Strongyloidiasis is an infection caused by a parasitic roundworm that lives in contaminated soil and can persist silently in the body for decades, yet it remains one of the most overlooked diseases affecting millions worldwide.

Epidemiology

Strongyloidiasis affects a significant portion of the global population, though the exact numbers remain uncertain due to limited data collection in many affected regions. Experts estimate that between 30 and 100 million people worldwide are infected with Strongyloides stercoralis, the parasitic roundworm responsible for most human cases. Some sources suggest the number could be as high as 300 to 600 million people globally.^[2][4][11]

The true burden of this disease is largely underestimated because many infected individuals show no symptoms at all, making it difficult to track how widespread the infection really is. This lack of visible signs means that people can carry the parasite for years without knowing they are infected, and healthcare systems often fail to identify or report cases.^[17]

Strongyloidiasis is most prevalent in tropical and subtropical regions of the world. Southeast Asia and the Western Pacific are particularly affected, as are countries in sub-Saharan Africa, the West Indies, Central and South America, and the Indian Ocean region. The infection thrives in warm, moist climates where conditions favor the survival of the parasite in soil.^[3][7]

However, strongyloidiasis is not limited to these areas. It also occurs in temperate climates, including parts of the United States. In the U.S., the infection has been documented in rural areas of the South and Southeast, particularly in Appalachia. Most cases diagnosed in the United States involve people who were infected in other parts of the world and later moved to the country.^[11][17]

The disease is particularly common in rural areas and places with inadequate sanitation facilities. Areas where human waste is not properly separated from human contact create ideal conditions for the parasite to spread. Long-term care facilities, agricultural settings, and communities with limited access to clean water and proper sewage systems also see higher rates of infection.^[17]

Causes

Strongyloidiasis is caused by infection with Strongyloides stercoralis, a type of parasitic roundworm also known as a nematode. This organism is unique among human parasites because of its complex life cycle, which includes both free-living and parasitic stages. There are more than 50 species of Strongyloides, but only a few infect humans. Strongyloides stercoralis is by far the most common cause of human disease.^[1][20]

The primary way people become infected is through direct contact with contaminated soil. When a person walks barefoot or has skin contact with soil containing infectious larvae of the parasite, the tiny worms can penetrate directly through the skin. Importantly, the skin does not need to have any cuts or breaks for this to happen. The larvae are capable of burrowing through intact skin.^[1][11]

Once inside the body, the larvae enter the bloodstream and travel to the lungs. From the lungs, they migrate up the airways, where they are eventually coughed up and swallowed. This brings them into the digestive system, where they reach the small intestine and mature into adult female worms. These adult worms live embedded in the wall of the small intestine and produce eggs through a process called parthenogenesis, meaning they reproduce without needing male worms.^[1]

The eggs hatch inside the intestine, releasing immature larvae called rhabditiform larvae. Most of these larvae are passed out of the body in feces, which contaminates the soil and continues the cycle. However, some larvae can develop into an infectious form while still inside the intestine or around the skin near the anus. This allows them to re-enter the body immediately, a process known as autoinfection. This unique ability means that once a person is infected, the parasite can maintain the infection indefinitely without any new exposure to contaminated soil.^[1][11]

In rare cases, strongyloidiasis can also be transmitted through other routes. Organ transplantation from an infected donor has been documented as a source of infection. There have also been reports of transmission in facilities for people with cognitive disabilities who need help with personal hygiene, as well as in long-term care facilities and daycare centers. One subspecies, Strongyloides fuelleborni subspecies kellyi, has been reported to spread to infants through breastfeeding.^[1][17]

Risk Factors

Certain groups of people and specific activities significantly increase the risk of contracting strongyloidiasis. Understanding these risk factors is important because the infection can remain hidden for many years and cause serious complications in vulnerable individuals.^[14]

Geographic location and travel history play a major role. People who live in or have traveled to tropical and subtropical regions, particularly Southeast Asia and the South Pacific, face higher risk. Rural areas with warm, wet climates are especially problematic because the parasite thrives in these conditions. Anyone who has spent time in these regions, even decades ago, may still harbor the infection due to the parasite’s ability to maintain itself through autoinfection.^[17]

Occupation and lifestyle factors matter considerably. Agricultural workers, farmers, and coal miners who frequently come into contact with soil are at increased risk. Walking barefoot on soil is a particularly important risk factor because it provides direct opportunity for the larvae to penetrate the skin. People who work or live in areas with inadequate sanitation, where human waste is not properly managed, also face higher risk of exposure.^[11][17]

Living situations can contribute to risk as well. Residents of long-term care facilities, institutional settings, and areas with limited access to clean water and proper sewage systems are more vulnerable to infection. The disease can spread more easily in environments where hygiene practices may be compromised.^[17]

The most critical risk factors involve the immune system. People who take corticosteroids (medications used to reduce inflammation and suppress the immune system) face dramatically increased risk of developing severe, life-threatening forms of the disease. Even individuals who took these medications years ago and have since stopped may still be at risk if they have a chronic, undetected Strongyloides infection.^[9][11]

