What is Trichinella?
Trichinella is a genus of parasitic nematode worms that can cause a potentially serious infection (trichinellosis or trichinosis) in humans following consumption of infected meat. Trichinella was first described as a cause of disease in man as early as 1865. Up to ten species (or genotypes) have been described, at least seven of which can infect man, but the principal species identified in human infection, and the species of most concern to the food industry is Trichinella spiralis. The other recognised species identified in human cases are T. britovi, T. pseudospiralis, T. nativa, T. murrelli, T. papuae and T. nelsoni, but these are less commonly found than T. spiralis and are usually associated with wild animals.
Trichinella species are found world wide and infect a wide variety of animal hosts, mostly carnivorous and omnivorous wild mammals, especially those that scavenge, such as foxes, bears, pigs and wild boar. Rodents, such as rats and mice, are also thought to play an important role as hosts in areas where the infection is endemic. The entire life cycle normally occurs within a single host species and consists of an adult worm and two larval stages. Humans are not definitive hosts, but may become infected by ingesting the infective second stage larvae, which may occur in cysts in the striated muscle tissue of infected animals.
What foods can be contaminated?
The infective second stage larvae of Trichinella occur in the muscle tissue of infected animals as very small, but detectable, cysts containing the larva. T. spiralis cysts are found in highest numbers in the diaphragm and tongue of the infected animal but can also occur in the skeletal muscles. Historically, infected pork from pigs fed with feed containing animal waste was the principal source of Trichinella infection in Europe and North America, but successful controls in pork production have greatly reduced the prevalence of infection in commercial herds. The prevalence in commercial pig herds in the EU has been estimated at less than 1 in 100,000 animals, with some variation between countries. In the USA, the prevalence of infection in commercial production had been reduced from an estimated 1.41% in 1900, down to 0.013% (13 in 100,000 animals) in 1995.
There is still some risk from home raised pigs and from pigs that are allowed to forage for food in the natural environment, which may include organically produced pigs. There is also a significant risk of infection from wild animals, especially wild boar in parts of Europe and bears in the USA. Imported horsemeat is also now a very significant source of infection in parts of Europe, especially France and Italy.
Raw, or undercooked meat is the principal vehicle for Trichinella infection in humans. The larvae do not survive effective cooking, and properly cooked pork and other meats present a negligible risk of infection. However, the larvae may survive in raw cured meats and some Trichinella species larvae are not killed by freezing. Therefore lightly processed and frozen pork or wild game products may still carry the risk of infection.
How does it affect human health?
The severity of trichinellosis in humans is highly variable. It may be asymptomatic in some cases, while in others complications may prove fatal. The severity of infection seems to be correlated with a number of factors, including the Trichinella species involved, the number of encysted larvae ingested and the strength of the immune response in the patient. The minimum infective dose is uncertain but has been estimated at between 100 and 300 live larvae.
After ingestion the larvae are released from the cysts by stomach acid and digestive enzymes and invade the lining of the small intestine, where they develop into adults. This process may be accompanied by gastrointestinal symptoms, including abdominal pain, vomiting and diarrhoea. Onset of symptoms typically occurs 24-48 hours after ingestion but may take longer. After about seven days the adult females release live larvae that migrate through the tissues to the striated muscles where they form cysts. This stage usually takes 4-8 weeks to complete and produces a different range of symptoms, including swelling of the face and around the eyes, fever, muscle pain, conjunctivitis and rashes. The production of the cysts usually causes muscle pain and weakness, but once it has been completed, the symptoms largely disappear.
However, in some cases potentially serious neurological and/or cardiovascular complications may occur, producing a variety of symptoms, such as headache, apathy, dizziness, chest pains and an irregular heartbeat. Rarely, complications may be fatal, especially in elderly people.
How common is illness?
It has been suggested that as many as 11 million people world wide could be affected by trichinellosis and an estimated 10,000 cases occur every year. However, the incidence of the disease in most European and North American countries has been decreasing for many years. For example, in the USA between 1947 and 1951, the average number of reported cases each year was 393 and 57 people died from the disease. But from 2002-2007 the annual average was only 11 cases, with no deaths. In the EU, there has been a general downward trend in the incidence of trichinellosis over the last 12 years, and the number of reported cases has been stable since 2000. However, incidence varies considerably between different countries. In 2008, 670 confirmed cases were reported in 13 countries, with the highest numbers being recorded in Romania (75%) and Bulgaria (10%). Many countries reported no cases, including the UK and Sweden. Elsewhere, relatively high incidences have been reported in Argentina (600 cases per year).
