Strongyloides stercoralis

Strongyloides stercoralis is a parasitic roundworm causing the disease strongyloidiasis in humans. It is a significant public health issue in tropical and subtropical regions, but cases have been reported globally due to increased travel and migration. The lifecycle of Strongyloides stercoralis is complex, involving both free-living and parasitic stages, which allows it to replicate within the host and persist for decades without external transmission.

Lifecycle

The lifecycle of Strongyloides stercoralis includes parasitic and free-living cycles. The parasitic cycle begins when filariform larvae penetrate the human skin, usually through contact with contaminated soil. These larvae then travel through the bloodstream to the lungs, ascend the bronchial tree to the throat, and are subsequently swallowed. Once in the small intestine, they mature into adult females. Unlike many other nematodes, S. stercoralis does not require a male for reproduction; the females reproduce through parthenogenesis, laying eggs that hatch into rhabditiform larvae. These larvae can either be excreted in the feces or transform into infective filariform larvae, perpetuating the cycle within the same host.

The free-living cycle occurs when rhabditiform larvae passed in the feces develop into free-living adult males and females, which reproduce sexually. The offspring of these free-living adults can develop into infective filariform larvae, capable of initiating infection in a new host.

Clinical Manifestations

Strongyloidiasis can range from asymptomatic to severe, depending on the host's immune status. In acute and chronic uncomplicated cases, symptoms may include abdominal pain, diarrhea, and rash. Severe infection, known as hyperinfection syndrome, can occur in immunocompromised individuals, leading to widespread dissemination of larvae and potentially fatal outcomes. Hyperinfection syndrome is characterized by severe gastrointestinal and pulmonary symptoms, sepsis, and meningitis.

Diagnosis

Diagnosis of strongyloidiasis is challenging due to the low sensitivity of conventional stool microscopy. Serological tests and molecular methods, such as polymerase chain reaction (PCR), offer higher sensitivity and specificity. Endoscopy and biopsy may also be used in cases of suspected gastrointestinal involvement.

Treatment

The treatment of choice for strongyloidiasis is ivermectin, which is more effective than albendazole. In cases of hyperinfection syndrome or disseminated strongyloidiasis, prolonged courses of ivermectin may be necessary. Preventive measures include wearing shoes in endemic areas and improving sanitation to reduce soil contamination with human feces.

Epidemiology

Strongyloides stercoralis is endemic in many tropical and subtropical regions, with an estimated 100-200 million people infected worldwide. However, due to its ability to complete its lifecycle within a human host and the potential for asymptomatic cases, the true prevalence is likely underestimated.

Prevention

Preventive strategies focus on reducing soil contamination with human feces and personal protective measures, such as wearing shoes in endemic areas. Education on the risks of walking barefoot and the importance of sanitation can help reduce the incidence of strongyloidiasis.