Strongyloidosis: causes and pathogenesis
Last reviewed: 23.04.2024
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Causes of strongyloidiasis
Strongyloides cause Strongyloides stercoralis (small intestinal nematode) is a type of Nemathelminthes, a class of Nematoda, a group of Rhabditida, a family of Strongyloididae. In the development cycle of S. Stercoralis, the following stages are distinguished: a free-living and parasitic mature specimen, an egg, a rhabditoid larva, a filarial larva (invasive stage). Development takes place without an intermediate host.
Sexually mature parasitic females 2.2 mm long and 0.03-0.04 mm wide have a colorless filiform body tapering toward the anterior end and a conical tail. Free-living females are somewhat smaller: 1 mm long and about 0.06 mm wide. Male free-living and parasitic have the same dimensions (0.07 mm in length and 0.04-0.05 mm in width).
The cycle of development of S. Stercoralis is complex, perhaps several variants. In the first variant of development, sexually mature helminths parasitize in the host (human) organism, larvae develop in the environment. In the second variant, sexually mature forms are formed in the environment, the multiplication and development of all helminth stages occur without the participation of the warm-blooded host. In the third variant, the larvae, without leaving the host, are transformed into sexually mature forms. Thus, the parasitic and free-living generations of the given helminth alternate.
In the human body, sexually mature worms parasitize in the upper parts of the small intestine, sometimes in the bile and pancreatic ducts, but more often in the cryber cries, where the females lay up to 40 eggs daily (about 0.05x0.03 mm in size), here they also leave the rhabditoid The larvae that enter the environment with faeces. The larvae have dimensions of 0.25x0.016 mm, the conically pointed posterior end of the body, the esophagus with a double extension (rhabdite-shaped esophagus). Under unfavorable environmental conditions, the rhabditoid larvae moult and after 3-4 days they turn into filarial larvae (invasive stage) having several larger sizes (0.5x0.017 mm), a slightly split posterior end of the body, a very thin esophagus. Larvae are able to move in the soil. In the human body, larvae penetrate actively through the skin or passively pass through the mouth with contaminated vegetables, fruits, water. For all types of infection, the filariform larvae migrate through the host organism, similar to the ascarid larvae. Females are introduced into the intestinal mucosa and 17-28 days after infection begin to lay eggs. When S. Stercoralis is invaded, the pathogenic effect is due to a strong sensitizing property of their antigens, especially in the stage of migration of larvae. At the same time, parasites cause a partial immune response to superinvasia, which limits their spread beyond the small intestine.
Under favorable environmental conditions (temperature and soil moisture), rhabditoid larvae give rise to the sexually mature generation (females and males). With a change in temperature, humidity, lack of nutrition, the rhabditic larvae become fusarium-like, infecting the host, and the worms pass to a parasitic lifestyle.
If the rhabditoid larvae linger in the intestine for more than 24 hours (in the presence of diverticula, constipation), their transformation into invasive filarial, which have the ability to immediately enter the intestinal mucosa or the skin of the perianal region. The phenomenon of self-infection (autosuperinvasia) determines the long-term course of strongyloidosis (sometimes tens of years) and high intensity of invasion.
[Pathogenesis of strongyloidiasis
In the early stage, pathological changes in tissues and organs along the migration routes of larvae are caused by sensitization of the body with products of helminth metabolism and their mechanical action. Parasitization of females and larvae causes an inflammatory reaction in the GI tract. During migration, larvae can enter the liver, lungs, kidneys and other organs and tissues where granulomas develop, dystrophic changes and microabscesses. When immunodeficient conditions caused by long-term use of glucocorticoids or cytostatics, HIV infection, hyperinvasia and disseminirovanny strongiloidosis occur. S. Stercoralis parasitize the host for many years. A perennial asymptomatic course of intestinal invasion, capable of quickly reactivating when suppressing cellular immunity, is possible.