop into nurse cells that support and protect the larvae from the host's immune system. For a more complete description of the life cycle that requires at least two hosts, please see http://www.dpd.cdc.gov/dpdx/HTML/Trichinosis.htm with further details and photographs.
How is trichinosis diagnosed?
There are no good tests for the early phase of infection of the intestines. The history from the patient telling the doctor that he or she ate raw or undercooked meat could be the first clue. Unfortunately, most patients do not seek physician help during the relatively short intestinal phase. During the muscle phase of the disease, a presumptive clinical diagnosis can be made in patients that have a history of eyelids swelling, pain, tenderness, and swelling in muscles, especially skeletal muscles and small hemorrhages (bleeding into tissues) under the fingernails and the conjunctivae of the eye that resemble splinters that occur a few weeks after eating raw or undercooked meat from pork or wild animals. Other laboratory findings that are elevated, but not specific for trichinosis, are creatine kinase, and lactate dehydrogenase, two enzymes that increase in the blood when muscle cells are damaged or destroyed. Also, a particular type of white blood cell, eosinophils, are usually increased several times their normal concentration after the muscle phase starts, but eosinophil increases can also occur in other parasitic infections.
More specific tests (indirect immunofluoresence, latex agglutination, enzyme-linked immunosorbent assays) are available that detect antibodies developed by the infected person's immune response to the parasites. However, these tests may not be positive until three or more weeks after infection and may be falsely positive in patients with infections with other parasites or autoimmune diseases. The best test for trichinosis is a biopsy of muscle that shows larvae in the muscle tissue. In general, biopsies are done infrequently and the diagnosis is based on presumptive clinical findings, patient history, and associated blood tests outlined above. In addition, trichinosis often occurs in outbreaks (a number of infections occurring at about the same time). For trichinosis, outbreaks occur when a number of people eat infected meat from the same source. For example, in 2007, over 200 patients were diagnosed with trichinosis in Poland when a meat-processing plant sold
Trichinella-contaminated pork to customers. Knowledge of the source of an outbreak can help identify and diagnose individuals that may be exposed to the parasites; it can also allow the source of the infections to be eliminated.
Next: How is trichinosis treated? »
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