Opinion of the Scientific Panel on Biological Hazards on the suitability and details of freezing methods to allow human consumption of meat infected with Trichinella or Cysticercus

Trichinellosis is a foodborne parasitic zoonotic disease caused by nematode worms of the genus Trichinella. All the species of the genus are known to infect mammals, mostly carnivores which are the main hosts of the parasite, rodents and omnivores including pigs, and occasionally herbivores such as horses. Transmission between animals occurs by ingestion of muscle tissue infected with the larval stage of the parasite, mostly by predators eating infected preys or scavenging carcasses, and by cannibalistic behaviour. In humans, the disease caused by the consumption of meat infected with trichinellae can be serious and life-threatening. Historically, most cases of human trichinellosis have occurred, and still occur, following consumption of pork world wide. However in the last fifty years it has become clear that also meat from other species - wild boar, bears, foxes, walrus, cougar, dog and horse - can be a vehicle of trichinellae. Trichinellae have been also exceptionally detected in cattle but their role in inducing disease in humans needs clarification. Freezing under officially controlled time and temperature conditions is accepted as an alternative method to microscopic inspection of porcine meat for trichinellae within the scope of the European Meat Hygiene legislation, as the cold treatment destroys the infectivity of Trichinella spiralis, the agent of domestic trichinellosis. However, the suitability of freezing is questioned in view of the occurrence of cold resistance in other Trichinella species, also taking into account the accession of new Member States and the prevalence of cold resistant Trichinella species in pigs and wild boar in some of those areas. This occurrence may require, in principle, the setting of different time/temperature combinations for the inactivation of the parasites in carcasses and meat, as cold resistant trichinellae might survive beyond the temperature/time limits known to inactivate T. spiralis, which is the Trichinella species that has been extensively employed for inactivation studies in meat. Among cold resistant trichinellae, Trichinella nativa is a species highly adapted to transmission among sylvatic carnivores in the arctic and subarctic zones of the Holarctic region and is highly resistant to freezing in the muscle tissues of carnivorous hosts. This species is very rare in nature in domestic pigs and wild boars, it has low or no infectivity to both hosts and is also relatively non-persistent in their muscle tissues. T. nativa has never been reported in naturally infected cattle, and a single experimental trial in this species has shown low infectivity and moderate persistence of this trichinella in the host muscle tissue. Trichinella T6 is a genotype closely related to T. nativa, which may be found in sylvatic carnivores of the Nearctic regions. Infectivity and persistence of this trichinella, which has never been found in nature in domestic pigs nor in wild boar, are very similar to those of T. nativa; for cattle, no reports of natural infection nor experimental data concerning Trichinella T6 are available. Preliminary experimental data suggest that the cold resistance of T. nativa and Trichinella T6 in pigs and wild boar is much lower than in wild carnivores, but its entity has not been fully assessed. No human trichinellosis cases by T. nativa and Trichinella T6 have ever been linked to the consumption of meat from pigs, wild boar or cattle.