ASCARIS SUUM: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE - BIOLOGY - 2024

Ascaris suum is a parasite belonging to the phylum nematoda that is characterized as the causative agent of ascariasis, mainly in pigs. It has many similarities with Ascaris lumbricoides, which is why it is sometimes confused with it.

It was described for the first time by the German zoologist Johann Goeze in 1782. Ascaris suum is a well-studied organism since, due to the damage it causes in pigs, it constitutes a harmful agent for the pig farming industry.

Ascaris suum specimens. Source: Alan R Walker

This organism can be considered representative of the genus Ascaris, since both its behavior and its life cycle are typical of its members.

Taxonomy

- Domain: Eukarya.

- Animalia Kingdom.

- Phylum: Nematoda.

- Class: Secernentea.

- Order: Ascaridia.

- Family: Ascarididae.

- Genus: Ascaris.

- Species: Ascaris suum.

characteristics

Ascaris suum is a eukaryotic pluriceular organism. This implies that it is made up of several types of cells that have their genetic material (DNA) delimited by a membrane (nuclear membrane), forming an organelle known as the cell nucleus.

Likewise, they are animals with bilateral symmetry. During their embryonic development they present three germ layers: mesoderm, ectoderm and endoderm. These layers give rise to all organs with specialized functions.

This member of the phylum nematoda is considered an endoparasite, because it lodges inside a host that it harms. In fact, it feeds on the nutrients that the host animal ingests.

These parasites have a type of sexual reproduction with internal fertilization. In this process, the male introduces his copulatory spicule into the female's genital pore. It is important to note that the females of this species have the ability to lay a large number of eggs, which are released through the host's feces.

Morphology

Because it belongs to the group of nematodes, Ascaris suum is a round worm, not segmented, and it presents sexual dimorphism. This means that there are marked morphological differences between female and male specimens.

In general, the adult specimens of this species of nematode have a fusiform shape and a pale color. Sometimes they have a yellowish appearance, and others, pinkish.

Female

Females have an approximate length that ranges between 22 cm and 50 cm, with a width of between 3 and 6 mm. Its rear end is conical in shape and ends in a rounded way. Likewise, on the lateral edges they present enlargements called postanal papillae.

Ascaris suum female and male specimens. Source: VlaminckJ

Males

On the other hand, as is typical in the organisms of the genus Ascaris, the male is smaller than the female. It can measure between 14 - 32 cm and have a width of 2 to 4 mm.

Its rear end ends curved. It presents here some extensions called spicules that can measure up to 3.5mm in length and are used in the copulatory act.

Similarly, the male has a series of papillae at its posterior end, of which 75 pairs are precloacal and 7 pairs are postcloacal. In addition, on the inner edge of the cloaca there is a single odd papilla.

At the anterior end of this parasite, in both males and females, there are three lips: one dorsal and two ventrolateral. Each of them has papillae at its base. The dorsal lip has two papillae, whereas the ventrolateral lip has a lateral papilla and a subventral double papilla.

The characteristic element that makes it possible to differentiate between Ascaris suum and Ascaris lumbricoides is that the inner edge of the three lips that the latter presents at its anterior end has a toothed edge.

Eggs

The eggs are surrounded by a capsule that in turn is made up of three layers: an external one that is yellowish brown in color, an intermediate one that is composed of proteins and chitin and an internal one, of the yolk type, which is made up of lipids.. The latter is waterproof, so it greatly protects the embryo, preventing the entry of any toxic substance.

The approximate size of the eggs is between 61 and 75 microns long by 50-55 microns wide. Its shape is rounded.

The eggs may or may not be fertilized. Unlike the fertilized ones, those that are not are more elongated and narrow. Internally they have a mass of unorganized granules.

Habitat

Ascaris suum is widely distributed throughout the world. It is especially abundant in humid, temperate and tropical climates.

Your main host is the pig. In this animal it is located specifically at the level of the small intestine, where it feeds on the nutrients ingested by the host.

Lifecycle

The life cycle of Ascaris suum is similar to that of other parasites of the genus Ascaris. When in the small intestine, the female releases a large number of eggs, an average of 300,000 daily. This number is relative, since cases have been reported in which a female can lay up to more than 600,000 per day.

Those eggs are released to the outside through the host's feces. In the external environment, when the environmental conditions of humidity and temperature are adequate, the larvae evolve into an infectious form, known as L2 larvae. This process can take between 23 and 40 days.

The pig, which is the main host of this parasite, ingests the eggs with the larvae in stage L2. In the small intestine, due to intestinal and gastric juices, the eggs hatch and the larva is released.

Ascaris suum life cycle. Source: US gov

The larvae do not remain in the small intestine, but rather penetrate the intestinal wall and enter the circulation. It is subsequently transported to the liver, where it evolves to the L3 larval stage.

Immediately, it enters the veins and through the venous return that ends in the inferior vena cava, the larva reaches the heart (right atrium and ventricle).

The larvae then reach the lungs via the pulmonary artery and its many branches. Here it undergoes another molt and transforms into an L4 larva. It then makes its way into the pulmonary alveoli and begins its ascent up the respiratory tract toward the bronchi and trachea. Upon reaching the epiglottis, it is swallowed and passes into the digestive system.

It finally reaches its definitive habitat, the small intestine. Here again it evolves into L5 larvae (young adult). This occurs approximately 25 days after the host has ingested the infecting eggs. It remains there until it reaches full maturity and is capable of producing eggs.

Approximately 60 days after infection, the female is able to release the eggs for the biological cycle to restart.