Infection with human T-cell lymphotropic virus type 1 (HTLV-1) is another major risk factor for severe disease. This virus affects the immune system in ways that make it particularly difficult for the body to control Strongyloides infection. People with HTLV-1 infection should be carefully screened for strongyloidiasis before their condition becomes critical.^[9][15]

Other conditions that weaken the immune system also increase risk. This includes people with hematologic malignancies (blood cancers such as leukemias and lymphomas), organ transplant recipients, and those taking other immunosuppressive medications. People living with HIV can develop disseminated strongyloidiasis or hyperinfection syndrome, though observational studies suggest their risk may not be as dramatically elevated as with corticosteroid use or HTLV-1 infection.^[9][13]

Additional risk factors include alcoholism and certain chronic health conditions. People who are malnourished or have conditions that affect their ability to fight infections may also be more susceptible to both acquiring the infection and developing complications.^[20]

Symptoms

One of the most challenging aspects of strongyloidiasis is that the majority of infected people experience no symptoms at all. Studies suggest that about 50 percent or more of infected individuals remain completely asymptomatic, never realizing they carry the parasite. This silent nature of the infection makes it particularly dangerous because people can harbor the worm for decades without seeking treatment.^[7][17]

When symptoms do occur, they often appear in stages that correspond to the parasite’s movement through the body. The first symptoms typically appear at the site where the larvae penetrate the skin. This can cause an itchy rash or red hives, particularly near areas that contacted contaminated soil. A distinctive rash called larva currens (meaning “racing larva”) can develop, appearing as a rapidly moving, snake-like track on the skin, often around the buttocks or thighs. This rash moves quickly because the larvae are traveling under the skin.^[11][18]

As the larvae migrate to the lungs, respiratory symptoms may develop. People may experience a dry cough, wheezing, shortness of breath, or a scratchy throat. These symptoms occur because the parasites are breaking through small blood vessels in the lungs and moving up the airways. The respiratory phase typically happens about two weeks after the initial skin infection.^[15][18]

Once the adult worms establish themselves in the small intestine, gastrointestinal symptoms become prominent. These can include upper abdominal pain or burning sensation, nausea, vomiting, diarrhea, or alternating periods of diarrhea and constipation. Some people experience bloating, heartburn, or stomach discomfort. Weight loss can occur, and in children, chronic infection may lead to malnutrition.^[4][11]

Skin manifestations can persist throughout the infection. Besides larva currens, people may develop urticaria (hives) or other types of rashes that come and go. These skin symptoms often appear intermittently over months or years as the autoinfection cycle continues.^[18]

In immunocompromised individuals, the disease can progress to much more severe forms. Hyperinfection syndrome occurs when the number of parasites increases dramatically because the weakened immune system cannot control them. This leads to severe gastrointestinal symptoms, respiratory distress, and potentially life-threatening complications. The larvae can invade many organs throughout the body in what is called disseminated strongyloidiasis.^[11][15]

Symptoms of hyperinfection and disseminated disease are much more serious and can include severe abdominal pain, persistent vomiting, bloody diarrhea, difficulty breathing, fever, and signs of sepsis (overwhelming infection in the bloodstream). The worms can carry bacteria from the intestine into the bloodstream as they migrate, leading to bacterial infections of the blood and potentially meningitis (infection of the membranes covering the brain and spinal cord). Without intensive treatment, these severe forms are fatal in 60 to 70 percent of cases.^[7][10]

Rarely, strongyloidiasis can cause symptoms in other organ systems, including arthritis, kidney problems, and heart conditions. The wide range of potential symptoms and the non-specific nature of many complaints make diagnosis challenging, particularly in people who do not realize they may have been exposed to the parasite years or decades earlier.^[17]

Prevention

Preventing strongyloidiasis relies primarily on avoiding contact with contaminated soil and maintaining good sanitation practices. Since the parasite enters the body through the skin, simple protective measures can significantly reduce the risk of infection.^[17]

The single most effective personal prevention measure is wearing shoes or other protective footwear when walking on soil, particularly in areas where the disease is known to occur. This creates a barrier that prevents the larvae from penetrating the skin. People who work in agriculture, mining, or other occupations that involve soil contact should wear appropriate protective clothing and footwear.^[17]

Avoiding direct contact with human feces or sewage is crucial. Using improved sanitation facilities, when available, ensures proper separation of human waste from human contact. This breaks the transmission cycle by preventing contaminated feces from reaching the soil where larvae can develop and infect new hosts. Communities that improve their sanitation infrastructure see dramatic reductions in soil-transmitted parasitic infections, including strongyloidiasis.^[17]

Access to clean water supports good personal hygiene, which helps prevent many routes of infection. Washing hands thoroughly, especially after any contact with soil or before eating, reduces the risk of infection through the fecal-oral route, though this is a less common way of acquiring strongyloidiasis compared to skin penetration.^[17]

Pet owners should be aware that dogs can also be infected with Strongyloides species and can potentially serve as a source of contamination in areas where they defecate. Cleaning up after pets promptly and properly disposing of animal waste helps reduce environmental contamination.^[17]

