Age affected: All ages, human risk.
Causes: 2-4 mm worm - Trichinella spiralis.
Effects: No effect in life, but undercooked meat can cause human diarrhoea, vomiting, even death.
Caused by a nematode worm, Trichinella spiralis (and some other species of Trichinella), which causes few, if any, clinical effects on the pig but represents a major zoonotic hazard to those consuming undercooked pork or some raw cured pork products. Infection occurs in countries with wild boar or infected carnivore populations. The larvae are eaten as cysts in meat or muscle, excyst in the small intestine, mate and the females burrow into the gut wall to give rise to larvae which encyst in the muscles. Encysted larvae may remain viable for 10 years and give rise to trichinosis in man when eaten.
Mode of transmission
Trichinella sp can be found in a number of mammalian and a few avian species. In all, infection follows the ingestion of larvae encysted in muscle, and the worms excyst, mate and produce new larvae which encyst in the muscles of the host. Although pig-to-pig transmission can result from cannibalism (tail biting) directly, in most cases it results from the consumption of infected muscle.
This can be of pig origin when carcases are consumed or from infected raw pork products used in pig feeds, but is usually from the consumption of rodent carcases or fresh rodents captured and eaten by pigs. Reservoirs of trichinella infection persist in wild animals and feral pigs, with foxes being one of the most important reservoirs of infection in the wild. Its introduction to domestic pigs in an area usually follows the introduction of contaminated meat from elsewhere or the infection of pig farm rodents from wildlife reservoirs.
None in the pig. Diagnosis is based on the presence of the worm in muscle at slaughter. Antibody can be detected in serum or in meat juice by ELISA methods, using excretory antigen from larvae and monoclonal antibodies. Antigen is present in the blood within 1-4 weeks and for 10-14 weeks after infection, and the Polymerase chain Reaction (RAPD-PCR) has been used. In countries such as the Netherlands which are essentially T. spiralis-free, the sampling of foxes which act as reservoirs of T. britovi has been suggested as being of greater value than trichinoscopy (direct muscle examination).
Pig carcases are sampled routinely for trichinella in most European countries and exports in trade from third countries are expected to be free from viable parasites. Infection is detected by laboratory means, at its simplest, using trichinoscopy (squashing the muscle to see the larvae with a microscope). In the EU, examination is by enzymic digestion of specified amounts of muscle from sites where larvae are most frequently found. Larvae are unaffected by this process and can be concentrated and seen by microscopy. Encysted larvae can be seen in muscle cells by histology, but this is not sufficiently sensitive or rapid to be of commercial use. The muscles of choice in the domestic pig and wild boar are those of the diaphragm, neck muscle or tongue.
Treatment and prevention
Albendazole has been shown to eliminate larvae at 10 mg/kg from live pigs. Viable larvae can be killed in meat and meat products by freezing the meat at least to -18°C for periods specified in regulations. Prevention is based on monitoring pig carcases at slaughter in areas where infection has been demonstrated in pigs, rodents or in carnivores, principally foxes. Infected carcases are destroyed or treated by freezing under state veterinary supervision. This procedure provides meat safe for human consumption. An alternative approach to ensuring that pork is safe is used in the US, where freezing of all carcases for a brief period is carried out.
Where infection occurs in a country, areas or farms may be considered free from infection provided that they have a record of freedom of the carcases produced from infection, monitor wildlife systematically, have enclosed farms and practise rodent control. Such programmes can only be carried out with the agreement of state veterinarians.
This condition is a serious health hazard to humans as infection is painful and sometimes fatal. It is a major consideration in international trade and requires constant vigilance.