Richard A. Oberhelman
Trichinella species are nematodes infecting the striated muscle of warm-blooded animals, and infection occurs by consumption of raw or insufficiently cooked infected meat. 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.1 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 Moslem 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. In the United States, trichinosis is most commonly diagnosed in far northern Native Americans, such as the Inuits, who acquire infection with Trichinella nativa from walrus meat or meat from other sea mammals. In this group, intestinal symptoms are the predominant clinical manifestations.
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 developed and developing countries.2 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 outbreaks attributed to sylvatic (wild animal–associated) Trichinella species, either through consumption of wild game or spillover to domestic animals. The disease is naturally perpetuated by cannibalistic rats consumed by higher carnivores, and the practice of feeding pigs garbage containing infected meat maintains the infection in pigs.
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 (eFig. 332.1 ). Adult nematodes mate in the intestine and fertilized eggs hatch in utero, so 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 Löeffler 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.
Clinical symptoms primarily depend on the number of worms ingested, number of larvae produced, and sites of invasion. Children tend to have typical symptoms, but with milder disease than adults, and they rarely present with severe complications. During the intestinal phase of infection, invading larvae and adult worms often cause acute gastrointestinal symptoms such as nausea, vomiting, and diarrhea, as well as fever, diaphoresis, and urticaria. These symptoms may begin within 24 hours of infection and may last up to 7 days. When larvae enter the general circulation, new symptoms may occur, including edema of the eyelids and face, conjunctivitis, splinter hemorrhages of the nailbeds, fever, and both cardiac and respiratory symptoms. Severe muscle tenderness, pain, and spasm occur during muscle invasion.
Primarily during the fourth to eighth weeks of infection, myocarditis may present (in about 20% of patients), leading to acute congestive heart failure and death in some cases. Arrhythmias are not common, but sudden death occurring in the second to fifth weeks of infection is attributed to arrhythmias. The electrocardiogram may show ST changes and T-wave inversion. Central nervous system symptoms include headache, stiff neck, and psychoses. More severe neurologic complications, such as paresis and paralysis, have been described as rare complications in patients with severe infections, usually occurring in the third week of infection and associated with larval migration to the central nervous system. Ocular involvement, particularly periorbital edema and chemosis, is typical and suggests the diagnosis. With uncomplicated disease, muscle tenderness is the only persistent symptom, and it gradually diminishes in 12 to 18 months.3
A rising eosinophilia beginning after 7 to 10 days and peaking at 20 to 21 days is a hallmark of this infection, with differentials that reach 20% to 60% eosinophils or higher. Leukocytosis is common. Creatine kinase, lactic acid dehydrogenase, and serum transaminase levels are elevated in more than 50% of patients. Hypoalbuminemia and hypergammaglobulinemia are common, and serum IgE concentration is markedly elevated.
DIAGNOSIS AND TREATMENT
Unless one can elicit a history of eating raw or partially cooked pork, early diagnosis is difficult. Eosinophilia in the presence of other characteristic features, such as periorbital edema, fever, and myalgia, should suggest the diagnosis, especially if a history of recent pork consumption is elicited. Although serologic diagnosis cannot readily be used before the third week of infection, changing titers strongly suggest acute disease. Serologic tests available through regional laboratories and the CDC include an enzyme-linked immunoassay, indirect hemagglutination test, indirect immunofluorescence test, a bentonite flocculation test, and a complement fixation test. Many serologic tests are not reliably positive until after the third week of infection, so testing of acute and convalescent sera may be useful. The enzyme-linked immunoassay is the most specific and most widely used of currently available immunoas-says, and a combined diagnostic approach using this assay and indirect immunofluorescence has been suggested to increase sensitivity of case detection.
Muscle biopsy may be necessary in some cases to confirm trichinosis. If needed, it should be performed after the second week of infection and taken from a tender muscle mass (see Fig. 332-1). Muscle biopsy is necessary only when other diagnostic modalities are unable to confirm the diagnosis.
Most patients, including those with severe disease, recover completely, and trichinosis rarely is fatal. The benzimidazole drugs, such as mebendazole, albendazole, and thiabendazole, are the mainstays of therapy. The first two drugs are preferred over thiabendazole due to the numerous side effects associated with the latter drug. Steroids generally should be avoided in uncomplicated disease, because animal studies indicate that they may increase the numbers of circulating larvae and prolong the infection. However, steroids (together with mebendazole or albendazole) are indicated for central nervous system disease and myocarditis to reduce symptoms from inflammation.
FIGURE 332-1. Trichinella larva encysted in a characteristic hyalinized capsule in striated muscle tissue. (Photo/Wadsworth Center, New York State Department of Health. Reprinted from CDC MMWR 53:606, 2004; public domain.)
Because infection primarily results from eating raw or partially cooked pork and pork products, proper education in preparing pork and pork products is necessary. The disease can be prevented by cooking the meat thoroughly, until it is no longer pink. The thermal death point of the encysted larvae is from 62°C (143.6°F) to 70°C (158°F). Trichinella larvae may also be killed by freezing pork at 23.3°C (10°F) for 10 days, although some reports indicate that Trichinella in Arctic wild animals can survive this procedure. The temperatures cited can generally be achieved with chest freezers, but may not reliably be achieved with upright home freezers or combination refrigerator/freezers.