HYMENOLEPSIS NANA: CHARACTERISTICS, HABITAT, DISEASES, TREATMENTS - BIOLOGY - 2025

Hymenolepis nana is a cestode of the order Cyclophyllidea. It is an obligate endoparasite of humans and rodents that may or may not use cockroaches as intermediate hosts, representing the only Cyclophyllidea species capable of infecting humans without the need to use the intermediate host.

It reaches up to 4 cm in length, presents a rhomboid-shaped scolex with 4 suction cups, an unvaginable rostellum of 0.3 mm in diameter, which is followed by a long neck and a strobile with up to 200 proglottids, each of which it has a bilobed ovary and 3 testes.

Hymenolepsis nana. Taken and edited from: Image contributed by the Georgia Division of Public Health..

Hymenolepsis nana infestation produces a disease called hymenolepiasis, whose symptoms can be abdominal pain, frequent evacuations with the consequent risk of dehydration, weight loss, malnutrition, insomnia, irritability, epileptic seizures, and has even been associated with the transmission of cancer.

It is a disease that occurs mainly in children, although it can affect anyone. Diagnosis requires microscopic identification of the eggs, and treatment consists mainly of praziquantel or niclosamide medication.

General characteristics

Hymenolepsis nana is an endoparasite of rodents and humans, with cockroaches as a non-obligate intermediate host and parasitizing the digestive tract of their hosts.

It lacks eyes, mouth, digestive system, and respiratory system. It is hermaphroditic and as an adult its body is divided into three regions:

Scolex

It constitutes the head of the parasite, has a diameter of 0.3 mm and is provided with four suction cups and a crown armed with 20 to 30 hooks that surround an untouchable rostellum.

Neck

Elongated structure and thinner than the scolex and the strobilus.

Strobilus

It is made up of 100 to 200 trapezoidal proglottids that measure 0.1 to 0.3 mm high by 0.8 to 1.0 mm wide and that increase in size as they move away from the neck.

Each proglottid is a segment-like portion of the body and contains the sex organs, represented by a bilobed ovary and three testes. The gonopores are located on the same side.

Taxonomy

Hymenolepsis nana is a flatworm (phylum Platyhelminthes) of the class Cestoda, belonging to the order Cyclophyllidea. This order groups tapeworms that are characterized by four suction cups on the scolex and a strobilus formed by numerous proglottids.

The Cyclophyllidea are subdivided into 12 families, among which is the Hymenolepididae family, whose members are characterized by having between one and four testes, genital pores located on one side of the proglottid, and a large external seminal vesicle.

The Hymenolepididae family contains at least 32 genera of cestodes, including the genus Hymenolepsis, described by Weinland in 1858, of which 23 species are currently recognized that use mammals or birds as definitive hosts.

The species H. nana was described by Siebold in 1852.

Scolex of a species of the genus Hymenolepsis. Taken and edited from: See page for author.

Habitat

Hymenolepsis nana is an obligate parasite that lives in the digestive tract of its host (endoparasite). It can survive for several days in the outside environment, but there it cannot reproduce or complete its life cycle.

The definitive hosts of the parasite are rodents and humans. You can also use cockroaches as an intermediate host.

Lifecycle

The eggs of the cestodes are surrounded by a protective structure called the embryophor, which allows the survival of the embryo in the environment for a period of two to three months. When ingested by humans or rodents, they hatch in the form of oncospheres, also called hexacanthos because they have six hooks.

In the definitive hosts, this larva is released in the upper part of the small intestine, penetrates the villi, where it develops until it becomes cysticercoid, which is characterized by having the scolex invaginate and in development, this phase lasts for four to six weeks.

Then it breaks the villi and travels to the lower part of the small intestine, where the scolex reaches its maximum development, evacuates and the parasite becomes an adult tapeworm.

If the eggs are ingested by cockroaches, the parasites only reach the cysticercoid stage and are abandoned at this stage in the feces of the insects. If these infested feces contaminate water or food and are ingested by rodents or humans, they continue to develop inside them.

