Introduction

Trichinosis, also known as trichinellosis, is a disease caused by the parasitic roundworm of the genus Trichinella. Trichinosis is categorised as a foodborne infection, and its transmission to humans occurs through the consumption of raw or undercooked meat that has been contaminated with the larval form of the parasite.¹ The primary sources of trichinosis are related to eating domestic pork. However, the parasitic roundworm can be found in other animal products, such as wild boar, wild game meat and horse meat.² It is one of the most widely distributed parasitic diseases that affect animals and humans globally.^3,4 While this disease is not common in recent times due to improved farming and cooking practices, symptoms of infected individuals may range from simple gut-related problems to severe health-threatening complications.⁵ For this reason, raising awareness about the way this parasite is transmitted and learning about symptoms and prevention methods is essential. 

Parasite life cycle

Examining the life cycle of parasites is essential for understanding how they invade the human body.⁶ The Trichinella parasite does not require more than one host and can complete its full life cycle in the human body. The life stages of the Trichinella parasite can be divided into several stages.

Infected meat ingestion

The infection begins when a person eats undercooked or raw meat, which carries the parasite. More specifically, the striated muscle of the animal that gets ingested contains cysts of the Trichinella larvae, which are the immature form of the roundworm.³

Excystation and invasion

Once eaten, the larvae move from the oesophagus to the stomach and finally into the small intestine of the human. At that stage, the cyst surrounding the larvae is broken down (excystation), and the larvae are released. The larvae then make their way into the intestinal mucosa (inner lining of the intestines) and settle into the epithelial cells (cells covering different surfaces of the body).³

Parasite larvae develop into adults

Within approximately 30 hours, the Trichinella larvae undergo four moults until they reach their immature adult worm stage, where they become either male or female.³

Adult Trichinella reproduce

Female and male Trichinella adults will mate and produce thousands of larvae, which are then deposited onto the intestinal wall.³

Migration of larvae across the human body

The Trichinella larvae will then begin their migration through the lymphatic system and enter the circulatory system, which facilitates their spread through the blood supply.³

Formation of cysts in the human body

Similar to the way Trichinella larvae were ‘stored’ in the muscle of the infected animal, they now form cysts in the striated muscle cells of different tissues, like the arms, diaphragm and mouth.³

The host muscle cell transforms into a nurse cell

At this stage, an intriguing phenomenon that is unique to this parasite occurs – the formation of a nurse cell. The parasite ‘tricks’ the human body into transforming the invaded cell into a nurse cell, which acts as a nutrient supplier for the parasite. This is achieved by the formation of blood vessels around the nurse cell to ensure it is well-nourished by its host. The life cycle of the parasite is finally complete, and the larvae will remain inactive in the human muscle cells until they are consumed by another host.³

Sources of contaminated meat

Trichinosis is an ancient disease transmitted between carnivorous animals and humans through the consumption of meat. Over the years, the parasite evolved to ensure it spreads to new host organisms without worrying about survival.

Today, Trichinella-contaminated meat typically comes from undercooked or raw sources.³

Meat that has not been treated with heat (e.g. through cooking) has a significantly high chance of causing disease.

The animals that display the most reported cases of trichinosis are:³

Domestic pork

Historically, most cases of trichinosis are related to the consumption of contaminated pork meat and products. This is due to the domestication of pigs and their widespread consumption by humans. In recent times, the risk of commercial pork carrying Trichinella parasites has shown a significant decline because strict regulations regarding farming and the diet of pigs are applied. This does not necessarily apply to pigs that are raised outdoors in rural areas or family-farmed pigs, where farmers may be unaware that their pigs have close contact with rodents or wild animals, or that they may consume animal carcasses.

Wild game meat (e.g., boar, bear)

Hunting of wild animals, such as boars, bears and seals, followed by ingestion of the raw or undercooked meat from wild game, can cause trichinosis. The risk of infection may be higher compared to domestic pork, as the diet of these animals is not controlled.

Contaminated equipment

A trichinosis infection may still occur if a person does not directly eat an infected animal, but rather meat that was processed or ground using the same equipment that Trichinella-carrying meat was used on previously.

Cured or fermented meat

The process of curing or fermenting meat, in the case of traditional sausage or dried meat products, may not ensure parasite eradication. Lower than necessary temperatures and pH levels, which allow the parasite to thrive during fermentation, may not be sufficient to kill the Trichinella larvae.

