Overview

Trichinosis (trick-i-noh-sis), now more commonly called trichinellosis (trick-i-nell-oh-sis), is a parasitic disease attributed to consuming raw or undercooked meat and is caused by nematodes (roundworms) of the genus Trichinella. Trichinella spp. (where “spp.” refers to either all Trichinella species or unknown species) has been infecting humans and animals for centuries. First observed in 1835 by Richard Owen and James Paget, in London, United Kingdom, Trichinella spiralis or T-spiralis (a type of trichinella) quickly became associated with the consumption of undercooked pork. In 1977, it was argued that Trichinella spp. larvae were recovered from a cyst in the muscles of an Egyptian mummified in approximately 1200 B.C.¹ Some researchers propose that Trichinella spp. may have been linked to carnivorous dinosaurs and prehistoric mammals.²

Cases today are increasingly linked to the consumption of pork and wild game meat, but general consumption of domestic and wild animals (including birds and reptiles) has also posed risks in the past.^3,4 Trichinellosis remains a public health concern in certain regions, particularly where food safety measures are inadequate. This article explores the epidemiology of trichinellosis, its transmission, risk factors, global distribution and strategies for prevention and control.

Background

What is trichinellosis

Human trichinellosis occurs when humans consume raw or undercooked meat containing the encysted larvae of Trichinella spp. parasites.⁴ Once ingested, stomach acids release the larvae from the cysts, and they mature into adult worms in the small intestine, where male and female worms reproduce. The worms live for about four weeks in the small intestine but release larvae after approximately 1 week. These larvae then migrate to muscle tissues, where they become encysted.

Trichinellosis infection can be divided into two stages:⁵

• Intestinal (enteral) phase:
  • If there are more than a few hundred larvae in the intestines, gastrointestinal complications may arise, such as gastroenteritis (diarrhoea, vomiting and abdominal pain) or constipation
• Muscular (parenteral or systemic) phase:
  • One week after infection, migration of larvae into muscle tissues can cause:
    • Fever
    • Swelling around the eyes and the rest of the face
    • Headaches
    • Photophobia
    • Muscle and joint aches and pains
    • Weakness
    • Splinter haemorrhages (small damaged blood vessels under the nails)
    • Rashes
    • Itchy skin
    • Peripheral eosinophilia (an increase in eosinophils, white blood cells which help the body fight infections and other foreign substances)
  • Symptoms intensify around the third week
• Minor infections may be asymptomatic
• In severe cases, the following complications can occur and can even lead to death:⁵
  • Myocarditis or heart failure
  • Blood clots
  • Central nervous system involvement, such as encephalitis or difficulty coordinating movements
  • Shortness of breath, persistent cough, respiratory failure or pneumonitis 
• Symptoms normally begin to ease 5 to 6 weeks post-infection⁵
• With no treatment, chronic trichinellosis, where worms continue to reproduce for months to years post-infection, can result in conjunctivitis, excessive sweating, numbness and muscle weakness for up to 10 years⁵ 
• Trichinellosis symptoms are not generally observed in animals⁵

Transmission routes

The life cycle of Trichinella spp. primarily involves carnivorous and omnivorous animals; however, herbivores (primarily horses, but can include sheep, cattle, beavers and moose) have been known to be “accidental hosts”.⁶ Potential transmission routes include:⁶

• Ingesting contaminated feed
• Attacking or predation
• Biting
• Cannibalism 
• Scavenging 

Animals, including pigs, rodents, bears, foxes and wild boars, become infected when they consume meat containing Trichinella spp. larvae. Humans acquire trichinellosis by eating undercooked meat from these infected animals.

Causative agents

In addition to T. spiralis, which infects many carnivorous and omnivorous animals worldwide, several other Trichinella species have been identified:

• Trichinella pseudospiralis: mammals and birds worldwide 
• Trichinella britovi: carnivores of Europe and western Asia
• Trichinella papuae: wild and domestic pigs in Papua New Guinea and Thailand
• Trichinella nativa: polar bears 
• Trichinella nelsoni: African predators and scavengers
• Trichinella zimbabwensis: found in crocodiles in Africa, but to date, there are no known cases of human infection

Global distribution and epidemiology

Cultural practices, including the consumption of traditional dishes which feature raw or partially cooked meat or meat-based products, significantly influence the epidemiology of trichinellosis.⁷ To ensure food safety, the slaughter of animals for food is now largely controlled by veterinary, public health and food safety agencies globally. 

Global prevalence

The epidemiology of trichinellosis varies widely between regions, depending on dietary habits, food safety regulations, and wildlife reservoirs. The World Health Organisation (WHO) estimates approximately 10,000 human trichinellosis cases occur worldwide every year, with the United States (U.S.) Centres for Disease Control and Prevention (CDC) reports a mortality rate of approximately 0.2%. Global prevalence data for trichinellosis is difficult to obtain due to the size of outbreaks, which tend to be small and related to a specific batch of meat or a single event.

North america

In North America, cases of trichinellosis have significantly declined due to stringent meat inspection and food safety practices. According to the CDC, in the U.S., people who eat raw or undercooked pork or wild game (especially bear, fox, wild boar, wolf, wildcat, walrus or seal) are at risk of developing trichinellosis.⁸ Canada also has a low trichinellosis incidence according to the Canadian Food Inspection Agency (CFIA), with occasional outbreaks related to the consumption of undercooked or raw meat from wildlife species such as walruses and bears.

Click here to read a case report on a 2022 trichinellosis outbreak in Minnesota, United States, following the consumption of undercooked bear meat sourced from Canada, as well as two individuals who consumed only vegetables and became infected due to cross-contamination. 

