Trichinella spiralis is a nematode parasite that is notorious for its transmission via consumption of undercooked meat, especially pork. Infection results in a disease called trichinosis. The larvae travel within the circulation and encapsulate within striated muscle cells of the host, mostly in diaphragm and intercostal muscle tissue. A person with trichinosis will experience chronic muscle pain that can last for several years, as well as possible disruption of organ function due to migrating larvae.
T. spirialis is one of the smallest nematode species to infect humans, with an adult worm being about 1.4 mm (male) to 3.0 mm (female) in size.
Life CycleEditAdult worms live and reproduce sexually in the small intestine, feeding on nutrients of the mucosa. These worms live for about six weeks, producing larvae which burrow through the intestinal wall and enter the circulation.
The larvae feed on blood and migrate until they reach striated muscle tissue; this is often diaphram or intercostal muscle, but can include other muscular tissue depending on the migration. There, a larva will enter a muscle cell and convert it into a nurse cell, an extended phenotype of the parasite that serves as protection and nourishment.
The harboring nurse cell becomes encapsulated, first by collagen then calcium. This complex is resistant to immune attack and stimulates the growth of nearby capillaries for increased circulation. An encapsulated larva can survive in muscle tissue for up to forty years.If muscle tissue containing these nurse cells is ingested by another animal, the capsules will be destroyed by HCl within the stomach. This releases the larvae, which then travel to the small intestine and burrow into the mucosa. There the larvae grow and mature into adults.
Pathology and SymptomsEdit
In the intestines, infection can lead to nausea and diarrhea due to disruption of the intestinal epithelium and resultant inflammation. Many symptoms arise when the larvae begin to migrate through the body, causing tissue damage and inflammation. During this phase, the host may experience fever, muscle pain, and weakness. Though uncommon, migrating larvae may enter tissues such as the heart, kidneys, or lungs, causing tissue damage. Even more rarely, the larvae can enter the central nervous system and cause neurological damage. When the larvae encyst within muscle tissue, their presence causes chronic pain that can last several years.
In trichiniosis diagnostic swelling (edema) is common, especially around the eyes. Another diagnostic symptom is hemorrhaging of the retina or under the fingernails.
There is currently no vaccine for Trichinella spiralis. If diagnosed early (within days of infection), antihelmintic drugs can be effective in preventing larvae from leaving the intestine and causing damage. However, most infections are not noticed until well after this period. Drugs such as mebenzadole and albenzadole are used to stop the development of hatched larvae, and can are effective in preventing infections from becoming worse. However, once larvae become encysted within muscle tissue, there is no way to eliminate them. The resulting chronic pain is treated with pain medication or steroids.
An interesting study by Aranzamendi, et al. (2013) has investigated Trichinella spiralis suppression of host immune response. Helminths such as nematodes are known to modulate immunity in order to survive within their hosts. In addition, a reduction of seasonal allergy symptoms has been found to be associated with helminth infection by Schistosoma sp., Necator americanus, and Trichinella spiralis in epidemiological studies. In the study by Aranzamendi, et al., a direct link between T. spiralis infection and allergy symptoms was investigated. Allergy severity was measured through experimental allergic airway inflammation (EAAI), by which an inflammatory allergen (in this case ovalbumin, or OVA) was introduced to the airway and resultant antibodies measured. Mouse subjects were either infected with T. spiralis, treated with OVA, or both. After the treatment, serum IgE antibodies were measured. The lung tissue was then dissected and analyzed through histology. It was found that in comparison to control mice (the group treated with the OVA allergen only), the T. spiralis-infected OVA-treated mice displayed lower serum antibody levels, lower cytokine concentration in bronchialveolar lavage cells, and reduced lung pathology. These results suggest that helminths peduce secretions that reduce antigen-specific and inflammatory immune responses to both the helminth and unrelated antigens. This could account for these worms' ability to remain within their hosts for extended periods of time without being destroyed. Further investigation of these mechanisms could provide new means of removing these infections, or have implications in the treatment of allergies.
The most common sources of Trichinella spiralis infection are domestic and wild pigs. However, other animals consumed by humans may harbor the parasite, such as bears, dogs, rats, horses,and seals. It is widespead, being seen throughout Europs as well as in the United States, Mexico, Argentina, and China. According to a study by Gottstein, Pozio, and Nockler (2009), the yearly number of human trichinosis diagnoses is about 10,000 cases, with a 0.2% death rate. Societies such as France and China that ingest raw or undercooked meat for cultural reasons are at higher risk of infection. Communities that often hunt and eat wild game are also at risk.
Prevention and ControlEdit
Governments have taken several actions to help reduce the prevalence of trichiniosis. Education about proper cooking and treatment of meat is very important. To ensure complete eradication of Trichinella larvae, meat must be thoroughly cooked to a temperature of 71* C (159.8 * F) for at least one minute. The color of the meat should turn from pink to gray. Due to uneven cooking, microwave ovens are not considered a safe method of meat preparation (Gottstein, Pozio, & Nockler, 2009). Curing, drying, and smoking meat are also considered unsafe. Contrary to popular belief, freezing is not a reliable method of destroying parasites; Trichinella larvae have been known to survive frozen for months or even years.
Governments have also passed laws controlling the farm-raising of pigs for commercial distribution. Pigs should be raised in a confined area or indoors to prevent them from roaming and possiblly consuming infected meat. Their diet and living conditions should be regularly monitored or inspected by qualified veterinarians. While this has been somewhat effective in developed countries, underdeveloped areas have limited infrastructure to regulate this. Pigs are free to explore and are also fed scraps which may contain decomposing meat. Also, a recent increase in "green" or "organic" meat sales has led to a higher prevalence of free-range farmers, whose roaming pigs are at risk of becoming infected with Trichinella.
In many developed countries, slaughtered pigs are regularly inspected for the presence of Trichinella larvae.
Aranzamendi, C., de Bruin, A., Kuiper, R., Boog, C.J.P, van Eden, W., Rutten, V., & Pinelli, E. (2013). Clinical & Experimental Allergy 43(1), 103–115.
Gottstein, B., Pozio, E., & Nockler, K. (2009). Epidemiology, Diagnosis, Treatment, and Control of Trichinellosis. Clinical Microbiology Review 22(1), 127-145.