CRYPTOSPORIDIUM PARVUM: CHARACTERISTICS, LIFE CYCLE, DISEASES - BIOLOGY - 2025

Cryptosporidium parvum is a parasitic microorganism belonging to the phylum Apicomplexa and which is responsible for the development in humans of a disease known as cryptosporidiosis.

Of all the species that make up the genus Cryptosporidium, this is the best known and studied by specialists on the subject. This is because it is the one that most frequently infects humans and causes symptoms of disease.

Cryptosporidium parvum specimens. Source: See page for author

This parasite is more common and frequent in underdeveloped countries and communities in which hygienic conditions are poor, which facilitates its prevalence and transmission. This is why it is vitally important to maintain the necessary hygiene measures when preparing food and to be careful not to ingest the water from the pools in which recreational baths are taken.

General characteristics

Morphology

The morphology of Cryptosporidium parvum is quite simple, compared to other parasites.

Their oocysts can be oval or round in shape, measuring approximately 7 microns in diameter. In addition to this, each oocyst is delimited by a double membrane, which, depending on where the oocyst will go, can be thin or very thick and resistant.

Within each oocyst are structures called sporozoites. In mature oocysts, there are four sporozoites. These are vermiform in shape and are released when the oocyst wall breaks.

Lifecycle

The life cycle of Cryptosporidium parvum is very interesting, because despite the fact that it takes place within a single host, the parasite undergoes a series of transformations that even involve the two types of reproduction that exist: asexual and sexual.

Reservoir

The place where infectious forms of this parasite are found is water. However, contrary to what one might think, it is not only the water that is ingested, but also any recreational body of water, in which people usually enjoy a bath. Pools and lakes are in the first place as infection sites for this parasite.

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As expected, the entry of the parasite to the host, which is the human being, occurs through accidental ingestion of water infested with this parasite. Ingestion can occur through food that has been washed or prepared using contaminated water, or by accidentally swallowing some contaminated water during a recreational swim in a pool.

Infectious form

The infective form of this parasite is represented by oocysts. These have a fairly resistant cover, which allows them to survive adverse environmental conditions for a certain period of time.

Within each oocyst there are about four sporozoites, which will be released once the oocyst reaches the host's intestinal tract. This stage of the parasite reaches the environment through two routes: through the feces, in which they are contained, or through the expectoration of respiratory fluids during a cough.

Inside the host

When oocysts are ingested, they are resistant to the action of digestive enzymes present in saliva, as well as gastric juices in the stomach.

In such a way that they reach the small intestine practically intact. There they rupture and then the sporozoites that were inside are released into the intestinal lumen.

However, sporozoites have a special predilection for cells of the intestinal epithelium. Due to this, immediately after being released from the oocysts, they invade these cells in order to continue their development within them. Here they reach the next stage, which is the trophozoite.

Asexual reproduction

The trophozoite is the stage of the parasite that subsequently undergoes asexual reproduction. This reproduction is called schizogony. As with many processes that are asexual, this consists of several mitotic divisions.

After the first division, the trophozoite transforms into a type I meront, which has 8 merozoites inside. Two things can happen here. First, the merozoites within the type I meront can be released and infect more cells of the intestinal epithelium, transforming themselves into type I meronts. Second, the type I meront can continue its transformation process and become in type II meront, within which are four merozoites.

Cryptosporidium parvum life cycle. Source: CDC / Alexander J. da Silva, PhD / Melanie Moser (PHIL # 3386), 2002

Sexual reproduction

Those four merozoites found within the type II meront are released and are the ones that undergo the sexual reproduction process. This process is known as gametogenesis and, as its name implies, it consists of the formation of gametes (sex cells), both female and male.

In the case of Cryptosporidium parvum, the male gamete is known as microgamont and the female gamete as macrogamont. When both types of cells are mature, the union of them occurs, better known as fertilization.

As expected, the product of fertilization forms a single cell called a zygote, which eventually becomes an oocyst.

In an infected individual, in this part of the parasite's life cycle, two types of oocysts are produced. Some have a poorly resistant cover and are kept inside the host, with the intention of keeping the infection latent.

The other type of oocyst that is generated has a rigid and very resistant cover. These are expelled to the outside through the feces. Their cover allows them to survive harsh environmental conditions, so they can survive there for an extended period of time.

Variations in the life cycle

Despite the fact that in most cases, the life cycle of Cryptosporidium parvum develops in the intestinal tract, there are occasions when the parasite enters the host's organism through inhalation of oocysts.

In these cases, the life cycle takes place at the level of the lungs, whose cells are apt to develop the parasite, such as the cells of the intestinal epithelium. After their development, the oocysts are released to the outside through expectoration during coughing or sneezing.

Diseases it produces

Because Cryptosporidium parvum is a pathogenic parasite, it has the ability to trigger a host of adverse reactions and symptoms that greatly affect its health.

All these symptoms that affect various organs of the body are known, collectively as cryptosporidiosis. This mainly affects the digestive tract, so most of the signs and symptoms manifest at this level.

