Strongyloides stercoralis

Systematics and general characteristics

In medical helminthology, this parasite is classified as follows:

• Order: Rhabditida
• Family: Strongyloididae
• Genus: Strongyloides
• Species: Strongyloides stercoralis (Bavay, 1876)

The parasite is a typical soil-transmitted helminth, meaning part of its life cycle necessarily takes place in the soil, where the larvae develop. Upon invading the human body, the causative agent causes a disease known as strongyloidiasis.

Morphological features of adult specimens

Adult specimens are small, filamentous, cylindrical nematodes. The anterior end of their body is rounded, while the posterior end tapers evenly. The structure of the digestive system includes an oral opening surrounded by small lips and a cylindrical esophagus occupying about a quarter of the body length. The anus opens in the lower third of the trunk, at the border of the transition to the tapered tail end.

Sexual dimorphism:

• Females: Reach up to 2.2 mm in length. They have a filariform esophagus. The internal anatomy includes an intestine, paired ovaries (occupying up to 3/4 of the body length) transitioning into two uteri. A characteristic chain of eggs can be found in the uteri. The uteri open into an unpaired vagina located at the border of the anterior and middle third of the body. It is the females that actively invade the mucous membrane of the host's small intestine.
• Males: Differ in smaller size (0.7 mm). The posterior end of the male's body is pointed and characteristically curved toward the ventral side, where copulatory appendages — spicules — are located.

Life cycle and developmental stages

The developmental cycle of Strongyloides stercoralis proceeds with obligatory migration through the host's organism.

The onset of the invasive process in the human body is associated with the localization of adult females within the thickness of the small intestinal mucosa. After fertilization, which can occur either in the intestinal lumen or in the lung tissues, the female lays eggs. Rhabditiform larvae hatch from the eggs directly in the intestine. At this stage, they are non-infective. Their esophagus, intestine, and genital primordium are already differentiated in their structure. During further development, filariform larvae are formed, which differ from rhabditiform ones by a modified esophagus shape and an increased total body length.

Human infection occurs through two main routes when the larvae are in the soil:

1. Percutaneous route — active penetration of larvae through intact skin.
2. Geo-oral route — ingestion of larvae along with contaminated soil particles.

Pathogenesis of strongyloidiasis

The pathological impact of the parasite on the body is divided into two main stages depending on the localization of the helminth during migration.

Migration stage:

• Site of entry: In percutaneous infection, pronounced allergic reactions accompanied by itching and dermatitis develop at the site of larval penetration.
• Respiratory system: Passing through the lungs, the larvae traumatize the respiratory tract. This leads to the occurrence of focal pneumonia, bronchitis, and laryngitis.

Intestinal stage:

• Localization: Adult specimens attach to the mucosa of the duodenum. With a heavy infection, the lesion zone expands: parasites can be found in the pyloric part of the stomach, cecum, and colon, and can also penetrate the bile ducts and pancreatic ducts.
• Damage mechanism: Helminths attach using cuticular teeth, causing trauma to small blood vessels and the formation of deep ulcers at the sites of attachment.
• Hematological changes: During feeding, the parasites secrete specific anticoagulants that prevent blood clotting. This provokes constant microhemorrhages, ultimately leading to the development of severe hypochromic anemia.

Diagnostics and preventive measures

Laboratory diagnosis of the disease is based on detecting parasites in the patient's biological materials. The main method is searching for helminth eggs in duodenal contents.

Prevention methods:

• Personal prevention: Strict adherence to hygiene rules, including mandatory hand washing before meals, as well as thorough washing of vegetables, fruits, and berries. In areas where the infection is endemic, wearing closed shoes is extremely important to prevent percutaneous infection.
• Public (sanitary) prevention: Includes active detection and treatment of infected individuals. An important role is played by the disinfection of feces and strict protection of the environment from fecal contamination, as well as the protection of food products from mechanical vectors such as flies and cockroaches.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphological differences between rhabditiform and filariform nematode larvae
• Mechanisms of development of allergic dermatitis during percutaneous larval penetration
• Specifics of the formation of focal pneumonia during the migration stage of strongyloidiasis
• Pathogenesis of hypochromic anemia in gastrointestinal parasitic infestations
• Features of the study of duodenal contents in parasitology

Other topics in the "Medical Helminthology" section

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