Trichinellosis develops after the ingestion of meat containing cysts of Trichinella (e.g., pork or other meat from a carnivore). Although most infections are mild and asymptomatic, heavy infections can cause severe enteritis, periorbital edema, myositis, and (infrequently) death.
Life Cycle and Epidemiology
Eight species of Trichinella are recognized as causes of infection in humans. Two species are distributed worldwide: T. spiralis, which is found in a great variety of carnivorous and omnivorous animals, and T. pseudospiralis, which is found in mammals and birds. T. nativa is present in Arctic regions and infects bears; T. nelsoni is found in equatorial eastern Africa, where it is common among felid predators and scavengers such as hyenas and bush pigs; and T. britovi is found in Europe, western Africa, and western Asia among carnivores but not among domestic swine. T. murrelli is present in North American game animals.
After human consumption of trichinous meat, encysted larvae are liberated by digestive acid and proteases (Fig. 256-1). The larvae invade the small-bowel mucosa and mature into adult worms. After ~1 week, female worms release newborn larvae that migrate via the circulation to striated muscle. The larvae of all species except T. pseudospiralis, T. papuae, and T. zimbabwensis then encyst by inducing a radical transformation in the muscle cell architecture. Although host immune responses may help to expel intestinal adult worms, they have few deleterious effects on muscle-dwelling larvae.
Life cycle of Trichinella spiralis (cosmopolitan); nelsoni (equatorial Africa); britovi (Europe, western Africa, western Asia); nativa (Arctic); murrelli (North America); papuae (Papua New Guinea); zimbabwensis (Tanzania); and pseudospiralis (cosmopolitan). CNS, central nervous system. (Reprinted from RL Guerrant et al [eds]: Tropical Infectious Diseases: Principles, Pathogens and Practice, 2nd ed, p 1218. © 2006, with permission from Elsevier Science.)
Human trichinellosis is often caused by the ingestion of infected pork products and thus can occur in almost any location where the meat of domestic or wild swine is eaten. Human trichinellosis may also be acquired from the meat of other animals, including dogs (in parts of Asia and Africa), horses (in Italy and France), and bears and walruses (in northern regions). Although cattle (being herbivores) are not natural hosts of Trichinella, beef has been implicated in outbreaks when contaminated or adulterated with trichinous pork. Laws that prohibit the feeding of uncooked garbage to pigs have greatly reduced the transmission of trichinellosis in the United States. About 12 cases of trichinellosis are reported annually in this country, but most mild cases probably remain undiagnosed. Recent U.S. and Canadian outbreaks have been attributable to consumption of wild game (especially bear meat) and, less frequently, of pork.
Pathogenesis and Clinical Features
Clinical symptoms of trichinellosis arise from the successive phases of parasite enteric invasion, larval migration, and muscle encystment (Fig. 256-1). Most light infections (those with <10 larvae per gram of muscle) are asymptomatic, whereas heavy infections (which can involve >50 larvae per gram of muscle) can be life-threatening. Invasion of the gut by large numbers of parasites occasionally provokes diarrhea during the first week after infection. Abdominal pain, constipation, nausea, or vomiting also may be prominent.
Symptoms due to larval migration and muscle invasion begin to appear in the second week after infection. The migrating Trichinella larvae provoke a marked local and systemic hypersensitivity reaction, with fever and hypereosinophilia. Periorbital and facial edema is common, as are hemorrhages in the subconjunctivae, retina, and nail beds (“splinter” hemorrhages). A maculopapular rash, headache, cough, dyspnea, or dysphagia sometimes develops. Myocarditis with tachyarrhythmias or heart failure—and, less commonly, encephalitis or pneumonitis—may develop and accounts for most deaths of patients with trichinellosis.
Upon onset of larval encystment in muscle 2–3 weeks after infection, symptoms of myositis with myalgias, muscle edema, and weakness develop, usually overlapping with the inflammatory reactions to migrating larvae. The most commonly involved muscle groups include the extraocular muscles; the biceps; and the muscles of the jaw, neck, lower back, and diaphragm. Peaking ~3 weeks after infection, symptoms subside only gradually during a prolonged convalescence. Uncommon infections with T. pseudospiralis, whose larvae do not encapsulate in muscles, elicit prolonged polymyositis-like illness.
Laboratory Findings and Diagnosis
Blood eosinophilia develops in >90% of patients with symptomatic trichinellosis and may peak at a level of >50% 2–4 weeks after infection. Serum levels of muscle enzymes, including creatine phosphokinase, are elevated in most symptomatic patients. Patients should be questioned thoroughly about their consumption of pork or wild animal meat and about illness in other individuals who ate the same meat. A presumptive clinical diagnosis can be based on fevers, eosinophilia, periorbital edema, and myalgias after a suspect meal. A rise in the titer of parasite-specific antibody, which usually does not occur until after the third week of infection, confirms the diagnosis. Alternatively, a definitive diagnosis requires surgical biopsy of at least 1 g of involved muscle; the yields are highest near tendon insertions. The fresh muscle tissue should be compressed between glass slides and examined microscopically (Fig. 256-2), because larvae may be missed by examination of routine histopathologic sections alone.
Trichinella larva encysted in a characteristic hyalinized capsule in striated muscle tissue. (Photo/Wadsworth Center, New York State Department of Health. Reprinted from MMWR 53:606, 2004; public domain.)
Most lightly infected patients recover uneventfully with bed rest, antipyretics, and analgesics. Glucocorticoids like prednisone (Table 256-1) are beneficial for severe myositis and myocarditis. Mebendazole and albendazole are active against enteric stages of the parasite, but their efficacy against encysted larvae has not been conclusively demonstrated.
TABLE 256-1Therapy for Tissue Nematode Infections ||Download (.pdf) TABLE 256-1Therapy for Tissue Nematode Infections
|Infection ||Severity ||Treatment |
|Trichinellosis ||Mild ||Supportive |
| ||Moderate ||Albendazole (400 mg bid × 8–14 days) or |
| || ||Mebendazole (200–400 mg tid × 3 days, then 400 mg tid × 8–14 days) |
| ||Severe ||Add glucocorticoids (e.g., prednisone, 1 mg/kg qd × 5 days) |
|Visceral larva migrans ||Mild to moderate ||Supportive |
| ||Severe ||Glucocorticoids (as above) |
| ||Ocular ||Not fully defined; albendazole (800 mg bid for adults, 400 mg bid for children) with glucocorticoids × 5–20 days has been effective |
|Cutaneous larva migrans || ||Ivermectin (single dose, 200 μg/kg) or Albendazole (200 mg bid × 3 days) |
|Angiostrongyliasis ||Mild to moderate ||Supportive |
| ||Severe ||Glucocorticoids (as above) |
|Gnathostomiasis || ||Ivermectin (200 μg/kg per day × 2 days) or |
| || ||Albendazole (400 mg bid × 21 days) |
Larvae may be killed by cooking pork until it is no longer pink or by freezing it at -15°C for 3 weeks. However, Arctic T. nativa larvae in walrus or bear meat are relatively resistant and may remain viable despite freezing.