Disease

Trichinellosis is caused by infection with the parasite Trichinella. The severity of disease is related to the infectious dose and host characteristics, such as immunological priming as a result of previous infection. Some reports have linked severity of disease to the infecting species of Trichinella, suggesting that certain species are more likely to cause severe disease than others. However, the pathogenicity of various species is difficult to define clinically without quantifying infectious dose.

The incubation period ranges from 1 to 4 weeks or more, depending on the infectious dose and possibly the species of parasite. The parasite larvae are released from meat during digestion and then penetrate the intestinal mucosa where they mature into adult worms. The adult worms mate and new larvae are produced which then migrate via the blood stream to skeletal muscle throughout the body. The host immune response leads to expulsion of the adult worms after several weeks; the larvae, once in the striated muscle cells, can persist for months or years, although clinical signs and symptoms typically wane after several months. The early signs of trichinellosis – diarrhea, fever, myalgia and edema, especially of the face – correspond to the new larvae migration through the body and can persist days to weeks. In addition to physical damage to affected tissues, larval penetration and tissue migration causes an immune-mediated inflammatory reaction and stimulates the development of eosinophilia. More severe manifestations include myocarditis, encephalitis, and thromboembolic disease.

Diagnosis of Trichinella Infection

The diagnosis of trichinellosis is based on history of consumption of potentially contaminated meat, the presence of compatible signs and symptoms, and identification of Trichinella larvae in biopsy muscle tissue or specific antibody in serum. For outbreak-associated cases, the diagnosis can be made in asymptomatic patients on the basis of positive results on laboratory testing and history of consumption of an implicated meat. Diagnosis can be challenging, particularly in non-outbreak settings. In the early stages of disease, signs and symptoms are non-specific (see Disease) and mimic those of many other illnesses. For example, the protracted diarrhea that can occur in the first weeks of Trichinella infection is also a common symptom of other foodborne diseases such as salmonellosis, and the high fever and myalgia in the acute phase of Trichinella infection are also suggestive of influenza virus infection.

Trichinella infections are most often diagnosed in the laboratory based on detection of antibodies to excretory/secretory Trichinella antigen in ELISA format. This antigen preparation will react with antibodies from other Trichinella species but may also cross react with non-specific antibodies.  IgG antibodies can be detected approximately 12 to 60 days post-infection. Antibody development depends on the amount of infective Trichinella larvae that are consumed. Levels peak in the second or third month post-infection and then decline, but may be detectable for 10 years or more following infection. At least two serum specimens should be drawn and tested weeks apart to demonstrate seroconversion in patients with suspected trichinellosis whose initial results were negative or weakly positive. Immunologic tests for Trichinella infection have the potential for cross-reactivity with sera from patients with other conditions, especially other parasitic infections. Two commercial EIA kits for trichinellosis antibody detection are available in the United States.

Muscle biopsies are infrequently performed, but they allow for the molecular identification of the Trichinella species or genotype, which is not possible with antibody testing. Usually, 0.2 to 0.5 grams of human or animal skeletal muscle tissue is collected and examined for Trichinella larvae via artificial digestion or histological analysis.

Serum and muscle biopsy specimens can be sent to CDC for confirmation. To obtain a specimen submission form for serologic or muscle biopsy testing, contact the Parasitic Diseases hotline at parasites@cdc.gov.

Treatment

Supportive care and treatment with steroids to help control the allergic inflammatory reaction to the parasite are critical to patient management. Prompt treatment with antiparasitic drugs can help prevent the progression of trichinellosis by killing the adult worms and so preventing further release of larvae. Once the larvae have become established in skeletal muscle cells, usually by 3 to 4 weeks post infection, treatment may not completely eliminate the infection and associated symptoms. Treatment with either mebendazole or albendazole is recommended. If treatment is not initiated within the first several days of infection, more prolonged or repeated courses of treatment may be necessary. Both drugs are considered relatively safe but have been associated with side effects including bone marrow suppression. Patients on longer courses of therapy should be monitored by serial CBC to detect any adverse effects promptly and discontinue treatment. Albendazole and mebendazole are not approved for use in pregnant women or children under the age of 2 years.