Geographical distribution of different Trichinella species

The most widespread species of the parasite that is responsible for serious illness caused primarily by pork across Europe, Asia and America is T. spiralis. T. britovi is linked to wild boar and other wild animals in Europe, Asia and Africa, while T. nativa is identified in Arctic and Subarctic mammals and is highly resistant to freezing temperatures. The last highly pathogenic species is T. pseudospiralis found in wild mammals and birds across Europe, Asia, Australia and North America.^3,6

Symptoms and health implications

Once the parasite is identified by the human immune system, a variety of symptoms may present as a reaction. Clinical features can be split into different phases according to the spread of the infection in the human body: intestinal, invasive and convalescent.³ 

Intestinal phase: the parasite migrates to the intestinal cells

Symptoms appear 1 or 2 days after the infected meat was ingested and they typically last between 2 to 7 days.

While some patients may be asymptomatic due to a low number of larvae in their body, symptoms of the intestinal parasite may include:

• Diarrhoea (most common)
• Nausea
• Vomiting
• Constipation
• Abdominal pain
• Anorexia
• Fever

These symptoms are common in most intestinal illnesses, which often leads to misdiagnosis. For this reason, many infected people do not receive medical attention at the early stages of the disease.

Invasive phase: parasite migrates to the circulation

Symptoms are more likely to occur during this phase, typically 2-6 weeks after ingesting the contaminated meat, and may last for weeks to months.

The parasite’s ability to invade tissues across the human body may lead to allergic reactions, which are different to the generic intestinal phase symptoms, helping doctors diagnose trichinosis.

Symptoms of the invasive phase include:⁸

• Face and hand oedema due to damaged blood vessels
• Muscle-related dysfunctions, such as weakness and difficulty across the body and mouth due to parasite migration to muscles important for walking, breathing, swallowing, etc
• High white blood cell count, as parasite-specific eosinophils are triggered by the immune system
• Heart and neurologic symptoms, such as myocarditis, headaches, vertigo, convulsions, and encephalitis

Convalescent phase: recovery

Finally, the convalescent phase marks the recovery stage of the patient, when the infection has been eradicated and the human body is healing. Recovering individuals may show few signs of illness 5-6 post-infection, and may still suffer from symptoms for up to a decade after recovering. Weight loss and psychological symptoms from the burden of the disease may persist, and parasitic larvae may remain in the cysts inside the nurse cells they created.

Diagnosis 

Trichinosis presents subtle early symptoms, making diagnosis complicated. When a patient suspects a trichinosis infection, the diagnostic process combines:

• Assessment of the dietary history
• Exclusion other infections
• Laboratory tests for Trichinella larvae (enzyme immunoassays for parasite-specific antibodies)
• Muscle biopsies (not preferred due to their invasive nature)

While a positive test can lead to a successful diagnosis, antibodies are usually detectable 3-5 weeks following the infection, meaning that early diagnosis is not always possible.⁹ Additionally, the antibodies may remain detectable in the human body for over a decade, making the standard diagnosis method not sufficiently sensitive.¹⁰

Prevention and treatment

Prevention of trichinosis, although seemingly simple, requires attention to detail when preparing and consuming animal meat. The following preventative measures will help lower the risk of infection in general:

• Cooking meat thoroughly: internal temperature should be at 71°C or higher to kill larvae
• Proper meat handling: disinfected equipment and hygienic conditions help reduce contamination risk.
• Avoid raw or undercooked meat: larvae are not visible to the naked eye
• Control animal environment: prevent contact with infected animals when farming, avoiding regions of established Trichinella populations when hunting
• Education & awareness: learning about the transmission of the disease and educating individuals who may be affected

Treatment options for trichinosis include anthelmintic drugs, such as albendazole and mebendazole.¹¹ These anti-parasitic medications are highly effective in killing adult worms, especially at the early phases of infection, but are not as successful at eliminating encysted larvae stored in muscle cells. Other options include glucocorticosteroids for symptom alleviation during treatment and probiotics for gut strength that effectively protects against infection.^5,12 Research on medicinal plant extracts for trichinosis therapy shows promising activity against migrating larvae, while vaccines that promote early immune response to parasite invasion and block their establishment in the body are under investigation.^13,14

Summary

Trichinosis is a foodborne infection that is caused by the parasitic roundworm Trichinella. A variety of species can cause the disease but in all cases, infection occurs through the consumption of undercooked meat coming from an infected animal. This is usually pork and wild game meat. Diagnosing trichinosis early can be complicated due to non-specific symptoms, but proper cooking and hygiene protocols when consuming meat, as well as educational and awareness programmes on ways in which this disease is transmitted, are key to prevention.