ECHINOCOCCUS GRANULOSUS: MORPHOLOGY, HABITAT, LIFE CYCLE - BIOLOGY - 2025

The Echinococcus granulosus, dog tapeworm or hydatid worm is a flatworm of the cestode class. It is one of the causative agents of cystic echinococcosis, also known as hydatidosis. Other species of cestodes of the genus Echinococcus are of medical importance, including E. multilocularis, E. oligarthrus, and E. vogeli.

Tapeworms or tapeworms are endoparasites of the digestive system of vertebrates. They have flattened shapes, similar to a ribbon. The body of these organisms is composed of three parts known as: the scolex, the neck and the strobilus.

This parasite is a small tapeworm that lives in dogs and other canids. Juvenile forms will develop in intermediate hosts, which includes man, among other mammals. Cysts can reach considerable sizes within their intermediate hosts, causing serious health problems.

This parasite is found worldwide, being a significant problem, not only at the clinical level, it also leads to significant losses in livestock. It is found most often in tropical areas.

Cystic echinococcosis is considered a rural disease, although it can occur in urban areas, when canids have access to livestock.

Morphology

E. granulosus belongs to the phylum flatworms. This group is characterized by not presenting coelom. They are organisms with the shape of worms flattened dorsoventrally. They present bilateral symmetry, the oral and genital openings are located in the ventral area. They lack an anus.

They have sensory and ciliated epidermis. The muscular system is of mesodermal origin and with several circular, longitudinal and oblique fibers under the epidermis.

Flatworm sperm have two flagella, contrary to the standard characteristic of these reproductive cells.

Cestodes can be differentiated from the rest of the flatworms by two specific features: absolute lack of digestive system and presence of microthric.

These are microvilli that function as projections to increase nutrient absorption. They help compensate for the lack of digestive system in these organisms.

Adult morphology

The adults are small worms measuring 3-6 mm in length. The parasite is divided into scolex, neck and strobilus:

Escolex

It is an organ of fixation. It features suction cups or hooks to be able to fulfill its purpose. The presence or absence and spatial distribution of these structures allows the identification of different species of cestodes.

In this species, the scolex measures 0.25 mm and the rostellum is of the non-retractable type. It has two crowns (or rows) with small hooks. The number of hooks varies between 20 and 50. It has four protruding oval-shaped suction cups.

Neck

Area where the development of new proglottids occurs.

Strobilus

It is a body sector composed of a linear series of organs. It is composed of three proglottids or segments, known as immature, mature and gravid.

These segments are marked on the outside by grooves. This species only has 3 to 4 proglottids.

Larva morphology

The larva can reach a diameter of 0.5 to 1 cm in approximately 6 months, although it can reach sizes larger than 10 or 15 centimeters.

It has a globular and opaque appearance. The cyst wall consists of three layers: parakeet, ectocyst, and endocyst. The endocyst can measure from 60 to 70 um.

The cyst has an internal fluid. It is a clear substance, rich in salts, carbohydrates and proteins.

Habitat

The adult worm lives in the small intestine of dogs and other canids, such as the fox. They can also be found in some felines.

The larval or juvenile stage, called cysticercus, develops in the form of a hydatid cyst. It is found in the viscera of humans and herbivorous hoofed animals, such as sheep, goats, cattle, and horses. They can also be found in some rodents.

Biological cycle

The adult worm is found in the small intestines of its definitive hosts, dogs, wolves, foxes, and other canids. The eggs pass through the feces to their final hosts.

Intermediate hosts, including humans and hoofed animals, acquire infection by ingestion of eggs. When a canid that has the parasite deposits its feces in the pasture, it favors the contamination of ruminants and other animals.

In humans, the main route of infection is the ingestion of eggs thanks to living with infected canids.

The egg hatches and releases the oncosphere. It penetrates the walls of the intestine and through the circulatory system is carried to various organs, including the liver, lungs, spleen and bones.

They can reach the heart through the venous circulation and in this way are taken to the lungs. In these organs the hydatid cyst develops.

When the definitive host ingests the cyst into the organs of the intermediate hosts, the protoscolices of the cyst are released. Later, the scolex can adhere to the intestine and develop as an adult.

The life span of these adult parasites is between 6 and 30 months.

Clinical features

In humans, hydatid cyst infection is generally asymptomatic. Symptoms develop when the cyst produces some type of obstruction or a pressure effect.

In most cases, the primary stage of the disease occurs in the liver. Another common site is the right lung.

In animals the manifestation of the disease is very rare. And if it does occur, it does so by manifesting non-specific symptoms.

Diagnosis and treatment

For the diagnosis of this cestode, serodiagnosis, molecular diagnosis (by means of the PCR technique) or by examination of the samples under the microscope can be used.

However, this technique cannot differentiate between the eggs of different species of tapeworms. Another type of diagnosis is by radiological images or ultrasound.

Treatment varies depending on the state of the disease. In the early stages, puncture, aspiration, injection, and re-aspiration can be carried out. This procedure, known as PAIR, is a non-invasive option to remove cysts.

They can also be removed through surgical treatments. Some commonly used medications are albedazole and praziquantel. The latter completely eliminates the parasite from infected dogs.

The disease can be prevented by taking appropriate hygienic measures. Among them, avoiding pet access to the entrails of the animals and continuously deworming the canids.

References

1. Berenguer, JG (2007). Manual of parasitology: morphology and biology of parasites of sanitary interest (Vol. 31). Editions Universitat Barcelona.
2. Larrieu, E., Belloto, A., Arambulo III, P. & Tamayo, H. (2004). Cystic echinococcosis: epidemiology and control in South America. Latin American Parasitology, 59 (1-2), 82-89.
3. Mahmud, R., Lim, YAL, & Amir, A. (2018). Medical Parasitology: A Textbook. Springer.
4. Pérez-Arellano, JL, Andrade, MA, López-Abán, J., Carranza, C., & Muro, A. (2006). Helminths and respiratory system. Archives of bronconeumology, 42 (2), 81-91.
5. Quiróz, H. (2005). Parasitology and parasitic diseases of domestic animals. Editorial Limusa.