Trichinella species are nematodes infecting the striated muscle of warm-blooded animals. Human infection with Trichinella (called trichinosis, trichinellosis, or trichiniasis) occurs by consumption of raw or insufficiently cooked infected meat. Trichinella spiralis is the first of at least 9 Trichinella species identified as responsible for disease and is the species most associated with domestic and wild swine. Most human infections are associated with undercooked pork, although horsemeat and wild carnivorous game, such as bear and walrus meat, may also be sources of infection.
PATHOGENESIS AND EPIDEMIOLOGY
When undercooked meat infected with Trichinella cysts is eaten, larvae excyst in the duodenum, invade the mucosa of the small intestine, and develop into tiny adults in 5 to 7 days (Fig. 327-1). Adult nematodes mate in the intestine, fertilized eggs hatch in utero, and larvae are discharged into the gut throughout the 1 to 4 months of the adult female’s life. By the second week, larvae are migrating throughout the body, and by the third week, encystment in striated muscle occurs. Here, the larvae may remain viable for years, but they usually die within 6 to 9 months and slowly calcify.
Mucosal petechiae and gastrointestinal bleeding are possible during the intestinal stage of the disease. The primary lesions are in striated muscle, where there is fiber hypertrophy, edema, and degeneration with an acute interstitial inflammatory exudate. The diaphragm is the most commonly involved muscle; infection is also common in the tongue, masseter, intercostal, extraocular, and laryngeal muscles. Eventually, larvae become trapped in an ovoid cyst. Although larvae do not encyst in the heart, their presence there during migration often causes acute myocarditis. Pathology in the central nervous system includes nonsuppurative meningitis or granulomatous inflammatory changes in the basal ganglia, medulla, and cerebellum. In the lungs, larval migration may produce a transient Löffler pneumonitis or pulmonary edema. Eosinophilia may reach 90% during the height of larval invasion. The host’s immunity is directed against both the adult and migrating larvae.
The disease occurs worldwide in both high- and low-income regions, with outbreaks reported in the United States, Mexico, Southeast Asia, and Europe. Because of the mode of transmission, disease is relatively uncommon in predominantly Muslim and Hindu countries where pork is rarely eaten. Most cases are linked to common source outbreaks from contaminated meat. Pork or pork products account for 75% to 80% of infections. While the worldwide prevalence of trichinosis appeared to decline in the 1970s and 1980s, from 1990 on, there has been a resurgence of this disease in both resourced and underresourced countries. This increase has been attributed both to the mass marketing of meat products, increasing the population at risk from single-source outbreaks, and to the growing proportion of ...