WHAT IS THE LOOS CYCLE? - BIOLOGY - 2025

The Loos cycle is the term used to designate the life cycle of nematodes within their host. The cycle consists of the stages that lead to maturity of the parasite in its host.

In mammals - including humans - the cycle involves a series of organs that nematode larvae often pass through to become mature entities. These generally reside in organs of the circulatory, digestive and respiratory systems.

Ascaris lumbricoides.

Source: Calicut Medical College

One of the most common nematodes that parasitize humans is Ascaris lumbricoides, popularly known as a roundworm, since its morphology is reminiscent of the common earthworm.

In man, this nematode is located in the small intestine. The parasite copulates in this cavity and the female releases the eggs. These go out into the environment, where they contaminate the soil.

If a person ingests the eggs in the infectious stage, they will hatch inside and the parasites will reach their intestines, completing the cycle.

What are nematodes?

The Loos cycle describes the passage of nematodes through their host. Therefore, it is necessary to know a little more about nematodes.

Members of the phylum Nematoda are ubiquitous vermiform pseudocoelomate animals commonly known as round or cylindrical worms.

There are more than 25,000 described species, although it is estimated that there are up to three times as many nematodes. They are considered the four richest animal group in species, surpassed only by arthropods, protozoa and mollusks.

Within nematodes, there are free-living species that have colonized a wide diversity of habitats, from fresh and salt water bodies to the polar regions. Many of these are capable of withstanding extreme environmental conditions.

Nematodes are characterized by their abundance. Fruits have been found to harbor extraordinary numbers of nematodes, more than 50,000 individuals in a decomposed fig!

Another abundant group of nematodes includes parasitic forms of medical and veterinary interest.

Life cycle of

The Loos cycle is a term that encompasses the life cycle of nematodes. Among the helminths, the Ascaris lumbricoides species is, without a doubt, the best known.

In addition, it is the nematode that grows larger. Females with lengths reaching almost 50 cm have been reported (females are often larger than males).

A single female can harbor more than 27 million eggs, producing up to 200,000 eggs daily. These are expelled in the stool.

Soil stage

When the eggs are outside and in favorable environmental conditions (temperatures not higher than 30 ° C and not lower than 15 ° C, partially humid soil and controlled solar radiation), the embryo of the egg undergoes two molts - even indoors of the shell - becoming an infectious biological entity.

The eggs remain in the ground for about two to four weeks. The passage to humans occurs through elements contaminated by eggs, generally by food or other utensils that have had contact with infected soils.

If the person ingests the egg in the non-larvated state, infection does not take place. The egg must be ingested in the infecting stage.

Male stage: small intestine

The man acquires the parasite by the consumption (oral route) of the eggs. These develop in the small intestine, thanks to enzymes that degrade the shell, a fact that favors hatching.

Later, the larvae cross the wall of the intestine and take the lymphatic or venous route to reach the heart. From this organ they pass to the capillaries of the lungs where they molt again. In this stage, the capillaries of the pulmonary alveoli break and undergo a molt.

The parasite ascends through the bronchi, the trachea and the pharynx, where they are swallowed and again reach the intestine, where they transform into an immature youngster.

Already in the small intestine, the parasites copulate and the female releases the eggs. The intestinal contents of the host carry the eggs, which are expelled into the environment. For the cycle to be completed, the eggs must reach the soils with the environmental characteristics previously described.

Symptoms and complications

The parasite lodged in the intestine does not pose major complications and they are not very aggressive. The damage includes mild intestinal dysfunction. When the volume of the parasite is massive, the complication can increase significantly causing subocclusion and intestinal occlusion.

It is common mainly in infants. In the most extreme cases, A. lumbricoides infection results in perforation, volvulus, and torsion of the small intestine. On some occasions, so-called erratic migrations can occur, where adults are housed outside the digestive tract.

The parasite can migrate to the upper parts of the digestive system, arising through various body orifices, including the mouth or nose. This migratory process can be complicated and the parasite is capable of obstructing the airways.

Diagnosis

An ascariasis (medical term used to designate the infection by this nematode) can be demonstrated by the expulsion of the parasite in the patient's stool, where they will be clearly visualized or by performing a stool examination. In infected patients, the parasite eggs will be visible and quantifiable.

The diagnosis of the parasite does not entail major complications, since the expulsion of the eggs in the patient's stool occurs constantly and very abundantly.

Treatment of infection

For patients with A. lumbricoides, the use of the drug piperazine is recommended. It has been observed that with the administration of 100 mg / kg / day for two days more than 90% of the cases are resolved.

Another suggested drug is pyrantel pamoate, mebendazole, kainic acid, and albendazole. In the most complicated cases, doctors resort to surgical removal.

Contagion prevention

As the active form of the parasite resides in soils, it is important to avoid soil contamination in order to effectively break the nematode's life cycle.

The most useful preventive measure is the proper handling of feces, properly sanitizing hands and utensils when consuming food.

References

1. Barnes, RD (1987). Invertebrate zoology. WB Saunders company.
2. Cabello, RR (2007). Human microbiology and parasitology: Etiological bases of infectious and parasitic diseases. Panamerican Medical Ed.
3. Lee, DL (2002). The biology of nematodes. CRC Press.
4. Murray, PR, Rosenthal, KS, & Pfaller, MA (2015). Medical microbiology. Elsevier Health Sciences.
5. Roberts, LS, & Janovy, J. (2009). Gerald D. Schmidt & Larry S. Roberts' Foundations of Parasitology. McGraw-Hill.
6. Zeibig, E. (2014). Clinical Parasitology-E-Book: A Practical Approach. Elsevier Health Sciences.