Trichinella in pork : current knowledge on the suitability of freezing as a public health measure

1Department of Infectious, Parasitic and Immunomediated Diseases, Istituto Superiore di Sanita, Rome, Italy 2Lab. of Zoology, Department of Ecology, The Royal Veterinary and Agricultural University, Frederiksberg, Denmark 3Centre for Food-borne and Animal Parasitology, Canadian Food Inspection Agency, Saskatoon, Canada 4UMR BIPAR INRA AFSSA ENVA UPVM, Maisons-Alfort, France 5Parasitologie, Hopital Cochin, Universite Descartes, Paris, France 6National Research Council, Washington, USA

Nematodes of the genus Trichinella are the causative agents of trichinellosis, a potentially severe disease in humans.Raw or undercooked pork, horse and game meat (predominantly wild boar and bear) poses a health risk to consumers.Various European and international regulations and guidelines have been developed to protect consumers from exposure to this parasite [1][2][3]; these regulations and guidelines cover both slaughter inspection and post-slaughter processing (e.g., freezing, cooking).Scientific studies have been conducted to validate these methods in pork, including an international study, which described the time and temperature requirements for the freezing process to inactivate Trichinella spiralis, the species of Trichinella most commonly associated with pork [4].Results of this study have been widely used to develop regulations governing the commercial freezing of pork and pork products [1,3].However, recent scientific information on the geographical distribution of species of Trichinella, other than T. spiralis, which can infect pigs, and the ability of some of these species to tolerate freezing, have raised doubts about the effectiveness of commercial freezing methods to kill trichinella larvae in pork intended for human consumption [5].

Freeze resistant species of Trichinella
More than 50 years ago, it was discovered that trichinella larvae (at that time all trichinella larvae were considered to be T. spiralis), present in the muscles of animals living in arctic and subarctic regions of the world (e g Greenland Canada Russia Siberia) were able to survive freezing for regions of the world (e.g., Greenland, Canada, Russia, Siberia), were able to survive freezing for months or even years.We now recognize eight species and three genotypes of the genus Trichinella [5].Of these, only muscle larvae of Trichinella nativa, its related genotype Trichinella T6, and Trichinella britovi are known to survive extended periods of freezing in the muscles of some of their natural hosts, including pigs [5].
From the perspective of food safety, freeze tolerant species of Trichinella are a potential concern as they might remain infective in pork following commercial freezing treatments.However, a number of experimental studies have demonstrated that T. nativa and Trichinella T6 larvae are only able to establish in very low numbers in the domestic pig [6,7].In general, the infectivity of T. nativa and Trichinella T6 for pigs is 10 4 lower than the infectivity of T. spiralis, and neither T. nativa nor Trichinella T6 has ever been found in a domestic pig in nature.These trichinella species pose a very low or negligible risk to consumers of pork from domestically reared pigs and therefore may not need to be considered in regulations governing freezing of pork and pork products.
Potential risks associated with freezing pork in areas where T. britovi is endemic T. britovi is found across Europe, Asia, Northern and Western Africa and has been shown in experimental studies to have moderate infectivity for the domestic pig [5,6].According to the database of the International Trichinella Reference Centre (http://www.iss.it/site/Trichinella/index.asp),36 of 200 (18%) of trichinella species isolated from domestic pigs in Europe were identified as T. britovi.
Freeze tolerance of T. britovi in pork is influenced by the age of the infection as well as the conditions of freezing and thawing (i.e.temperature and time) [8].Data shown in Table 1 [9][10][11], demonstrate the high variability of survival of T. britovi larvae in frozen meat of domestic pigs and wild boar (Sus scrofa).
Table 1.Infectivity of T. britovi larvae after freezing of pork of naturally or experimentally infected swine.The information provided on the Eurosurveillance site is designed to support, not replace, the relationship that exists between a patient/site visitor and his/her physician.Our Website does not host any form of commercial advertisement.

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