Europe

In Europe, trichinellosis cases have declined due to rigorous meat control measures, for example, mandatory testing of pork for Trichinella spp. larvae. However, sporadic outbreaks still occur, particularly in Eastern Europe where home-slaughtered pork and wild boar are commonly consumed.³ The European Centre for Disease Prevention and Control (ECDC) stated that in 2022, there was a 49% decrease in trichinellosis cases compared with 2021. 8/28 countries in the European Union/European Economic Area (EU/EEA) accounted for a total of 39 cases of trichinellosis, with Latvia and Bulgaria accounting for the highest notification rates in the EU/EEA at 0.16 and 0.13 cases per 100,000 population, respectively. France had the highest number of cases, representing 38% of all cases reported. Eating undercooked pork from free-range or non-confined pigs, as well as wild boar hunted in the wild, poses the greatest risk of contracting trichinellosis in the EU/EEA. Most European countries (including the United Kingdom which has not seen a human trichinellosis case from UK-sourced meat in over 30 years) reported no cases of human infection in 2022. 

Asia and oceania

The People’s Republic of China (PRC) remains one of the countries with the highest incidence of trichinellosis, particularly in rural areas where backyard pig farming and wild meat consumption are prevalent.⁹ Between 1964 (when the first case of human trichinellosis was documented in Tibet) and 2009, 557 outbreaks of human trichinellosis, with 25,125 cases and 251 deaths, have been reported in China.⁴ Conversely, though there are no documented outbreaks associated with domestic pig consumption, outbreaks have been linked to undercooked or raw bear meat consumption in Japan and raw soft-shelled turtle shell meat in Taiwan.³ 

Trichinellosis outbreaks following consumption of raw and undercooked domestic pigs and wild boar continue to occur in pockets of Tibet and Southeast Asia (Cambodia, Laos, Vietnam and Thailand), where traditional raw pork dishes are popular and often consumed by mountainous tribes during religious celebrations and feasts (e.g., larb)

Though trichinellosis is rarely reported in the Middle East (potentially due to religious proscription prohibiting the consumption of pork), occasional cases are seen, particularly in regions where wild boar meat is consumed (e.g. Christian communities in Lebanon).¹⁰ 

Similarly, there is a lack of reporting on trichinellosis outbreaks in Oceania, though sporadic infections have been documented in New Zealand and Australia.¹¹ Interestingly, in the Western region of Papua New Guinea, where the consumption of wild boar is common, 10% of the population living there were found to have anti-Trichinella antibodies.¹⁰ 

South and central america

Most countries in the Caribbean, Central America, and South America have not reported Trichinellosis infections in humans, pigs or wildlife, but this is likely due to lack of data as opposed to absence of disease.¹¹ Argentina and Chile have the highest number of human cases due to the consumption of traditional pork products. In Argentina, outbreaks have been associated with homemade pork sausages, leading to periodic spikes in cases.

Africa

There is limited recent data on human trichinellosis in Africa (though there exists plenty of data on wildlife infections), but sporadic cases have been reported - mainly linked to bushmeat consumption.¹¹

The arctic

Many populations in the Arctic rely on hunting game meat, a cultural and traditional practice essential for survival in the region.¹³ Freezing, which would typically inactivate other species of Trichinella spp., does not inactivate arctic-adapted species of Trichinella.¹³ Drying, smoking and fermenting meat is typically preferred over cooking meat, with the consumption of polar bear and walrus meat posing a significant risk of infection in this region, despite polar bears being a protected species.¹³

Trichinellosis risk factors

• Consumption of undercooked meat: eating raw or insufficiently cooked pork, wild game, or bushmeat increases the risk of infection
• Hunting and meat processing practices: hunters and those handling wild game are at higher risk if they do not follow proper meat safety measures
• Traditional and cultural food practices: certain dishes, such as raw, dried or fermented meat products, are associated with trichinellosis outbreaks
• Poor animal husbandry and food safety standards: Inadequate meat inspection and improper feeding of livestock (e.g., allowing pigs to scavenge on infected carcasses) contribute to disease transmission

Prevention and control measures

Meat inspection and food safety regulations

Strict meat inspection practices have been pivotal in reducing trichinosis cases. In the EU/EEA and the UK, regulations mandate Trichinella spp. testing for all slaughtered pigs, significantly lowering human infections. In the U.S., commercial pork is rarely a source of trichinellosis as a consequence of regulated farming practices, veterinary medicine and inspection methods.

Surveillance systems and reporting

Reliable surveillance systems are essential for gathering and preserving precise prevalence data. Approaches include outbreak analysis, laboratory and clinical-based investigations, and case documentation. Factors such as underreporting, lack of healthcare-seeking, poor healthcare infrastructure, weak veterinary practices and varying diagnostic standards pose challenges to accurate reporting.

Safer cooking practices

Cooking meat to an internal temperature of at least 71°C (160°F) kills most Trichinella spp. larvae. Freezing pork at -15°C (5°F) for at least three weeks also inactivates most Trichinella species, although some species found in wild game are resistant to freezing.

Public health education

Raising awareness about safe meat handling and cooking practices is essential, particularly among high-risk populations such as hunters and indigenous communities. Campaigns targeting traditional food preparation methods have successfully reduced infections in some regions.  

Summary

While trichinosis has declined in many high-income nations due to improved food safety measures, it remains a concern in regions where traditional dietary habits and inadequate meat inspection persist. Public health initiatives, including stringent meat testing, proper cooking education and awareness campaigns, are crucial in preventing future infections. As global food consumption patterns evolve, continued vigilance and research are needed to mitigate the risks associated with Trichinella spp. infections.