Introduction

Nematodes of the Trichinella genus are the aetiological agents of the human disease called trichinellosis (or trichinosis), which can be fatal in severe cases. These parasites are widespread in all continents excluding Antarctica,¹ and their transmission occurs through the ingestion of live Trichinella larvae in the muscle tissue of infected animals.² Thus, the main source of transmission is the consumption of raw or insufficiently cooked meat, especially pork and its byproducts. This causes an important public health issue in countries where the consumption of pork is high, although there are other animal hosts such as cats, dogs, brown rats, and armadillos.³

There are different species of Trichinella, the most common being Trichinella spiralis. Hoever, other species are also capable of causing human disease.⁴ Despite its global distribution, trichinellosis remains easy to misdiagnose due to its nonspecific symptoms, which can mimic other conditions.⁵ Regardless, it is crucial to study the Trichinella species in order to identify the possible source of infection, the geographical origin of the parasite and to assess the risk of infection for domestic pigs and humans.⁶

Taxonomy and classification

The genus Trichinella comprises 12 recognised species groups, which can be classified into two main categories based on whether they develop a protective collagen capsule in muscle tissue. The encapsulated group, including the main relevant species of medical significance, such as  T. spiralis, T. nativa, T. britovi, presents the characteristic capsule-like cyst around the larvae in the host’s muscles. In contrast, the non-encapsulated group, which includes T. pseudospiralis, does not form such a capsule after muscle cell dedifferentiation. Even though the life cycle of the two groups is quite similar, they show some differences in how they affect the host’s body. For example, they have distinct parasitological, pathological and immunological processes. Moreover, another key difference lies in their host range. Encapsulated species complete their life cycle only in mammals, which maintain body temperatures between 37°C and 40°C. In contrast, a broader range of hosts, including mammals, birds, and reptiles, can be infected by the non-encapsulated species. This is mainly because they are capable of developing at body temperatures from 25°C to 42°C. Thus, the type of vertebrate host plays a crucial role in activating different physiological and immunological mechanisms that influence parasite development and survival.⁷

Epidemiology

All 13 Trichinella species are distributed worldwide, excluding Antarctica, with cases reported in wildlife across 66 countries and 55 countries for humans. Historically, T. spiralis in domestic pigs was the main source of human infection, but improved biosecurity has significantly reduced this risk, making wildlife the primary concern.

Here’s a list of the most significant species for human health and their distribution:

• T. spiralis, found worldwide, is still present in domestic and wild pigs. Nearly eradicated in many developed countries, but remains a concern in regions with low biosecurity
• T. nativa, found in Arctic carnivores like polar bears and walruses
• T. britovi is common in wild carnivores in Europe, North Africa, and Asia
• T. pseudospiralis is globally distributed, infecting mammals and birds
• T. papuae and T. zimbabwensis, found in Papua New Guinea, Australia and Africa, infecting mammals and reptiles, are transmitted through wild pig and crocodile meat⁸

Life cycle

The life cycle of Trichinella species takes place within the intestine of the host. The first phase of the cycle, known as the enteral phase, occurs after human or animal ingestion of contaminated meat containing the infective muscle larvae. Once in the stomach, the gastric acids digest the protective cyst, followed by the subsequent release. They reach the upper small intestine and invade the epithelial layer, where they develop ⁹. After four moults, the larvae mature into reproducing adults. Females can deposit up to 1,500 newborn larvae 4-5 days after eating the infected meat, before being expelled from the host’s immune system. Afterwards, the infection progresses to the next stage, in which the newborn larvae migrate into tissues, entering the lymphatic vessels and general blood circulation, which determines the peripheral or systemic stage. This allows for a wide distribution of the larvae in the tissues until they reach the muscle fibres through the capillaries. It is here that the muscular phase of the infection begins: once in the muscle, depending on the species, the larvae can encyst, develop and become infectious within 15 days, then persist for months or even years.¹⁰

Clinical manifestations

The first symptoms of trichinellosis to occur are associated with gastroenteritis, linked to the first stage of the infection. They manifest 2 to 7 days after contact with the parasite, and include:

• Diarrhoea, nausea and vomiting, all due to the presence of the worms in the small intestinal mucosa
• Muscle pain, often felt in the mid-abdomen, face and chest
• General malaise
• Low-grade fever

In most cases, these symptoms are so painful that they can lead to difficulties in performing daily activities.⁴

However, enteral symptoms are often misdiagnosed because they are similar to those of common gastrointestinal conditions.¹⁰

Approximately 2 to 6 weeks later, the symptoms of the parenteral phase appear, which consist of the migration of the newborn larvae into the muscle tissue of various organs. These symptoms are often the first to be clinically detected, and include: 

• Fever
• Chills 
• Weakness and fatigue 
• Diffuse myalgia 
• Periorbital or facial edema 
• Conjunctivitis and subconjunctival hemorrhages
• Skin rash, most commonly in the form of urticaria
• Splinter hemorrhages on nail beds
• Retinal hemorrhages
• Paralysis-like state
• Prolonged diarrhea, may occur in some cases

Other less common manifestations:

• Headache
• Cough 
• Dyspnea 
• Dysphagia⁴ 

In some individuals, trichinellosis can result in serious complications, such as myocarditis, which could result in death, despite the fact that the newborn larvae don’t directly invade the heart muscle.¹¹ Many other severe cases present conditions such as meningitis, encephalitis,⁹ respiratory myositis,¹² renal failure,¹³ and more.

