Parasitic Organisms 1847 Life Cycle and Epidemiology

The most important features of this helminth's (nematode) life cycle is its obligatory transmission by ingestion of meat; there is no free-living stage, as exists in many other parasitic nematodes. T. spiralis is cosmopolitan in its distribution. Recent research has shown that the concept of only one species is wrong and that there are at least six to seven genetic types, some with distinct host specificities (12-14). The type that is normally associated with domestic swine is unique and appears to have evolved along with the domestication of Sus scrofa (pig) (12). Although there is some controversy as to the proper taxonomic status and host specificities of various isolates of Trichinella from domestic and wild animals, all appear to be infective for humans.

After ingestion, the larvae are digested out of the muscle capsule in gastric fluid and enter the small intestine, where within 4-6 days they develop into sexually mature males and females. Their offspring (newborn larvae) migrate via the circulatory system throughout the body, invade striated muscles, and eventually (17-21 days after initial infection) become infective, encapsulated larvae. Although larvae may invade smooth muscle and other tissue, they eventually die in these sites. The encapsulated larvae may persist for years in muscle, until they become calcified and die.

The degree of clinical disease in human trichinosis is somewhat dependent on the number of muscle larvae ingested. The ingestion of 500 or more larvae can produce moderate-to-severe and even life-threatening illness. During the first few weeks, if large numbers of worms are present, illness may be reflected by gastrointestinal signs such as nausea and abdominal pain. Subsequent to the production of the muscle-invading larval offspring, the acute muscle phase may be seen. This is usually characterized by muscular pain, facial edema, fever, and eosinophilia.

As pointed out above, the major source of T. spiralis for humans is the domestic pig, although game accounts for about a third of the annual cases. Periodic prevalence studies on swine have revealed a rather marked decline in infection rates among pigs in the United States during the past 85 years, although this varies considerably from one region to another. The prevalence in slaughter hogs has decreased from about 1.4% around the turn of the century to 0.63% in 1952 and 0.13% in 1970 (3). These rates are influenced by the preponderance, in the samples, of grain-fed Midwestern hogs, which account for the majority of production in the United States; prevalence surveys in the Midwest indicate only 0.001% of hogs are infected. Recent surveys (15) indicate that the rates in swine raised in the eastern United States, however, range from 0.5 to 1.0%; in this region swine husbandry includes more high-risk management practices, such a garbage feeding.

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