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Strongyloides Stercoralis
Updated: Jul 24, 2008
Introduction
Background
Strongyloides stercoralis is a common enteric helminthic parasite of worldwide significance. Typically, the infection is asymptomatic or manifests as mild gastrointestinal symptoms. However, in immunocompromised persons, the infection can be devastating and carries a 60-85% mortality rate.
Pathophysiology
Strongyloides larvae exist in 2 forms: filariform infective larvae and a free-living rhabditiform larvae that lives in soil independent of a human host. Infection occurs when exposed skin contacts contaminated soil. The larvae penetrate the skin and migrate via the lymphatics and venules toward the pulmonary circulation. After penetration into the alveoli, the larvae continue their migration up the respiratory tract until they are swallowed. Filariform larvae rest in the small intestine, mature into adult females, and undergo parthenogenic reproduction. Each adult female may live up to 5 years and continue the reproductive cycle. Eggs typically mature into rhabditiform larvae within the intestine. Strongyloides is the only helminth to secrete larvae (and not eggs) in feces. Typically, larvae appear in feces approximately 1 month after skin penetration. The excreted rhabditiform larvae may again live freely in soil or be transformed into filariform larvae awaiting another human host.
Under certain conditions (eg, constipation, decreased bowel motility, diverticular disease), the larvae do not exit the host in feces and instead molt into the infective filariform larva within the intestinal lumen. These larvae are then capable of penetrating the bowel wall and traveling throughout the body. The CNS, liver, and lungs are the most common destinations of the autoinfectious larvae. This autoinfectious cycle can be accelerated in immunocompromised hosts and results in a frequently fatal condition known as hyperinfection.
Frequency
United States
The true prevalence of S stercoralis is likely underestimated because infection is often subclinical. Currently, an estimated 100-200 million persons are infected worldwide in 70 countries.
Strongyloides is endemic in the Appalachian United States, especially in eastern Tennessee, Kentucky, and West Virginia. Populations at risk also include those who have recently traveled to or immigrated from endemic areas and veterans of World War II and Vietnam. Southeast Asian immigrants living in Washington, DC, were found to have a 38% incidence of infection. Similarly, a Canadian epidemiology study of Southeast Asian immigrants to Canada demonstrated infection in 11.8% in the Vietnamese population and a 76.6% seroprevalence in Cambodian immigrants. Sudanese Lost Boys and Girls and Somali Bantu refugees demonstrated 46% and 23% respective seropositive rate. Five percent of Vietnam veterans reporting mild symptoms demonstrated S stercoralis infection.
International
Strongyloides is endemic in tropical and subtropical areas with sporadic occurrence in temperate areas. Sub-Saharan Africa, South and Southeast Asia, Central America, and South America, and parts of Eastern Europe are considered endemic areas of Strongyloides infection. Worldwide prevalence is estimated as 2-20% in endemic areas.
Mortality/Morbidity
Infection can range from asymptomatic to multiorgan failure. The mortality rate for patients requiring hospitalization with Strongyloides infection is 16.7%. In disseminated strongyloidiasis, the mortality rate can be as high as 70-90%.
Race
No racial predilection is apparent for Strongyloides infection. However, Southeast Asia appears to have the highest endemic percentage. Larva currens is most frequently seen in white patients with an infection acquired in Southeast Asia.
Sex
No gender predilection is demonstrated.
Age
Strongyloides infection is represented in all ages. However, infection may most frequently initially occur in childhood, as children are most likely to play outdoors in contaminated soil with bare feet.
Clinical
History
Most Strongyloides infections are asymptomatic and can survive decades undiagnosed. Symptomatic infections typically manifest in gastrointestinal, pulmonary, and dermatologic systems.