For healthcare and public health systems, prevention includes screening high-risk populations before they undergo treatments that suppress the immune system. Healthcare providers should be particularly diligent about testing people who are about to start corticosteroid therapy or other immunosuppressive treatments, those with HTLV-1 infection, people with blood cancers, and those being considered for organ transplantation. Identifying and treating asymptomatic infections before immune suppression begins can prevent life-threatening hyperinfection syndrome.^[9][13]

Broader prevention strategies require addressing social and economic factors. Improving living conditions, ensuring access to adequate sanitation and clean water, and providing health education to at-risk communities are all essential components of comprehensive prevention programs. These interventions not only reduce strongyloidiasis but also help control many other parasitic and infectious diseases.^[3]

Pathophysiology

Understanding how Strongyloides stercoralis causes disease in the human body requires examining its unique life cycle and the changes it creates in normal bodily functions. The parasite’s ability to reproduce both outside and inside the human host sets it apart from other parasitic worms and explains why infections can last a lifetime.^[1]

The disease process begins when infectious filariform larvae penetrate intact human skin. These tiny worms, invisible to the naked eye, secrete enzymes that help them burrow through the skin layers. This penetration triggers an inflammatory response in the skin, causing the itching and rash that some people experience. Once through the skin, the larvae enter small blood vessels and lymphatic channels.^[18]

The bloodstream carries the larvae through the venous circulation to the right side of the heart and then into the lungs. In the lungs, the larvae break through the walls of tiny blood vessels called capillaries to enter the air spaces. This physical disruption of lung tissue causes inflammation and can trigger coughing, wheezing, and other respiratory symptoms. The body’s immune system responds by sending white blood cells, particularly eosinophils (a type of white blood cell that fights parasites), to the area, which contributes to the inflammation.^[15]

From the lungs, the larvae crawl up the airways toward the throat. The body’s natural response is to cough up this foreign material, but the larvae are then swallowed along with mucus and saliva. This brings them into the digestive system, where they travel through the stomach (surviving the acidic environment) and reach the small intestine, their final destination.^[1]

In the small intestine, specifically in the duodenum and jejunum (the first two sections), the larvae undergo two molts to become adult female worms. These adult worms are about 2 to 2.5 millimeters long and live partially embedded in the intestinal wall, specifically in the mucosa and submucosa (the layers just beneath the intestinal surface). They attach themselves to the intestinal lining and feed on blood and tissue fluids.^[1][15]

The adult female worms reproduce through parthenogenesis, producing eggs without needing fertilization by male worms. The eggs are deposited in the intestinal tissue and hatch almost immediately, releasing rhabditiform larvae. These larvae are then released into the intestinal lumen (the hollow space inside the intestine) and are normally passed out with feces. Under the right conditions of warmth and moisture in soil, these larvae can either develop directly into infectious filariform larvae or can mature into free-living adult male and female worms that reproduce sexually in the soil, producing more larvae that eventually become infectious.^[1]

The most critical aspect of Strongyloides pathophysiology is autoinfection. Some rhabditiform larvae do not leave the body but instead transform into infectious filariform larvae while still in the intestine. These newly infectious larvae can penetrate the intestinal wall directly (internal autoinfection) or can penetrate the skin of the perianal area, buttocks, or thighs if they are passed in feces that contaminates these areas (external autoinfection). Once they re-enter the body, they follow the same migration pathway through blood, lungs, and back to the intestine, continuing the cycle indefinitely.^[1][11]

In immunocompetent people (those with normal immune systems), the body’s immune response, particularly the Th2-type cellular immunity involving eosinophils and specific antibodies, keeps the parasite burden under control. The number of worms remains relatively low, and symptoms may be minimal or absent. However, this balance is disrupted when the immune system becomes suppressed.^[15]

When a person takes corticosteroids or other immunosuppressive drugs, or when they have conditions like HTLV-1 infection that specifically impair Th2-type immunity, the body loses its ability to control the autoinfection cycle. The rate of autoinfection accelerates dramatically, and the number of larvae increases exponentially. This leads to hyperinfection syndrome, where massive numbers of larvae are found in organs that normally contain them (lungs and gastrointestinal tract).^[13][15]

In disseminated strongyloidiasis, the larvae migrate beyond their usual sites to invade virtually any organ in the body, including the liver, heart, brain, and other tissues. As they burrow through the intestinal wall in large numbers, they physically create holes that allow intestinal bacteria to escape into the bloodstream. This leads to polymicrobial bacteremia (multiple types of bacteria in the blood) and can cause bacterial meningitis. The bacteria commonly carried by the larvae include gram-negative organisms normally found in the gut, which can cause severe sepsis.^[10][15]

The overwhelming burden of parasites causes extensive tissue damage and inflammation throughout affected organs. The lungs may develop acute respiratory distress syndrome (ARDS), a life-threatening condition where fluid accumulates in the air sacs. The intestines can develop obstruction, bleeding, or perforation. The combination of massive parasite burden, extensive tissue damage, bacterial superinfection, and systemic inflammatory response creates a medical emergency with high mortality rates even with intensive treatment.^[10]