TRICHINELLA SPIRALIS: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE - BIOLOGY - 2024

Trichinella spiralis is a roundworm belonging to the phylum nematoda that causes a disease known as trichinosis in humans and other mammalian animals. It was first described in 1835 by the English biologist Richard Owen, who also described trichinosis and its mechanism of infection.

Trichinella spiralis is a parasite that requires a host, specifically a mammal, to develop. The host is often the pig. Likewise, it is a parasite with a wide distribution in the world. However, the highest number of infections have been reported in Europe and North America, while in South America and Africa it is rare. Despite this, cases in these latter places have been on the rise in recent years.

The pig is the main host for Trichinella spiralis. Source: Pixabay

The disease caused by this parasite is mild, as it usually resolves on its own after a few months. However, in some cases there may be certain complications that compromise the life of the person who is infected.

Taxonomy

- Domain: Eukarya.

- Animalia Kingdom.

- Phylum: Nematoda.

- Class: Adenophorea.

- Order: Trichurida.

- Family: Trichinellidae.

- Genus: Trichinella.

- Species: Trichinella spiralis.

characteristics

Trichinella spiralis is a roundworm, the cells of which are eukaryotic. This means that their genetic material is enclosed in the cell nucleus. Likewise, they are triblastic, since during their embryonic development three germ layers are formed: endoderm, mesoderm and ectoderm. From them the tissues and organs of the animal are formed.

Regarding symmetry, this worm has bilateral symmetry, which implies that if a line is drawn along the longitudinal plane, the two halves obtained are exactly the same.

In the same way, Trichinella spiralis is an endoparasite because in order to develop it must be inside a host, taking advantage of it and causing damage.

This is a dioecious species because the sexes are separated, that is, there are female and other male individuals. Likewise, it is a viviparous species since, unlike other nematodes, the larvae develop inside the female and then be expelled.

Morphology

Trichinella spiralis are smaller parasites, if compared to most of those belonging to the phylum nematoda.

Like all roundworms of this phylum, the body of Trichinella spiralis is surrounded by a kind of resistant layer, known as the cuticle. It also has a complex longitudinal musculature system, which is of vital importance in the displacement process of this parasite.

Importantly, your esophagus occupies the first third of the body. It has a tubular part that is surrounded by cells called esticosites, which together make up what is called a esticosome. The esticosites have the function of synthesizing antigenic polypeptides that are secreted in granules.

Adult female

As in most nematodes, the females are larger than the males. They are approximately 3mm long by 60-90 microns wide. Likewise, the vulva is located very close to the esophagus, specifically in its middle part.

Similarly, they have a single uterus which is divided, in an imaginary way, into two sections: an anterior section, which contains juvenile larvae that have hatched from the eggs; and a posterior section, in which there are developing eggs. Near the anal opening is the ovary. The eggs it produces have 3 chromosomes.

Adult male

The male of Trichinella spiralis is smaller than the female. It has an average length of 1-1.5 mm and 30-40 microns wide. The front end is flatter than the rest of the body. The anus is located at the terminal end.

Among its most outstanding morphological characteristics, it can be mentioned that it has two lobular caudal appendages, which are very useful during the act of copulation with the female.

It also has an open cuticle, nerve ring, esticosomes and an oral opening. The intestine is divided into three parts: midgut, hindgut, and cloaca. It is monorchid, which means that it has only one testicle. Their sperm are characterized by lacking flagella and having between 2 and 3 chromosomes.

Larvae

The larvae are approximately 0.9mm by 1mm. They are generally found rolled in a lemon-shaped capsule.

Those that are female are characterized by presenting a telogonic ovary. In this type of ovary, germ cells proliferate in a single part of the ovary, and not in its entirety. They also present outlines or primordia of the uterus and the seminal receptacle.

On the other hand, the larvae that are male have a long rectum, approximately 50 microns. Likewise, the anterior end of the testicle is curved backwards.

The characteristics mentioned are distinctive of each larva, so they are used by specialists to differentiate them as accurately as possible.

Biological cycle

Like all parasites, Trichinella spiralis requires a host in order to carry out its life cycle. Sometimes there may be an intermediate host and a definitive host. In most cases, the definitive host is the pig, while the intermediate host can be a rodent such as rats.

Life cycle variations

According to specialists, the life cycle of this parasite presents three variations depending on the host it has.

This is how there is a domestic life cycle in which the host of the parasite is a pig. There is also the wild life cycle, whose hosts are wild animals such as fox, bear and wolves, among others. And finally, in the semi-domestic life cycle the hosts are usually domestic animals such as cats, dogs and some rodents.

Sequence

The life cycle begins with the entry of the larvae into the host's body. This occurs in most cases due to the ingestion of raw or undercooked meat, in which cysts of the parasite are present.

When cysts are ingested, they are exposed to the action of various digestive enzymes and the low pH of gastric juices, which results in the release of the larvae. These reach the small intestine, where the cycle continues.

In the small intestine, the larvae penetrate the intestinal mucosa and undergo various molts, passing from L2 larvae to L5 larvae, until finally reaching maturity. Adult parasites invade the cells of the intestinal epithelium. It is in these cells where the copulation between male and female occurs.