In the mature parasite, proglottids will form at the base of the neck by a process called strobilation. Each new proglottid displaces the previous one, so the more mature ones are found in the most distal portion of the parasite and the immature ones near the neck.

These parasites can cross-breed or self-fertilization can occur between different proglottids or in the same proglottids. When the ovules of the proglottid have been fertilized, it is released from the strobilus and can escape with the host's feces.

The eggs are released from the proglottid, which disintegrates, and is ready to infest a new host and restart the cycle.

Diseases

Hymenolepiasis

Hymenolepsis nana is the organism responsible for the disease known as hymenolepiasis, which affects rodents and humans. This disease occurs worldwide, with the highest incidence in the American continent, the Middle East, Australia and in Europe, where it occurs mainly in the Mediterranean countries.

Although it can affect people of any age, it is mainly given to children in areas with hot and humid climates. The form of infestation can be by ingestion of water or food contaminated by fecal material from intermediate or definitive infested hosts.

Mice and rats act as reservoirs for the disease, since they are also final hosts for the parasite, while some insects, mainly cockroaches, are intermediate hosts.

The severity of the disease depends on the host's parasite load, as well as its immunological and nutritional status. Endogenous and exogenous autoinfestation processes help to aggravate the clinical picture.

Endogenous autoinfestation occurs when the eggs are released from the proglottids and hatch in the host's intestine, becoming adults inside the host. Exogenous autoinfestation, on the other hand, occurs when the eggs are effectively eliminated in the feces and infest the same individual from which they come.

The diagnosis of the disease depends on the finding of cestode eggs in stool samples. These eggs are difficult to detect and the examination often must be repeated several times.

Worldwide, the World Health Organization has estimated that there are 44 million people parasitized, with a prevalence of hymenolepiasis caused by H. nana that can be close to 60%.

Cancer

Recently this species has been associated with the transmission of cancer in people who carry the human immunodeficiency virus (HIV).

Hymenolepsis nana egg. Taken and edited from: Image contributed by Georgia Department of Public Health..

Symptoms

Depending on the intensity of the infestation, the state of health, nutrition and the age of the host, an infestation may occur without apparent signs of disease (asymptomatic), or there may simply be diffuse abdominal discomfort.

Enteritis occurs in the intestinal area, which is probably due to the absorption of the parasite's metabolic wastes by the host.

In case of moderate infestations, among other symptoms, pain in the upper stomach area or in the area around the navel, circumanal itching, eosinophilia, distention of the abdomen due to gas accumulation, nausea, vomiting, loss of appetite, abundant diarrhea, dehydration or weight loss.

Agitation, irritability, difficulty falling asleep, and urinary incontinence during sleep (enuresis) may also occur.

If the infestation is more severe, the symptoms become stronger, the diarrhea is more profuse and there is a sudden loss of weight, with malabsorption syndrome, and episodes of epilepsy can sometimes even occur.

Treatments

Praziquantel

Praziquantel is a broad spectrum antiparasitic anthelmintic compound. Treatment with this medicine consists of a dose of 25 mg / kg / day orally over a period of 10 days.

Among the advantages of this drug are that it lacks toxicity, in addition to having no side effects. If present, these are light and of short duration, in addition to achieving a level of healing close to 100%.

Niclosamide

It is a specific medicine for tapeworm infestations. This treatment has the disadvantage that it must be associated with a laxative, and is given in a first dose of 2 g on the first day, then 550 mg / day must be administered for a period of 5 days to a week.

Nitazoxanide

This drug is a synthetic derivative of broad spectrum sialicidamide that is effective against bacterial infections and helminthic infestations.

Prevention

There are prevention mechanisms that can help both to avoid the particular infestation and the spread of hymenolepsis and other pathologies associated with Hymenolepsis nana. Among them are the practice of good hygiene and eating habits, such as washing your hands before eating.

People should also wash food well before preparing it, avoid contact with flies and cockroaches, boil water.

Proper disposal of sewage and feces is also necessary.

A public health program that includes the control of pests that are vectors or reservoirs of the disease, such as cockroaches, mice, and rats, would also aid in disease control.

References

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