Many outbreaks of trichinellosis have been reported all around the world. In the EU there have been significant outbreaks in the last 20 years. Most of these have occurred in Spain, France, Italy and Germany and were caused either by horsemeat imported from third countries, wild boar, or non-intensively raised pigs. An outbreak affecting 124 people in Poland in 2003 was believed to have been caused by wild boar meat and a large outbreak in Romania in 2008, in which 108 people needed hospital treatment, was associated with pork sold without veterinary control. However, 52 cases reported in Germany in 1998-99 were linked to commercially produced raw sausages and minced meat.
Outbreaks in the USA have also been reported. In 1990, 105 people were affected in two outbreaks associated with raw sausages made from commercially produced pork. However, since that time, most outbreaks have involved foods prepared from wild game meat, including wild boar and bear.
Where does it come from?
Two distinct cycles for Trichinella are recognised by epidemiologists. The natural, or sylvatic cycle occurs in wild animals, especially carnivores that scavenge or exhibit cannibalistic behaviour. In this cycle, a number of the recognised Trichinella species are involved. The parasites develop in one host and infective encysted larvae are passed to another when infected tissues are consumed. In the domestic cycle, Trichinella (most commonly T. spiralis) circulate in farm raised pigs that are fed with feed containing infected animal tissue, or are allowed to come into contact with other infected animals.
The domestic cycle is now much less important in developed countries than was once the case, following improvements in pig husbandry and in statutory controls. For example, in the USA between 1997 and 2001, 72 cases of trichinellosis were reported and only 12 of these were associated with commercial pork products. The remaining cases were caused by consuming wild game, or pork raised under unregulated conditions. In the EU, the most important sources of trichinellosis are now wild boar meat, and horse meat imported from Eastern Europe. Some EU countries, including the UK, Ireland and Sweden, have not reported cases of human trichinellosis caused by locally produced meat products for at least 20 years.
How is it affected by environmental factors?
The encysted larvae of Trichinella species are extremely persistent in the live host and may survive for many years in striated muscle tissue. Encysted larvae of T. spiralis are not resistant to freezing and are killed by rapid freezing and storage at -20 oC or below for at least 48 hours. However, this may not be the case for other species of Trichinella. Infective Trichinella species larvae have been found in frozen bear meat after storage for more than two years. The larvae may also be able to survive some curing processes used for pork products. They are not heat resistant and are killed by temperatures above 60oC for 2 minutes.
How can it be controlled?
The principal control for Trichinella in commercial meat products is inspection by a recognised direct detection method, usually tissue digestion followed by microscopic examination of the remaining sediment. This is mandatory for pork, horsemeat and game in the EU and in other developed countries. Infected meat is designated unfit for human consumption.
For primary producers
Improved animal husbandry has been very effective in reducing Trichinella infection in commercial pig herds. Measures include ensuring that all pig feed is adequately heat processed to destroy infective larvae, effective separation of pigs from rodents and other potentially infected animals and good on-farm hygiene practices.
For food processors
The larvae of T. spiralis can be destroyed by freezing, cooking and by some curing procedures. The USDA has produced specific freezing and cooking times and temperatures for pork products and has also specified curing methods. Freezing times and temperatures are dependent on the size of the pieces of meat involved, but for cooking processes, fresh pork should reach a minimum internal temperature of 71oC. The EU has also specified several freezing treatments that can be used to kill T. spiralis larvae in meat. These are detailed in the relevant legislation (see below)
Freezing cannot be relied upon to destroy the larvae of other Trichinella species that may be found in game meat and horses.
Are there rules and regulations?
In EU legislation measures to protect consumers against trichinellosis are contained in a Commission Regulation (EC) No 2075/2005. This covers inspection of meat at slaughter, detection methods and freezing procedures.
The US Code of Federal Regulations contains similar requirements and includes recommendations for freezing, cooking and curing of pork products.
Many countries have introduced legislation regulating aspects of animal husbandry, meat inspection and pork processing designed to protect consumers from trichinellosis.
Where can I learn more?
CDC parasitic disease information – trichinellosis
New Zealand Ministry of Health – Trichinosis fact sheet
Food Research Institute Briefing – Foodborne Parasites