Definition
Trichinella spiralis is a parasite that infects swine, other carnivorous animals, and people. The parasites are usually ingested through consumption of infected meat.
Occurrence
Most carnivorous, warm-blooded animals can become infected with the parasite. Infected animals of special importance in human trichinellosis include animals whose flesh is consumed raw or incompletely cooked (feral pigs, bears, several sea mammals, and horses). Species of particular importance, as sources of infection for swine, include previously infected pigs, rodents, and scavenged wildlife carcasses. Meat scraps from infected, slaughtered animals in uncooked garbage can remain infectious and may end up being a source of infection for commercial swine in some countries.
The incidence of infection in swine is low in countries that routinely inspect meat for T. spiralis. In the United States, food safety and eradication efforts have resulted in a prevalence so low that it is nearly undetectable with current diagnostic tests.
Historical information
Trichinellosis once caused outbreaks of severe illness in people but control measures were developed and adopted by many pork-producing countries. In some countries T. spiralisinfection of swine is near eradication and this has suppressed outbreaks. In the United States, less stringent measures have greatly reduced trichinellosis in both pigs and people but the disease still occurs occasionally. In recent years, most of the US human cases are from eating game meat (especially bear meat) and not from pigs.
The FDA has approved irradiation of pork, beef and lamb. Irradiation kills trichinae as well as most bacteria that cause food poisoning. Research on T. spiralis and irradiation of pork products are driven by public health concern and a desire to expand foreign markets for pork. Swine production with trichinella-free certification is becoming a reality.
Etiology
The parasite is Trichinella spiralis. Four separately named strains of Trichinella spiralis have been named. They have minor anatomical differences and may simply represent different ecotypes of the same species.
Mature T. spiralis females are three to four mm long and are found in the duodenal and jejunal mucosa. A fertilized female produces 500-1000 larvae over a period of two to six weeks.
The parasite’s life cycle begins with ingestion of infected muscle that contains encysted T. spiralis larvae. Trichinae within muscle fibers are liberated when cyst walls are exposed to gastric secretions after consumption. The larvae reach maturity in the small intestine in about four days. They mate and mature females penetrate into the mucosa and liberate larvae in lymphatics. Some larvae escape into the lumen of the gut and are eliminated in feces. Recently voided feces may be infectious but this is considered a minor mode of transmission.
Larvae in the lymphatics soon gain access to the systemic circulation. Many are destroyed by body defenses. Those that gain access to muscle fibers penetrate them, usually one per fiber. A larva within a fiber often induces a change in the fiber that causes it to enlarge and act as a nurse cell. In two to three months the larva reaches a length of over 100 µm and is enclosed in a cyst-like structure in the modified muscle cell. Here it can survive for up to 11 years.
Larvae have a predilection for certain striated muscles, generally those that normally are quite active, e.g. diaphragm, intercostal, lingual, laryngeal, masseter and ocular muscles. The larvae occasionally localize in cardiac muscle fibers. Once encysted, larvae must be ingested by another mammal for the cycle to be repeated.
Epidemiology
Survival of T. spiralis is promoted by the wide range of animals that serve as intermediate hosts, by the long survival of larvae in muscle fibers, and by the scavenging, cannibalistic nature of pigs.
In swine, transmission often occurs when pigs scavenge trichinella-infected carcasses of swine, rats or other animals. Transmission also occurs when uncooked garbage containing pork or other meat scraps that harbor T. spiralis are fed to pigs. Tail-biting of infected pigs and ingestion of larvae in feces are other possible sources of exposure.
Pathogenesis
Trichinella spiralis has a relatively low pathogenicity for swine but can cause severe clinical signs in people.
Clinical signs
Clinical signs seldom are observed in swine naturally infected with T. spiralis. Experimentally infected swine show signs of intense muscle pain and have decreased weight gains but most recover and weight gain improves. In infected miniature swine, hypergamma-globulinemia and eosinophilia have been reported.
Lesions
In swine, gross lesions are usually not visible unless there is white streaking from calcification of affected muscle fibers. On microscopic exam, the encysted parasites are easily observed. If there are degenerate or necrotic muscle fibers, an inflammatory reaction with many eosinophils may be present around them.
Diagnosis
The prevalence in the US swine population is low (0.007% in 2000) and usually no special effort is made to detect T. spiralis in slaughtered swine. In many countries, techniques used for surveillance include trichinoscopic examination of muscle (often diaphragm muscle) likely to contain T. spiralis larvae. In this test, the muscle is homogenized and enzymatically digested in order to release the larvae from their cysts for efficient visualization by the technician. An enzyme-linked immunosorbent assay (ELISA) test and PCR are also available. Trichinella-free production certification in the US currently relies on an ELISA test.
Control
In the US, an educational program that places emphasis on thorough cooking of pork has been reasonably successful in minimizing the incidence of human infection. Also, there are regulations that govern the way ready-to-eat pork products must be processed to ensure destruction of T. spiralis. Irradiation of pork and other meats was legalized in 1988 and now is providing additional protection. A Pork Quality Assurance (PQA Plus) program, encourages good production practices that make T. spiralis infection in swine unlikely. It has been implemented by many producers and is required by some pork processors. In late 2008, the United States Department of Agriculture’s Animal and Plant Health Inspection Service (USDA-APHIS) established a voluntary Trichinae Certification Program for US pork. Most recent reports of human trichinellosis in the US have been attributed to the consumption of undercooked wild game meat rather than commercial swine.
No effort should be made to treat animals suspected of having trichinellosis. They should be reported to state public health and animal health authorities.