Overview

Trichinosis, formerly known as Trichinellosis, is a parasitic infection caused by roundworms (nematodes) belonging to the Trichinella species.¹ This disease is prevalent in all parts of the world except Antarctica.

It affects both domestic and wild animals as well as humans. The Trichinella parasite uses birds, reptiles, amphibians, and mammals as reservoirs. In animals and humans, infection occurs due to the consumption of meat that contains Trichinella larvae. Upon entering the host body, these larvae encyst in striated muscle tissues, particularly in areas such as the tongue and diaphragm.⁵ Studies have shown that infected animals exhibit clinical symptoms that are very mild or sometimes absent. Whereas human symptoms can be severe and occasionally fatal.

The significance of this disease lies in its impact on animal health, food safety, and the risk of transmission to humans. Veterinarians play a vital role in diagnosis, symptom management, and the prevention of trichinosis.

The causative agent of Trichinosis

Trichinella worms are among the world’s smallest species of nematodes. There are currently ten different identified species of this genus,⁶ among them, T. spiralis (T1) is highly prevalent and distributed globally.

The life cycle of Trichinella in the host has 3 stages:

The Enteral stage: The encysted larvae enter the host via consumption of infected meat. In the stomach, the gastric acid and pepsin dissolve the capsule and release the larvae.

The Parenteral stage: The freed larvae enter the small intestine and mate. The young larvae (0.1mm) are released into the lymphatic channels for circulation. They enter the highly vascularized striated muscles found in the tongue, pectoralis major, and gluteus maximus.⁷

The Encysting stage: Upon entering the striated muscle cells, the larvae grow in size and encapsulate. Further, with the help of host vascular endothelial growth factor, the encapsulated cysts form nurse-cell complexes and remain contained in the striated muscles.⁸ The entire life cycle of Trichinella takes approximately two weeks to complete. 

The ecology and epidemiology of trichinosis

Studies show that Trichinosis is globally distributed ⁹ in domestic and wild animals. The main factors that contribute to this are

• Physiological adaptations of the encysted larvae ¹⁰
• Hunting practices of abandoning animal carcasses ¹¹

_{The domestic and sylvatic live cycles of Trichinella species. United States Centers for Disease Control and Prevention (CDC). Available at: https://www.cdc.gov/dpdx/trichinellosis/. Public domain.}

Transmission of Trichinella species occurs by either the domestic cycle or the sylvatic cycle. In the domestic cycle, most often, pigs and rodents are infected due to cannibalism and by eating meat scraps. Later, predators feed on these rodents and continue to be the reservoirs. In the sylvatic cycle, animals like bears and wild boars are infected due to scavenging and by preying on the primary hosts, such as pigs. Later, these animals often become sources of human infection. Given the complexity of trichinosis transmission, eradication is technically impossible.¹² Only routine screenings and staying alert about possible outbreaks can help contain the spread.

The clinical manifestations of trichinosis in animals

Most of the animals infected with Trichinella do not show any symptoms at all. But in some cases, when there is acute infection, the following symptoms manifest:

• Gastrointestinal distress, leading to irritation of the gut lining¹³
• Muscle pain, weakness, and stiffness, usually at the diaphragm, tongue, and limbs¹⁴ 
• Reduction in the growth rate and decline in reproductive efficiency in livestock ¹³
• Generalised inflammation, fever, and lethargy in wild and domestic animals ⁴

Usually, the infected wild and domestic animals have a subclinical expression of the disease and act as a silent reservoir, helping in spreading the parasite to other animals and humans. Due to this zoonotic nature of the Trichinella parasite, crucial measures to stop its spread are essential.

Diagnosis of trichinosis in animals

There are two main ways to detect Trichinella larvae in animals

1. Diagnosing infections in live animals to prevent disease transmission
2. Examining meat to ensure food safety

The following methods are used for this purpose:

Microscopy-based techniques

Trichinelloscopy

This method of direct detection is used in surveillance situations, particularly to examine wild and domesticated animals for Trichinella infection. It is also used in meat processing industries and slaughterhouses to certify the safety of meat for consumption. 

A tissue sample of 0.5–1.0 g is obtained through biopsy from the affected animal. The diaphragm, the forearm muscles, and the tongue are the recommended sampling sites, as per the International Commission on Trichinellosis.¹⁵ The tissue samples are compressed between two thick glass slides and examined via an optical microscope to visualise the encysted larvae.^17,18

Even though this method shows direct evidence of the parasite infection, Trichinelloscopy is not favored, as it often fails to detect larvae of non-encapsulating species or larvae that have yet to encapsulate. Also, it is influenced by the sample size and type.¹⁶ Due to these drawbacks, trichinelloscopy cannot be highly considered as the main method of certification.¹⁹

Magnetic stir bar method (MSM) 

Compared to direct microscopy, MSM is a more sensitive technique. The meat samples are treated chemically to break down tissues and release encysted Trichinella larvae. To ensure that there is no clumping and uniform digestion, a magnetic stir bar is employed. After a series of filtrations, the sediment is examined for Trichinella larvae.

