The genus Trichinella has genetically distinguished but taxonomically still undetermined genotypes other than usual species
The biggest morphological classification is based on the presence/absence of collagen capsule surrounding the pathogen in cysts in infected muscles
Encapsulated
Non-encapsulated
Infect only mammals
Infect birds and mammals
Trichinella spiralis
Trichinella nativa
Trichinella nelsoni
Trichinella britovi
Trichinella murrelli
Trichinella patagoniensis
Trichinella genotype T6
Trichinella genotype T8
Trichinella genotype T9
Trichinella pseudospiralis
Trichinella papuae
Trichinella zimbabwensis
※Manson's Tropical Infectious Diseases 24th ed. (published in 2023) describes that T. spiralis has several subspecies but according to NCBI Taxonomy Browser and the following articles subspecies written in Manson's are classified as species.
Pozio, E., Rosa, G. la, Murrell, K. D., & Lichtenfels, J. R. (1992). Taxonomic Revision of the Genus Trichinella. The Journal of Parasitology, 78(4), 654. https://doi.org/10.2307/3283540
Zarlenga, D., Thompson, P., & Pozio, E. (2020). Trichinella species and genotypes. Research in Veterinary Science, 133, 289–296. https://doi.org/10.1016/j.rvsc.2020.08.012
Epidemiology
Since Trichinella infections often cause asymptomatic or mild disease and no serological tests with high performance is available, true epidemiology of human trichinellosis is thought still underestimated.
Trichinellosis distributes worldwide from arctic region through the tropics.
Human trichinellosis in developed countries has been dramatically decreased due to improvement of farming and slaughtering of domestic pigs and shrinkage of backyard pig farming in private facilities.
Yayeh, M., Yadesa, G., Erara, M., Fantahun, S., Gebru, A., & Birhan, M. (2020). Epidemiology, diagnosis and public health importance of Trichinellosis. Journal of World’s Poultry Research, 10(3), 131–139. https://doi.org/10.36380/scil.2020.ojafr18
Transmission to human occurs by ingestion of raw or undercooked meat including pigs, wild bores, horse, dog, bear, polar bear, badger and soft-shelled turtle (スッポン).
Rostami, A., Gamble, H. R., Dupouy-Camet, J., Khazan, H., & Bruschi, F. (2017). Meat sources of infection for outbreaks of human trichinellosis. Food Microbiology, 64, 65–71. https://doi.org/10.1016/j.fm.2016.12.012
The world-first report of trichinellosis originated from soft-shelled turtle was published in Japan in 2009 (but only in Japanese and neglected from English literature).
In 2-7 days incubation, larvae penetrate duodenal and jejunal mucosa
Nausea, vomitting, abdominal colic, fever
Maculopapular skin rash and pneumonitis may accompany
Migration (invasion) phase
Larvae invade blood vessels and migrate toward striated muscle cells in diaphragm, masseters, intercostals, laryngeal, tongue and ocular muscles
Severe myalgia, difficulty of mastication, difficulty of breathing, dysphagia, periorbital edema, paralysis of extremities, high fever, petechiae in nails and conjunctivae
Eosinophilia arises but subsides in a week
In some case myocardial complication, neurological complication occurs
Encystment phase
Weeks after infection, larvae encyst in striated muscles they arrived
Cachexia, edema, extreme dehydration
In 6 months calcification of cysts takes place
Inside calcified cysts, 'nurse cells' which is transformed from normal striated muscle cells by larvae secretion encapsulate and nourish larvae
Encapsulated larvae can survive months to decades in human striated muscles
The larger number of larvae infect, the more severe symptoms are
<10 larvae: asymptomatic to mild
50-500 larvae: moderate
<1000 larvae: severe to fatal
Diagnosis
Case definitions by ECDC
Clinical
Laboratory
Epidemiological
At least 3 of
Fever
Myalgia
Gastrointestinal symptoms
Facial edema
Eosinophillia
Subconjunctival, sublingual and retinal hemorrhage
At least 1 of
Trichinella larvae in muscle biopsy specimen
Trichinella-specific antibody by ELISA or Western blot
At least 1 of
Ingestion of laboratory-confirmed contaminated meat
Ingestion of potentially contaminated meat from laboratory-confirmed infected animal
Epidemiological link to laboratory-confirmed human case with the common source
Gottstein, B., Pozio, E., & Nöckler, K. (2009). Epidemiology, Diagnosis, Treatment, and Control of Trichinellosis. Clinical Microbiology Reviews, 22(1), 127–145. https://doi.org/10.1128/CMR.00026-08
Trichinoscopy
Encystment phase begins 1 week after infection at the shortest
Muscle biopsy specimen is thin-sliced and pressed between two slides without any stain and cysts are observed