However, it is very important to recognise that factors such as host immunity, age, sex, and overall health play a crucial role in determining the outcome of the disease.¹⁰

Diagnosis

The clinical diagnosis of trichinellosis is very difficult:  early infection lacks pathognomonic signs or symptoms,  while later chronic forms are not easy to diagnose.

The delayed diagnoses may lead to the establishment of larvae in muscle tissue and to the development of a collagen capsule, which leaves the larvae resistant to drugs.¹⁴

As Trichinella infection progresses, laboratory tests show increasing eosinophilia (>7% of total white blood cells or >1000 eosinophils/µL).

Other nonspecific markers are:

• Elevated creatine phosphokinase levels (>0.5 mg/mL) indicate muscle damage
• Elevated lactic acid levels (>2.0 mmol/L), which indicate sepsis
• An elevated troponin level (>0.23 ng/mL) indicates myocardial damage

Serologic diagnosis is based on detecting Trichinella-specific antibodies using an immunosorbent assay (ELISA). IgE appears early but is often undetectable due to its short half-life. Anti-Trichinella IgG detection via ELISA is the most sensitive method, recommended in acute and convalescent phases (tests 14 days apart), as IgG may only appear 20-35 days post-infection. 

To confirm the Trichinella species and avoid cross-reactivity, molecular techniques (PCR) targeting specific genetic regions are necessary, but are mainly available at state labs. Indirect immunofluorescence tests, based on nematode surface antigens, are less sensitive due to frequent cross-reactions in patients with autoimmune diseases and other helminthiases.

Microscopic parasitological studies, including muscle biopsies, squash preparations, or touch smears, help to confirm the presence of encysted larvae. In infected meat, parasite burden is microscopically quantified by counting encysted larvae per gram of muscle tissue.¹⁵

Treatment and management

Trichinellosis is treated with anthelmintic drugs, which include:

• Albendazole: oral administration, recommended dose is 400 mg to be taken twice daily for 10-14 days
• Mebendazole: oral administration, recommended dose is 200-400 mg to be taken three times daily for 5-7 days
• Pyrantel: recommended for children and pregnant women, given as a single dose of 10-20 mg/kg of body weight, but it is only effective against intestinal larvae and not against muscle larvae

Therapy should begin immediately, if possible, within the first three days of infection as it would prevent muscular invasion and the progression of the disease.³

However, treatment generally begins after the appearance of symptoms caused by larval invasion and encystment in muscle, such as fever, rash, periorbital and facial oedema, and severe muscle pain, which is the reason why an adjunctive therapy with analgesics is usually required.¹⁵

Prevention and control

To prevent the infection with larval worms of Trichinella spiralis, it is recommended to freeze cuts of pork less than 15 cm thick for 20 days at -20°, but this is not effective against the larvae of other species, which are freeze-resistant, such as Trichinella britovi and Trichinella nativa. Wild game meat must be cooked thoroughly to an internal temperature of 71 to 74°C or higher to kill encysted larvae. It is also recommended that people who handle raw meat should wash their hands with soap and water before and after touching it. They should also clean all tools, like knives, cutting boards, and tables, that were used to prepare the meat. Therefore, hunters play an important role in controlling Trichinella infections in wild and domestic animals. To help stop the spread of the disease, hunters should not leave animal carcasses or waste in the field after skinning or throwing away unwanted meat. This helps prevent the disease from spreading to other animals.¹⁵

Summary

Trichinellosis is a disease transmitted to humans through the ingestion of raw or undercooked meat infected with parasites of the Trichinella genus. 

Different species can cause health issues, but the most common one is Trichinella spiralis. 

Trichinellosis can present with mild symptoms, including digestive discomfort, in the initial stage of infection. However, it can progress to serious conditions leading to heart and brain inflammation. Thus, it is crucial to diagnose the disease as early as possible, although this remains very difficult due to the lack of specific symptoms which are very similar to those of other gastroenteritides. Treatment involves antiparasitic medications, but their effectiveness decreases if the larvae have already settled in the muscles. However, preventing trichinellosis can be done by simply following some general hygiene rules, such as cooking meat thoroughly, mainly pork and game meat, as they are the main source of transmission. In addition, monitoring animals and educating consumers can be very useful to reduce the risk of infection and the spread of the parasite.