With regard to this, specialists have discovered that females secrete a chemical substance that has a pheromone function, which exerts an attraction on the male, causing it to move to where the female is for fertilization to occur.

When fertilization finally occurs, the male dies, while the female penetrates the intestinal mucosa. There, after a few days (approximately 10), it releases the larvae, which can measure up to 0.008 mm with a diameter of 7 microns.

Life cycle of Trichinella spiralis. Source: CDC

The larvae enter the blood vessels, specifically the veins and through the venous return reach the heart (right atrium and ventricle), passing from there to the lungs, returning again to the heart (left atrium and ventricles) to be distributed through the arterial blood.

Myocytes: from muscle cells to nurses

Through general circulation, the larvae can be distributed in various organs of the body. However, they have a predilection for striated muscles that are particularly active, such as those found in the lower limbs (quadriceps) and upper limbs (biceps), as well as those related to speech and chewing (masseter and language).

Inside the muscle cells, the larvae begin to grow and transform into L1 larvae. Likewise, the larvae cause a series of structural and functional changes within these cells, which become nurse cells.

Among the modifications that muscle cells undergo, we can mention: increase in the number of mitochondria, loss of the organization of contractile fibers, and increase in catalytic activity. That is, the myocytes completely lose their function and adopt the ideal conditions for the larvae to thrive.

The whole of the nurse cell with the larva inside is what is known as a larval cyst. Each cyst can contain up to three coiled larvae, and these can remain dormant for up to two years.

Finally, when another animal ingests meat containing the larval cysts, the cycle begins in another host again.

Diseases

Infection by individuals of Trichinella spiralis is known as trichinosis. The infection mechanism is through the ingestion of raw or poorly cooked meat from an animal that is infested with cysts of this parasite. The main source of infection for humans is contaminated pork.

This infection is more frequent in rural areas, while in urban areas the cases that may occur are very rare.

Symptoms

Sometimes, Trichinella spiralis infection is asymptomatic: that is, a person may have ingested the cysts of the parasite, but not show any symptoms or signs of this. However, in most of the person various symptoms appear, as the larvae develop.

Initial phases

In the early stages of infection, when the larvae are released from the cyst, the following symptoms may occur:

- Nausea.

- vomiting

- Frequent liquid stools.

- Diffuse abdominal discomfort.

- General tiredness.

Advanced phase

Later, when the larvae that were produced in the intestine pass into the bloodstream and begin to invade other tissues apart from the intestine, some symptoms that together are called systemic symptoms occur, among which may be:

- Intermittent fever (more than 39 ° C).

- Intense headache.

- Edema (swelling) facial or periorbital (around the eyes).

- Pain and discomfort at the muscular level.

- Photosensitivity.

- Ocular petechiae at the level of the conjunctiva.

- Inflammation of the conjunctiva.

- General tiredness and weakness.

As the larvae become encyst, the symptoms begin to subside until they finally disappear. However, the individual remains with the cysts in their muscle tissue.

Graphic image of the larvae encysted in the striated musculature. Source: Trustedperson

Sometimes the infections are more serious than normal and can trigger serious diseases, such as myocarditis.

Diagnosis

The diagnosis of trichinosis is made in two ways: a blood test or a biopsy of muscle tissue.

- Blood test: this test looks for signs that indicate that there is an active Trichinella spiralis infection. These signs involve an increase in eosinophils (a type of white blood cell) and the presence of antibodies to this parasite. The latter appear in the bloodstream approximately 5 weeks after infection.

- Muscle biopsy: a sample of the muscle that is believed to be affected is taken so that it can be analyzed by a doctor who specializes in pathological anatomy. This will look for the presence of cysts in the tissue.

Although both tests are considered complementary, muscle biopsy is performed very rarely. The doctor usually makes the diagnosis based on the clinical picture and the blood test.

Treatment

Treatment for people with trichinosis is similar to other parasite infections.

The most commonly used medications to treat this infection are anthelmintics such as albendazole and mebendazole. However, these drugs are effective only when the larvae are still in the intestine and have not passed to other tissues. In other phases of the infection, this treatment is ineffective.

Likewise, to alleviate the pain caused by the larvae it is common to prescribe analgesics.

Once the larvae encyst, they end up calcifying. When this happens, the symptoms and signs subside until they disappear.

References

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3. Despommier, D. (1990). Trichinella spiralis: Te worm that would be a virus. Parasitology today. 6 (6) 193-196
4. Murrel, K. (2016) The Dynamics of Trichinella spiralis epidemiology: out to pasture ?. Veterinary Parasitology. 231.
5. Pozio, E., Paterlini, F., Pedarra, C. and Sacchi, L. (1999) Predilection sites of Trichinella spiralis in naturally infected horses. Journal oh Helminthology. 73 (3). 233-237.
6. Romero, J., Enríquez, R., Romero, J. and Escobar, M. (2009). Trichinellosis. Medical bulletin of the Hospital Infantil de México. 66 (3)