Trichinosis

TRICHINOSIS

EPIDEMIOLOGY

Trichinosis is widespread in carnivores. Among domestic animals, swine are most fre-quently involved. They acquire the infection by eating rats or garbage containing cyst-laden scraps of uncooked meat. Human infection, in turn, results largely from the consumption of improperly prepared pork products. In the United States, most outbreaks have been traced to ready-to-eat pork sausage prepared in the home or in small, unlicensed butcheries. Disease incidence is highest in Americans of Polish, German, and Italian de-scent, presumably because of their custom of producing and eating such sausage during holidays. Recent outbreaks have been reported among Indochinese refugees, apparently related to undercooking of fresh pork. Clusters have also followed feasts of wild pig in California and Hawaii. At present, nearly one third of human cases in the United States, particularly those in Alaska and other western states, have been attributed to consumption of the meat of wild animals, particularly bears. Outbreaks among Alaskan and Canadian Inuit populations have followed the ingestion of raw T. nativa–infected walrus meat. Several recent outbreaks in Europe have involved horse meat or wild boar. Each year, a few cases are acquired from ground beef intentionally but illegally adulterated with pork.

Human infections occur worldwide. In the United States, the prevalence of cysts found in the diaphragms of patients at autopsy has declined from 16.1 to 4.2% over a period of 30 years. This decline has been attributed to decreased consumption of pork and pork products; federal guidelines for the commercial preparation of such foodstuffs; the widespread practice of freezing pork, which kills all but arctic strains of Trichinella; and legislation re-quiring the thorough cooking of any meat scraps to be used as hog feed. Nevertheless, it is estimated that more than 1.5 million Americans carry live Trichinella in their musculature and that 150,000 to 300,000 acquire new infection annually. Fortunately, the overwhelming majority are asymptomatic, and only about 100 clinically recognized cases are reported an-nually to federal officials. In other areas of the world, infection is more commonly acquired from sylvatic sources, including wild boar, bush pigs, and warthogs.

PATHOGENESIS AND IMMUNITY

The pathologic lesions of trichinosis are related almost exclusively to the presence of larvae in the striated muscle, heart, and central nervous system. Invaded muscle cells enlarge, lose their cross-striations, and undergo a basophilic degeneration. Surrounding the involved area is an intense inflammatory reaction consisting of neutrophils, lympho-cytes, and eosinophils. With the development of specific IgG and IgM antibodies, eosinophil-mediated destruction of circulating larvae begins, production of new larvae is slowed, and the expulsion of adult worms is hastened. A vasculitis demonstrated in some patients has been attributed to deposition of circulating immune complexes in the walls of the vessels.