- Skin penetration by infective larvae can elicit ground itch, a cutaneous eruption of pruritic papulovesicular lesions. Typically, skin penetration is on the feet but may be at any site that contacted infected soil. Larva currens (racing larvae), the pathognomonic rash of Strongyloides infection, is a serpiginous urticarial rash that creeps 5-15 cm/h up the body. This rash, likely an allergic response to the migrating larvae, often manifests as a pruritic wheal or linear urticaria. This dermatologic manifestation may last hours to days but in autoinfection cycles can recur over weeks, months, and years. Rarely, in disseminated strongyloidiasis, a petechial purpuric eruption may be present secondary to vessel injury during larval migration.
- Gastrointestinal symptoms are vague: anorexia, weight loss, nausea, chronic diarrhea, constipation, bloating and, rarely small bowel obstruction. Strongyloides is an important cause of failure to thrive and cachexia in immunocompetent children. Malabsorption syndromes may occur in chronic infections.
- Initial infection may trigger wheezing and mild cough. Hyperinfection and disseminated disease are frequently associated with wheezing, dyspnea, cough, pleuric pain, tachypnea, hemoptysis, acute respiratory distress syndrome (ARDS), and may require mechanical ventilation.
- Hyperinfection syndrome
- Hyperinfection and disseminated disease occur during amplification of the autoinfective life cycle. Classically, the syndrome presents in a chronically infected person after immunosuppressive therapy is initiated for an underlying condition. With diminished T-cell immunity, larvae migrate in frequently massive numbers to the lungs and often aberrantly to other tissues, triggering local inflammation and antigen stimulation. Bacteremia, meningitis, and gram-negative sepsis can occur, as intestinal flora attached to the larvae also migrate throughout the body. Fever is almost always present in disseminated disease.
- Risk factors for disseminated Strongyloides include immunosuppressive therapy, transplantation, hematologic malignant disease, human T-lymphotropic virus type 1 (HTLV-1) infection, human immunodeficiency virus, malnutrition, diabetes mellitus, chronic renal failure, hypogammaglobulinemia, anti-TNF receptor therapy, and chronic alcohol consumption. HTLV-1, the retrovirus associated with adult T-cell leukemia, has a bidirectional relationship with Strongyloides. Co-infection of Strongyloides shortens the preleukemic phase of HTLV-1 infection. The Strongyloides antigen accelerates leukemogenesis, and treatment of the infection may actually decrease HTLV-1 viral load. Recent studies suggest that Strongyloides infection may be associated with increased incidence of GI lymphoma.
Physical
Fever is typically present in disseminated disease.
- Dermatologic
- Ground itch - Papulovesicular pruritic rash, usually on the feet
- Larva currens - Serpiginous urticarial rash often on the trunk and buttocks
- Generalized urticaria
- Cutaneous granulomas (with chronic autoinfection)
- Petechial/purpuric rash (with disseminated disease)
- Case reports have been made of multiple atypical dermatologic presentations.
- Gastrointestinal
- Bloating, distension
- Diffuse abdominal pain
- Diarrhea, typically nonbloody
- Pulmonary
- Wheezing
- Cough
- Hemoptysis (in disseminated disease or hyperinfection syndrome)
- Central nervous system - Meningeal symptoms may be present in disseminated disease.
- Reproductive: A case report has demonstrated infertility as a presentation for disseminated strongyloidiasis with larvae found in ejaculate and conception occurring after treatment. Another patient experienced years of recurrent abdominal pain and fever with recurrent eosinophilic oophoritis who had positive strongyloides serology and clinical response to treatment.
Causes
Infections are initiated when exposed skin contacts contaminated soil. Autoinfection commonly occurs allowing infection to persist decades. The longest documented asymptomatic infection was more than 65 years. Hyperinfection typically is triggered by drug-induced or disease-associated defects in cellular immunity, which allows a massive increase in parasite burden and dissemination to nearly all organ systems.
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| References |
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References
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Further Reading
Keywords
Strongyloides stercoralis, Strongyloides infection, strongyloidiasis, larva currens, parasite intestinal helminth, systemic eosinophilia, S stercoralis, intestinal helminth, systemic eosinophilia
