Strongyloides is an opportunistic pathogenic parasitic nematode. Adults parasitize in the small intestine of cats, dogs and humans. The larvae can invade lung, brain, liver, kidney and other tissues and organs, causing strongyloidiasis. The disease is mainly prevalent in warm and humid tropical and subtropical regions. In recent years, due to the continuous increase of HIV/AIDS infection rate in the population, patients with strongyloidiasis are also common, and the clinical symptoms of these severely infected patients with low immunity are complex, and misdiagnosis cases often occur.
Morphology and Life History of Strongyloides
The female size of the parasitic generation of Strongyloides is about 2.2mm × 0.03-0.074mm. Its tail end is pointed and thin, the end is slightly tapered, the body is translucent, the body surface has fine horizontal stripes, and the mouth It is short and the pharynx is slender, about 1/3 to 2/5 of the length of the worm. The size of the female worms of the spontaneous generation is about 1.0mm×0.05-0.075mm, and the tail is relatively tapered. The mature individuals have eggs at different developmental stages arranged in a single row in the uterus. The size of the male is about 0.7mm × 0.04-0.05mm, the tail end is curled to the ventral surface, and there are 2 mating spines. The eustachian tube of the self-generation female and male worms is rod-shaped, and the vulva is located slightly behind the middle of the ventral surface of the worm. Strongyloides is a facultative parasite, with a more complex life cycle than other nematodes. It can switch between the spontaneous generation and the parasitic generation, and has the ability of self-infection and self-reproduction in vivo. The rod-shaped larvae of the autogenous generation molt twice to develop into infectious filamentous larvae (direct development), or molt four times to develop into the male and female adults. After mating, the female ovulates, and the hatched rod-shaped larvae It can enter a new spontaneous generation or develop into filamentous larvae, which invade the human body through the skin and start a parasitic life. Filamentous larvae are transferred to the lungs through the small circulation, burrow into the alveolar cavity, migrate along the bronchi to the pharynx, and reach the small intestine with the swallowing activity of the host, where they molt twice to develop into adults. The female parasites living in the intestinal mucosa perform parthenogenesis to produce eggs, hatch rod-shaped larvae, which can be excreted with the feces, or develop into filamentous larvae and burrow into the intestinal mucosa (self-infection) or anus peripheral skin causes autoinfection. The skin, lungs and intestines are mainly attacked by Strongyloides. Bare-handed field workers or mine workers are highly susceptible to infection. The clinical symptoms vary according to the intensity of infection and the immune function of the host. It can manifest as an asymptomatic carrier or a severe disseminated infection, which eventually leads to the death of the host due to multiple organ failure. Autoinfection occurs mainly in immunocompromised humans, monkeys and dogs. Experiments have shown that immune-competent gerbils do not develop auto-infection. When gerbils are injected with non-steroidal immunosuppressive agents (tacrolimus), auto-infection can be carried out in their bodies.
The pathogenicity of Strongyloides is closely related to the degree of infection, the site of invasion and the state of human immune function. There are three types of human infection: the first type of mild infection can be eliminated by the body through an effective immune response without clinical symptoms; the second type is due to chronic persistent self-infection with intermittent gastrointestinal symptoms; the third type Disseminated severe infections can occur in patients with long-term use of immunosuppressants, cytotoxic drugs, corticosteroids, or HIV/AIDS. The main clinical symptoms of the latter are caused by the migration of larvae, which can cause damage to the skin, lungs and digestive tract. The larvae migrate to other organs with the bloodstream, causing damage to the corresponding organs.
Diagnosis of Strongyloides Disease
The current methods for diagnosing strongyloidiasis include duodenal drainage, immunological tests (IFA, IHA, EIA, ELISA) and stool examination. The key to diagnosis is to find the pathogen, so repeat stool testing is the best method. The current methods for fecal larval detection are Kato-Katz method, direct smear method, fecal smear salt-Lugol’s iodine staining method, formalin-acetoacetic acid concentration method, Harada-Mori filter paper technology culture, nutrient agar plate Culture and Baermann funnel technique. However, stool inspection is expensive and time-consuming, and the detection is limited by factors such as equipment and professionals, so it is not widely used in the field of batch rapid inspection. However, the detection method of testing the serum of subjects by immunological ELISA is more and more used in the diagnosis of Strongyloides because of its simple operation, large-scale rapid detection, and very reliable sensitivity and reliability.