Trichinosis 

Trichinosis is the result of infection by the nematode Trichinella spiralis. Humans are infected incidentally when they eat inadequately cooked meat that contains larvae of Trichinella species. Most infestations do not cause symptoms, although heavy exposure can cause various clinical manifestations, including diarrhea, fever, myalgias, and prostration.

Although 8 species of Trichinella currently exist and are described taxonomically based on genetic, biochemical, and biological data, an additional 4 genotypes are acknowledged in the genus, but their taxonomic level is uncertain.^[1] The table below describes the taxonomically described species, including distribution, major host reservoir, infectivity of humans, resistance to freezing, and pathogenicity to humans.^{[1, 2]}

Table. Important Characteristics of Trichinella Species (Open Table in a new window)

Trichinella species require 2 hosts to maintain their life cycle. After development in a single host, they spread to the next through ingestion of infected flesh, as opposed to the traditional arthropod intermediate host. Trichinella species have 3 major life cycles in nature: pig-to-pig, rat-to-rat, and by carnivorous or omnivorous animals in the wild. Rats and pigs are the animals most commonly associated with trichinosis; however, depending on the region, walruses, seals, bears, polar bears, cats, raccoons, wolves, and foxes may also be infected. Life cycle of Trichinella species parasite is depicted in the image below.

The life cycle begins when raw or inadequately cooked meat is eaten that contains viable larvae housed inside a cyst wall, known as a nurse cell. The acidic environment in the host's stomach releases the larvae from the cyst wall. The free larvae migrate into the small intestine and attach to and penetrate the mucosa at the base of the villi. After 4 molts and over a period of 30-36 hours, they develop into adult worms and become obligate intracellular organisms. The adult male measures 1.5 X 0.05 mm, and the adult female measures 3.5 X 0.06 mm. Approximately 5 days after infection, the female begins shedding live newborn larvae (L1 stage). The female remains in the intestine for 4 weeks, releasing up to 1500 larvae. After an adequate inflammatory response develops in the intestine, the female is eventually expelled in the feces.

The newborn larvae enter the lymphatics and blood circulatory system and migrate to well-vascularized striated skeletal muscle. The parasite has a predilection for the most metabolically active muscle groups; therefore, the most frequently parasitized muscles include the tongue; the diaphragmatic, masseteric, intercostal, laryngeal, extraocular, nuchal, intercostal, and pectoral muscles; the deltoid; the gluteus; the biceps; and the gastrocnemius. In tissues other than skeletal muscle, such as the myocardium and brain, the parasites soon disintegrate, causing intense inflammation, and are then reabsorbed.

The larvae continue to grow over the next 2-3 weeks until they reach the fully developed L1 infective stage, when they increase in size up to 10-fold. The adult worms are viviparous. The larvae coil and develop a surrounding cyst wall, or nurse cell (except for T pseudospiralis, which does not encyst). The complete cycle takes 17-21 days. The larvae within the cyst wall reach an average size of 400 X 260 µm; however, lengths of 800-1000 µm have been described. The nurse cell–L1 complex may persist for 6 months to several years before calcification and death occur. The life cycle is complete when a compatible host ingests the infected muscle.

The intensity and frequency of exposure to infected meat determine the severity of the disease. The degree of infection is categorized as light (0-10 larvae ingested), moderate (50-500 larvae ingested), and severe (>1000 larvae ingested).

Frequency

United States

From 1997-2001, 72 cases of trichinosis were reported to the Centers for Disease Control and Prevention (CDC).^[3] Most cases were associated with eating wild game (43%), although 17% were associated with commercial pork products and another 13% from noncommercial pork products. Infections may also occur during foreign travel, especially to Mexico and Asia. The percentage of infected domestic swine in the United States is 0.001%; however, one autopsy study documented a 4% incidence of old infection. Data have also shown the presence of T murrelli in raccoons and coyotes.^[4]

International

In Europe, where pork inspection is mandatory, most cases of trichinosis are associated with horse or wild boar meat. In Latin America and Asia, domestic pork is the chief source of infection. The rate of Trichinella infection in swine in China is as high as 20%. Studies have also reported increase rates of trichinosis in former European countries such as Romania due to political changes and regional food habits.^[5] In addition, the European Centre for Disease Prevention and Control and the European Safety Authority reported 779 human cases of trichinellosis in the European Union found in farm animals and wild animals, particularly in the latter.^[6]

Mortality/Morbidity

• Although Trichinella infections are most likely underreported in the United States, fewer than 25 cases are documented per year, with a very low mortality rate.
• Patients with light infection are usually asymptomatic. Those with mild symptoms improve in 2-3 weeks. Symptoms associated with heavy infections may persist for 2-3 months.
• Factors that may affect morbidity include the quantity of larvae ingested, the species of Trichinella (most notably T spiralis), and the immune status of the host. Patients succumb to exhaustion, pneumonia, pulmonary embolism, encephalitis, or cardiac failure and/or arrhythmia. Death from trichinellosis usually occurs in 4-8 weeks but may occur as early as in 2-3 weeks.

Race

Trichinella infections are related to cultural differences in food cooking and storing methods, specifically the inadequate cooking or freezing of meat.

Sex

• No differences in the rates of trichinosis between males and females are reported.
• Pregnant patients milder trichinosis symptoms than patients who are not pregnant; however, abortions and stillbirths have been reported.
• Symptoms of trichinosis are typically worse in females who are lactating than in females who are not.

Age

Children appear to be more resistant to Trichinella infection; however, their symptoms may be more intense. Children also have fewer complications and recover more rapidly.