Diseases

Ascaris suum is considered an almost exclusive parasite of pigs. However, it can sometimes infect humans and cause infections that occur in a similar way to those caused by Ascaris lumbricoides.

Likewise, when it comes to infection in pigs, this infection is very important. This is because it represents considerable economic losses for the pig feed industry.

The reason is that during the transit that this parasite continues in its life cycle through the host's organism, it causes serious damage to the tissues it passes through, mainly in the lungs.

Infection in humans

Although rare, this parasite can cause parasitic infections in humans. When a person ingests infective Ascaris suum eggs, the larvae follow the same path that they take in the pig's body, causing damage to these tissues.

The symptoms that a person manifests as a result of this parasite are the same as those related to Ascaris lumbricoides, such as frequent liquid stools, abdominal pain, vomiting and nausea. Likewise, at the pulmonary level there is evidence of symptoms that resemble pneumonia.

Infection in pigs

In pigs this infection is also known as ascariasis and it is very common in the farms that are responsible for raising these animals. This is because the parasite can be found in many places such as soil, water, food, grass and the skin of the breasts, among other places. Because of this, it is very easy for an animal to become infected.

Inside the animal's body, the parasite causes damage at the intestinal level, mainly by the action of its lips, which exert a certain harmful effect on the intestinal mucosa. This results in a condition known as catarrhal enteritis.

Likewise, for each tissue that the parasite larvae travels, it causes certain damage, mainly of an irritating nature. This triggers an inflammatory response in the host by the tissues.

Similarly, some specialists consider that the larvae of this parasite favor some bacterial infections, since in its passage from the intestinal lumen to the bloodstream it allows some bacteria such as Salmonella to reach the blood.

In the same vein, this parasite also exerts a toxic action, because it releases certain substances that are considered toxins. These can trigger a very serious allergic reaction in a sensitive individual.

Symptoms

The presence and severity of signs and symptoms depends on the amount of parasites found in the intestine of the animal. Sometimes, when the infection is not so intense, there are no symptoms. However, when symptoms do occur, they can be the following:

- Frequent liquid stools.

- Increase in body temperature.

- Prominent belly.

- Jaundice (yellow skin and mucous membranes).

- Delay in growth.

- Rickets.

- Cutaneous dystrophy with evidence of hyperkeratosis.

- Altered appetite.

- Seizures (in severe cases).

Likewise, some animals may show symptoms that indicate the development of pneumonia, such as a persistent cough with bloody sputum and high fever.

Diagnosis

The diagnosis of this pathology includes the procedures already known to diagnose any disease caused by an intestinal parasite. These methods include the following:

- Stool examination: through a microscopic examination of the stool it is possible to determine the presence of eggs in them. Even if the parasite is very intense, an adult specimen of the parasite can be captured.

- Clinical diagnosis: this is based on the expertise and experience of the specialist who reviews the affected animal, as well as on the observation and presence of certain signs. For example, the presence of a parasite in the stool or a larva in the sputum.

In the case of animals that die without having an exact cause of death, a post-mortem examination can be performed. In this it is possible to observe the lesions caused by this parasite in the various organs. For example, on the liver there are white spots known as "milk spots".

These are nothing more than scars that show the migratory passage of the larvae during their life cycle through the liver.

Likewise, in the small intestine of the animal, it is possible to study the lesions that this parasite leaves, as a result of chronic irritation of the intestinal mucosa.

Treatment

Treatment for this type of parasite encompasses several important aspects that, taken together, contribute to eradicating the harmful agent.

First, what must be done is to deworm all animals, regardless of whether they have shown symptoms or not. Likewise, a deep and significant cleaning of the pens and farrowing pens where they are should be carried out.

Similarly, because feces are considered a source of infection, they must be incinerated, since they may contain eggs with infective capacity.

Finally, it is important to apply a drug therapy that includes antiparasitic drugs, such as the following:

- Albendazole.

- Piperazine.

- Mebendazole.

- Oxfendazole.

Prevention

To avoid Ascaris suum infections, it is important to follow the following recommendations in pig farming sites:

- Regularly examine all pigs, even if they show no signs of infection.

- Use effective disinfection methods, such as the use of formaldehyde and hot steam.

- Carry out deep cleaning in the places where the pigs are habitually, such as feeders and drinkers.

In the case of humans, to avoid becoming infected with this type of parasite, it is important to adopt and practice hygiene measures such as washing hands after using the bathroom, properly washing food and avoiding direct contact with any soil that may be infected.

References

1. Costa, M., De Castro, I., Costa, G., Millena, L., Luciani, F., Kanadani, A. and Victor, J. (2015). Ascaris suum in pigs of the Zona da Mata, Minas Gerais State, Brazil. Brazilian Journal of Veterinary Parasitology 24 (3).
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
3. Dold, C. and Holland, C. (2010) Ascaris and ascariasis. Microbes and infection. 13 (7). 632-637.
4. Leles, D., Lyell, S., Iniguez, A. and Reinhard, K. (2012). Are Ascaris lumbricoides and Ascaris suum a single species ?. Parasites & vectors. 5 (1)
5. Loreille, O. and Bouchet, F. (2003) Evolution of ascariasis in humans and pigs: a multi - disciplinary approach. Memories of the Oswaldo Cruz Institute. 98.
6. Vlaminck, J., Levecke, B., Vercruysse, J., Geldhof, P. (2014). Advences in the diagnosis of Ascaris suum infections in pigs and their possible applications in humans. Parasitology 141 (14) 1-8