Risk factor's

A risk factor is everything that predisposes a person to be susceptible to contracting or manifesting any disease or pathology. Risk factors have to do with genetics, biology, and lifestyle.

The risk factors that can make it easier for a person to get Cryptosporidium parvum are the following:

- The ingestion of water contaminated with the parasite or of doubtful origin.

- Few hygiene measures during the preparation and cooking of food.

- Frequent bathing in swimming pools or places that a large number of people go.

- Maintain contact with animals, mainly pets. Farm animals can also carry the parasite and transmit it to humans, although this is rare.

- Direct contact with people who have diarrhea and who, of course, are infected with the parasite.

The people most prone to contagion are professionals who hold positions within the health system or those who work in daycare centers caring for young children.

Contagion

As already mentioned, the infecting form of this parasite is found in water, which is reached by the product of infected feces. In this sense, the most common form of contagion of this parasite is through the ingestion of contaminated water.

This can occur when taking a recreational bath in a pool or lake. Likewise, transmission can also occur through the ingestion of food that has been contaminated by being washed with water infested by oocysts.

Swimming pools are the main source of contagion. Source: Pixabay.com

The cases in which the contagion of this parasite occurs from person to person are very rare and fortuitous. In far fewer cases, contagion can also occur from an animal to humans. Although unlikely, but some cases have been documented.

The most common way is having water as a transmission vehicle.

Symptoms

As has already been stated, the parasite's oocysts enter the host in the vast majority of cases, through ingestion, so they remain in the digestive tract causing damage to the tissues and organs to which it adheres.

Sometimes there are those who are infected by the parasite and do not show any type of symptoms, so the microorganism can coexist quietly in the host's body for a long time.

Among the symptoms that occur most frequently in people infected with Cryptosporidium parvum, the following can be mentioned:

-Frequent liquid stools for two or more days

-Sickness

-Frequent vomiting

-High fever

-Loss of appetite

-Intense abdominal discomfort, characterized by colic and acute pain.

The duration of the standard clinical picture is approximately 2 weeks.

More complicated cases

When a person has a weakened immune system from an underlying condition, such as AIDS, symptoms tend to persist. This is because the mechanisms that the body has to fight infections do not work optimally and efficiently.

In the case of these people, the symptoms persist over time and can cause various complications. These include:

- Dehydration product of the loss of liquid through the evacuations.

- Decreased body weight and malnutrition. This is due to the loss of appetite on the part of the infected person and the poor absorption of nutrients by the intestinal epithelial cells that are infested with parasites.

- Deterioration of the general state of the organism, especially of the organs compromised by the chronic infection by Cryptosporidium parvum.

Diagnosis

When a doctor receives a patient with symptoms similar to those already described and suspects that they may be infected by this parasite, they may decide to perform some diagnostic tests. These include:

Acid fast stain

It is the definitive diagnostic method for this parasite. Cryptosporidium parvum belongs to a group of organisms known as acid resistant. This means that when it is subjected to a special dye and subsequently washed with an acid solution, it is able to retain the dye.

For this test, a sample of stool or intestinal tissue is taken and undergoes the aforementioned process. At the end, the sample is observed under the microscope and then it is possible to determine the presence or not of the parasite or oocysts.

Stool culture

It consists of taking a stool sample and observing it under a microscope in order to determine the presence of any foreign body in them. Although microcysts can be observed, the definitive diagnosis is given by the previous method.

Treatment

In general, Cryptosporidium parvum infections have a successful resolution within about two weeks. The care that the patient should have are those recommended for someone with diarrhea, such as good hydration.

However, when symptoms persist and the general condition of the infected person deteriorates over time, it is necessary to resort to various medications that, although they will not kill the parasite, can mitigate its adverse effects on the body.

Antiparasitic drugs

Like for example nitazoxanide. These types of drugs interfere with the metabolic activity of the parasite, thus stopping the damage that they may be causing to intestinal cells.

Antimotility drugs

They are drugs that reduce the movement of the intestinal tract. This allows the nutrients to stay longer in the intestine, so the absorption process is carried out optimally.

References

1. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
2. De la Parte, M., Bruzual, E., Brito, A. and Hurtado, M. (2005). Cryptosporidium spp. and Cryptosporidiosis. Journal of the Venezuelan Society of Microbiology. 25 (1).
3. Díaz, M., González, H., Leyva, E. and Mata, V. (2003). Incidence and viability of Cryptosporidium parvum in the drinking water of Ciudad Obregón, Sonora, Mexico. International Journal of Environmental Pollution.
4. Hernández, N., Hernández, L. and Cortés J. (2018). Cryptosporidiosis and a health. Journal of Public Health. 20 (1)
5. Hijjawi, N., Melloni, B., Ng'anzo, M. and Ryan, U. (2004). Complete development or Cryptosporidium parvum in host cell-free culture. International Journal of Parasitology. 34 (7).
6. Hijjawi, N. and Ryan, U. (2004). New developments in Cryptosporidium International Journal of Parasitology. 304 (6)