Trichinellosis (Trichinosis)

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Learning resources

• 旋毛虫 in Japanese

• THE TRICHINELLA PAGE
• Trichinellosis - DPDx by US CDC
• Illustrated Trichinella written by Yuzo Takahashi (not for sale)

Pathogen and Taxonomy

• 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

• Distribution of species (directly linked from THE TRICHINELLA PAGE)

Life cycle and Transmission

• Life cycle is maintained amongst host mammals and birds.
  • Pigs and rats (domestic cycle) or wild bores, wild bears, polar bears, rats and birds (sylvatic cycle).
  • Humans are accidental (deadend) hosts for Trichinella
  • Only humans develop clinical symptoms by Trichinella infection
• Refer to DPDx - Trichinellosis

• 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).
• Transmission to horse (obligate grazer) is speculated that pasture or hay may be accidentally contaminated by infected carcass (rodents etc.).

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).
• The transmission route is speculated that soft-shelled turtles were fed by carrions of pigs dead by diseases and contaminated through the carrions.

前田卓哉, 藤井毅, 岩本愛吉, 長野功, 呉志良, & 高橋優三. (2009). スッポンを感染源とする旋毛虫症例. 病原微生物検出情報, 30(10), 272–273. https://idsc.niid.go.jp/iasr/30/356/kj3563.html

Human disease

• Humans are accidental (deadend) hosts.

1. Ingestion of larvae-infected meat
2. Enteric phase
   1. In 2-7 days incubation, larvae penetrate duodenal and jejunal mucosa
   2. Nausea, vomitting, abdominal colic, fever
      1. Maculopapular skin rash and pneumonitis may accompany
3. Migration (invasion) phase
   1. Larvae invade blood vessels and migrate toward striated muscle cells in diaphragm, masseters, intercostals, laryngeal, tongue and ocular muscles
   2. Severe myalgia, difficulty of mastication, difficulty of breathing, dysphagia, periorbital edema, paralysis of extremities, high fever, petechiae in nails and conjunctivae
   3. Eosinophilia arises but subsides in a week
      1. In some case myocardial complication, neurological complication occurs
4. Encystment phase
   1. Weeks after infection, larvae encyst in striated muscles they arrived
   2. Cachexia, edema, extreme dehydration
   3. In 6 months calcification of cysts takes place
   4. Inside calcified cysts, 'nurse cells' which is transformed from normal striated muscle cells by larvae secretion encapsulate and nourish larvae
   5. 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

Nurse cell

• Refer to Nurse cell formation in THE TRICHINELLA PAGE or the following article

Wu, Z., Sofronic-Milosavljevic, L., Nagano, I., & Takahashi, Y. (2008). Trichinella spiralis: nurse cell formation with emphasis on analogy to muscle cell repair. Parasites & Vectors, 1(1), 27. https://doi.org/10.1186/1756-3305-1-27

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
• ELISA for antibody detection is common
• Multiplex PCR is also used

Differential diagnoses

• Trichinellosis is a great mimicker
  • Typhoid, encephalitis, myositis, tetanus, Katayama syndrome, hookworm infection, strongyloidiasis, periarthritis nodosa, rheumatoid arthritis

Treatment

• Albendazole and mebendazole

Prevention and Control

Heat	Freezing
Meat core temperature ≥71°C for ≥1 min Color to gray, muscle fibers separated	Up to 15cm thickness meat block ≤-15°C for ≥3 weeks
	Up to 50cm thickness meat block ≤-15°C for ≥4 weeks
	※Applicable only to T. spiralis in pork

※Other species are more resistant to cold temperature

• T. britovi in pork survived for 3 weeks at -20°C
• T. spiralis in horse survived for 4 weeks at -18°C
• Game meat like bear harbors freeze-resistant Trichinella
  • 5 years in bear meat