B255 - TRICHINELLOSIS

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B255 - TRICHINELLOSIS

Nature of the disease
Trichinellosis (trichinosis) is a parasitic disease of mammals caused by the nematode parasite Trichinella spp. It is an important zoonosis with humans becoming infected through the eating of raw or inadequately cooked infested meat.

There are 7 species of Trichinella including the most recently discovered T. papuae found in Papua New Guinea in 1998. The potential importance of trichinellosis as a zoonosis in the Pacific Region is investigated by the Regional Animal Health Service of SPC under the Zoonoses Regional Project.

Classification
OIE List B disease
Susceptible species
All Trichinella spp have relatively low host specificity and all mammals are susceptible to trichinellosis, including humans. Birds (Raptors, passeriform birds, and chickens) are susceptible to T. pseudospirallis.

Infestation are more common in omnivores (Horses, Humans, Pigs, Rats...) and carnivores (Cats, Dogs, Seals...). Pigs and rodents seem to play the most important in the epidemiology of the disease.

Distribution
Trichinellosis is more common in temperate regions than in tropical regions. It occurs in North America, South America (Argentina and Chile), northern and eastern Europe, Kenya, Egypt, Lebanon, Nepal, Thailand, Indonesia.

In the Pacific region, Trichinellosis occurs in the North Island of New Zealand, and serological evidences of Trichinella spp. have been found in Fiji, Kiribati, Palau, Samoa, Solomon Islands, Tonga and Wallis and Futuna. T. pseudospirallis has been found in Australia and T. papuae in PNG.

Clinical signs 
The pathogenicity of all the different species of Trichinella has not yet been totally explored. For the most common, T. spirallis, the following clinical signs have been found.

Trichinellosis is rarely detected clinically in animals. In case of heavy burden, the following signs are seen:

  • Diarrhea,
  • Anorexia,
  • Fever, 
  • Weakness
  • Sometimes myositis causing unwillingness to move.

In humans there is an initial phase dominated by gastro-intestinal symptoms (vomiting and diarrhea), followed by a stage lasting about 2 months in which there is:

  • Fever,
  • Sub-cutaneous oedema,
  • Muscle pains,
  • Cachexy,
  • Myocardiosis,
  • Weakness,
  • Deaths are common (up to 40%) either due to anaphylactic choc or due to the consequences of the myocardiosis.
Post-mortem findings 
Cysts may be present in any voluntary muscle but are best seen in thin muscles such as the diaphragm, the tongue, the masseters, the laryngeal muscles, and eyes muslces. In case of heavy burden (1,500 parasites/g), the muscle can have a grey appearance directly observable. There are no cysts for the two non encapsulated Trichinella: T. pseudospirallis and T. papuae.

Usually the lesions (cysts and larvae) are only visible under microscopic observation. 

NB People handling potentially infested tissues should wear rubber gloves and wash thoroughly before eating or handling food.

Differential diagnosis 
Trichinellosis cannot be diagnosed on clinical grounds. Microscopically the cysts have to be differentiated from those of other parasitic, protozoan or fungal infections.
Specimens required for diagnosis 
Usually the disease is not recognised as a clinical disease and is only diagnosed in animals at slaughter. 

For identification of the parasite, different techniques can be used:

  • Observation under trichinoscope, which is a special microscope use for the diagnosis of trichinosis, it consists of a compressorium to press muscles samples whose image is projected on a screen. That techniques requires muscle samples collected from several sites including diaphragm, cheek muscles and the tongue. Samples are cut into at least 28 pieces of 2x10 mm in size.  This technique is recommended by OIE.
  • Digestion of muscles by pepsin and HCl to release the parasite from the cyst and microscope observation after filtration of the sample (numerous methods). These techniques require samples of at least 100g of muscles (diaphragm pillars or tongue). The 100g can consist of pool from different animals. Depending on local legislation and suspected level of prevalence, 1 to 5g of the pool will be finally examined. Using 1g of muscle, the technique allows to detect infection superior to 3 larvae/g. This technique is recommended by OIE.
  • PCR test allow the differentiation between the different species of Trichinella. However such tests require the development and amplificaiton of specific DNA-probes. These tests will be further used and developed for the diagnosis of T. papuae under the zoonoses project.

For serological identification of the parasite, ELISA tests have been developed. Their sensitivity is very good (detection of infection level such as 1 larvae/100g of muscles) but they may fail to diagnosis early and late stage of the disease, due to lack of immune response. Thus they are more useful for herd screening than for individual diagnosis. An ELISA test for T. papuae infection will be validated under the zoonoses project.

N.B. The selection of muscles for samples require a good knowledge of the parasite location, for T. papuae this is beeing investigated under the zoonoses project.

Transmission   
Disease in pigs is perpetuated by swill feeding, eating infected rodent' carcases, tail-biting, infestation by faeces from newly infested animals

Transplacental transmission of larvae occurs in mice and humans, but not in pigs.

Human transmission is caused by ingestion of raw, or improperly cooked, infected meat. Pigs, and where eaten, horses, dogs and rats are the source of infection for humans. Trichinella organisms are destroyed by storing meat at -15oC or heating to a core temperature of 60°C.

Risk of introduction   
Trichinellosis could be introduced with infected pigs or pig meat from endemically infected countries. Freezing will destroy the encysted larvae in pig meat. Recommended temperatures and times to destroy T. spirallis are:
  • -15°C for 20 days;
  • -23°C for 10 days;
  • -30°C for 6 days; 
  • -35°C for 40 minutes.

However introduction by infected rodents could result in pig transmission which emphasize the importance of de-ratting aircraft and ships.

Control / vaccines  
No vaccines have been developed so far. Treatments exist for humans if diagnostis is done in time.

To prevent trichinosis in pigs (main host at risk for human health in the region) measures involve the control of vectors and the destruction of the parasite in infected meat. If swill is fed to pigs, it should be cooked at 100°C for 30 minutes to inactivate T. spirallis. 

References
  • AMBROISE-THOMAS et al. (1990). Trichinose In Parasitologie - Mycologie à l'usage des étudiants en médecine, 4th ed, La Madeleine, p.68-73
  • ANGUS SD and THOMPSON A (2001), Review of Trichinellosis in the Pacific Region In Zoonoses Project Proposal to ACIAR, not published, p.10-12, 16-18, 21-25.
  • BUSSIERAS J, CHERMETTE R, Helminthologie In Parasitologie Vétérinaire, Ecole Nationale Vétérinaire D'Alfort, 1992, p 103-105; 243-249
  • CORWIN RM and STEWART TB (1999), Internal Parasites, In Diseases of Swine, Iowa state University Press, Ames, Iowa, USA, p.713-730
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, p. 403-407
  • LJUNGSTRÖM I, MURRELL D, POZIO E, WAKELIN D (1998), Trichinellosis In Zoonoses, ed by SR PALMER, Lord SOULSEY and D.I.H. SIMPSON, Oxford University Press, Bath Press, Avon, 1998, p.326-328
  • Office International des Epizooties, 2002
  • SOULSBY EJL, Helminths, Trichinella In Helminths, Arthropods and Protozoa of Domesticated Animals, Lea and Febiger Inc, 7th ed, 1982, Philadelphia, p 330-334
  • Trichinellosis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 517-518