T. spiralis is a roundworm with an unsegmented body. The adult female has a body size of 1.4 to 4 mm, and adult males are 1.4 to 1.8 mm with muscle larvae measuring about 1mm. The parasites have a long region of specialized, densely packed and well-ordered cells near the front portion of the worm collectively referred to as the stichosome. Muscle larvae live within host cells and can be identified in coiled form in a cyst within the striated muscle cell, which in turn is surrounded by a collagen sheath in a fully reprogrammed host cell. T. spiralis is the cause of most of the human trichinella infections and deaths around the world, and its pathogenicity is higher than that of other trichinella species due to the higher number of newborn larvae produced by the females and for the stronger immune reaction induced in humans. The life cycle of the parasite begins when a person or an animal eats contaminated meat containing larvae. Digestive juices from the stomach dissolve the capsule-like cyst and release the parasites. The larvae then penetrate into the intestine where they mature and mate. Male T. spiralis are capable of inseminating at least 4 females.
The normal habitat of T. spiralis adults is in the outer layer (epithelium) of the mucosal lining of the small intestine of its host. Copulation and insemination occurs between 30 and 32 hr post-infection in the epithelial layer of the mucosal lining. A majority of the adults are completely embedded in the epithelium during the deposition of the motile larvae. Female worms then pass the motile larvae into the blood stream where they make their way through the capillaries (tiny blood vessels) into the muscle fibers. Once in the muscle fibers, they form cysts again and begin a long life. Motile larvae can also migrate through small veins and lymphatic vessels to new muscle sites.
-- Adapted from Pozio and Zarlenga, 2005, Parasitic roundworm diseases, NIH-NIAID.
Trichinella spiralis shows a wide distribution in temperate and equatorial climatic zones, because it has been passively imported into most continents due to its high infectivity to swine and rats. The predominant hosts are domestic and wild swine (Sus scrofa), and other animals that live in conjunction with humans, such as the brown rat, the armadillo, cats, dogs, and a broad range of wild carnivores. The geographical distribution of this species in wild mammals appears around current or past foci of infections originating from the domestic cycle i.e. pig transmission, but quickly infect the surrounding animal population.
A sequencing plan submitted to the NHGRI has recently been approved. The plan calls for a BAC fingerprint map, BAC end sequencing, 8-fold sequence coverage in plasmids, end sequence of a fosmid library at 0.3-fold coverage, and two rounds of directed sequence improvement ("pre-finishing"). The BACs and the data will be available to the community. The individual traces will be available from the NCBI Trace Archive, and the sequence assembly will be available upon internal evaluation for quality. Funding for the sequence characterization of the Trichinella genome is being provided by the National Human Genome Research Institute (NHGRI), National Institutes of Health (NIH).