Strongyloides stercoralis
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p660-661
2025-10-26
57
GENERAL CHARACTERISTICS
Infection with Strongyloides stercoralis is less common than other intestinal nematodes. The organism is endemic in the tropics and subtropical regions of Asia, Latin America, and Africa. A limited geographic distribution exists in the United States and Europe.
S. stercoralis, commonly referred to as the threadworm, may inhabit the intestine or exist as a free-living organism in the soil. The life cycle can be classified as direct, indirect (free-living phase), or autoinfective (Figure 1). The filariform (infective larvae) penetrate the skin and migrate via the circulatory system to the heart and lungs. The organism enters the bronchial tree and then is swallowed, where it lives in the digestive tract and matures into an adult worm. In the intestine the filariform larvae may also penetrate the mucosa, resulting in autoinfection. The female worm produces eggs by parthenogenesis (a form of asexual reproduction where growth and development occur without fertilization), because parasitic adult male worms are nonexistent. Within the indirect life cycle, the rhabditiform (noninfective) larvae develop into mature males and egg-producing females (Figure 2). The free-living life cycle may revert to the production of infective larvae at any time.
Fig1. Strongyloides stercoralis life cycle.
Fig2. Strongyloides stercoralis rhabditiform larva, iodine stain.
EPIDEMIOLOGY
S. stercoralis is transmitted via direct penetration in endemic areas. Person-to-person transmission occurs within institutionalized groups, in day care centers, and among homosexual men.
PATHOGENESIS AND SPECTRUM OF DISEASE
Infections may be asymptomatic or consist of a variety of disseminated strongyloidiasis syndromes. Reinfection is more commonly associated with immunocompromised patients. Acute infections may develop a localized pruritic, erythematous papular rash. Some patients develop a macropapular or urticarial (red and raised) rash on the buttocks, perineum, and thighs. The migration of larvae may cause epigastric pain, nausea, diarrhea, and blood loss. Hyperinfection, an increased worm burden within the lungs and intestines, may occur. Disseminated infections may also result in larvae within the central nervous system, kidneys, and liver.
A second species, Strongyloides fuelleborni, a primate parasite, has been isolated from humans in Africa and causes a severe life-threatening condition called “swollen belly syndrome.
LABORATORY DIAGNOSIS
The rhabditiform larva is the primary diagnostic stage for strongyloidiasis in humans through microscopic examination of stool. The larvae are 250 to 300 µm long with a short buccal capsule, a large bulb on the esophagus, and a prominent genital primordium (Figure 3). The filariform larvae are larger (up to 500 µm) and have a notched tail with an esophageal to intestinal ratio of 1 : 1. The eggs, which are rarely identified, are segmented with a thin shell.
Fig3. Rhabditiform larvae. A, Strongyloides. B, Hookworm. C, Trichostrongylus. bc, Buccal cavity; cb, beadlike swelling of caudal tip; es, esophagus; gp, genital primordia. (Illustration by Nobuko Kitamura.)
S. stercoralis larvae are the most common found in human stool specimens. Depending on the fecal transit time though the intestine and the patient’s condition, both rhabditiform and rare filariform may be present. If stool examination is delayed, embryonated ova may be present. Parasite recovery from stool may be enhanced by the Baermann funnel technique. The basic method is to wrap the sample in a paper tissue or cloth and sub merge it in a funnel filled with water. The nematodes will clump and sink to the bottom of the funnel where they can be recovered.
Culture of feces for larvae is useful to (1) reveal the presence of parasites when they are too scant to be detected by concentration methods; (2) distinguish whether the infection is due to S. stercoralis or hookworm based on rhabditiform larval morphology by allowing hookworm egg hatching to occur, releasing first-stage larvae; and (3) allow development of larvae into the filariform stage for further differentiation. In the agar culture method, a stool sample is placed on a nutrient agar dish and incubated for 48 hours. The larvae crawl over the top of the agar, leaving tracks in the bacterial growth. The modified Harada-Mori filter paper technique is the recommended culture method. This test uses filter paper smeared with fecal material inserted into a test tube containing distilled water. The capillary flow of water up through the filter paper provides soaking of the material. The capillary action provides a mechanism to move the soluble elements to the top of the paper, capturing hatching ova and developing larvae. Because of low recovery of larvae, repeated examinations of stool may be required.
Serologic testing is indicated when infection is suspected and the organism cannot be isolated by repeated stool examinations, string test, or duodenal aspirates. The Centers for Disease Control and Prevention offers a highly sensitive (>95%) cross-reacting enzyme-linked immunosorbent assay (ELISA) with other parasites, including filaria, hookworm, Paragonimus, and Echinococcus.
Although not available in routine laboratories, real time polymerase chain reaction (PCR) methods have been developed that amplify the small subunit of the rRNA gene. The assay is used to detect DNA in fecal samples and has a demonstrated sensitivity and specificity of 100%. A high throughput multiplex assay has also been developed that includes primer and probe pairs for S. stercoralis as well as other intestinal nematodes and protozoa.
Additional specimens such as sputum, body fluids, and tissues may be used for the diagnosis of hyperinfections.
THERAPY
Ivermectin is the recommended treatment for uncomplicated infections. Albendazole is an alternative, but has not proven to be as effective. Hyperinfection and disseminated conditions require anthelmintic therapy in combination with broad-spectrum antibiotics to prevent secondary bacterial enteric infections. In addition, patients taking immunosuppressive medications should discontinue use during infection and treatment. Follow-up examinations are indicated and treatment should be reinstituted if larvae are identified within 2 weeks following cessation of therapy.
PREVENTION
Immunocompromised individuals and patients taking immunosuppressive medications should avoid contaminated beaches and other areas.
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