Laboratories performing MSM need to follow the strict International Commission on Trichinellosis (ICT) standards ²⁰ and International Organisation for Standardisation (ISO) guidelines^21,, due to which the test is internationally accepted as the “gold standard method” for surveillance of Trichinellosis ²². Studies have shown that this method can detect very minute amounts of larvae.²³

Despite being highly sought-after, MSM still has loopholes, like false implication of results due to larva washout ²⁴ and hazardous long-term exposure to HCl and pepsin.²⁵

To overcome these drawbacks, a complementary testing procedure for MSM called Artificial Antigen Detection (AAD) Technology ²⁶ has evolved. The procedure utilises an immunoassay-based approach to identify Trichinella antigens and is the popular choice.

Serological testing 

When Trichinella larvae enter the host tissues, they release excretory/secretory (ES) antigens for which the host produces IgG antibodies. An ELISA assay is performed to detect these IgG antibodies in the serum samples of the infected animals.²⁷ Due to high sensitivity (99.6%) and specificity (98%)²⁸ it is used for surveillance studies ²⁷.

But ELISA cannot differentiate between active and past infections ²⁷ nor can it distinguish between different species. To overcome these drawbacks, the positive ELISA serum samples are combined and tested with western blot.^29,30,31

An agglutination assay can be performed to test for the presence of ES antigens, especially in the digested meat samples (from food processing units) and serum samples of the live infected animals.³² A high sensitivity of 80% was achieved in this test, as per a study.³³

Molecular testing 

These tests for the diagnosis of Trichinella infections focus on amplifying specific DNA sequences of Trichinella, making it highly accurate. The most targeted gene for PCR is the conserved regions of the subunit ribosomal DNA -internal transcribed spacers.³⁴

Many variations of Polymerase Chain Reaction (PCR), including Quantitative PCR (qPCR)³⁵, Loop-Mediated Isothermal Amplification (LAMP)³⁶, and Lateral Flow-Recombinase Polymerase Amplification (LF-RPA)³⁷ are used for the detection. Out of which, the Quantitative PCR (qPCR) is the most promising.³⁸

These molecular testing methods offer quick results and are highly sensitive, they can detect even species, making diagnosis easier. But these testing techniques need a high investment initially and are also not feasible for field conditions.

Management and control of trichinosis 

Trichinella infections in domestic animals and wildlife do not require a treatment plan. But a control strategy is required due to its public health significance. 

For domestic animals, this involves reducing the risk of infection by preventing access to infected meat, cannibalism, and exposure to infected rodents or wildlife.³⁹ Regular surveillance in the farms and slaughterhouses can help to stop the spread. But the same cannot be done for wildlife.

To prevent transmission of Trichinella into humans, regular serological examination of carcass samples at slaughterhouses⁴⁰, proper meat preparation⁴¹, testing of Wildlife carcasses, and educating about the risks of scavenging and cannibalism of game meat needs to be undertaken. 

However, Trichinella is one problem that requires multiple disciplinary actions. Management is possible only on a peripheral level to reduce the risk. 

Summary 

Trichinosis is a disease caused by a nematode called Trichinella. The disease affects both domestic and wild animals, as well as humans. Trichinella Infection occurs due to the consumption of meat containing encysted larvae, which migrate to striated muscles, particularly the diaphragm and tongue. Animals often show mild symptoms, but infection in humans can be severe.

The life cycle of this parasite Trichinella has three stages: larvae ingestion, intestinal development, and encystment. Once encysted, the parasite transmission occurs through either the domestic cycle involving pigs and rodents, or the sylvatic cycle involving wild carnivores like bears and wild boars.

Animals infected with Trichinella usually show subclinical symptoms, but in severe cases may undergo gastrointestinal distress, muscle pain, weakness, fever, and reduced reproductive efficiency.

Diagnosis of Trichinella involves detecting larvae in meat samples or ES antibodies in serum. Microscopy-based methods such as trichinelloscopy and the Magnetic Stir Bar Method (MSM) are used in meat processing industries. Serological tests like ELISA detect antibodies but cannot differentiate active from past infections. Molecular testing, including PCR and qPCR, provide high sensitivity.

There is no treatment for Trichinella in animals, nor is eradication possible due to its complex epidemiology, but its spread can be controlled. Control measures include preventing exposure to infected meat, rodents, and cannibalism in domestic animals. Regular farm and slaughterhouse surveillance, proper meat cooking, and testing wildlife carcasses help prevent human infections. Public education on the risks of consuming undercooked game meat is the need of the hour.

Due to its zoonotic nature, managing trichinosis requires a multidisciplinary approach focused on reducing transmission risks.