Strongyloidiasis

Strongyloidiasis is an infection caused by a parasitic worm that can persist in the body for decades, often without symptoms. While many infected people remain unaware of their condition, the infection can become life-threatening in those with weakened immune systems.

Table of contents

• What is Strongyloidiasis
• The Causative Agent and Related Species
• How the Infection Develops
• Where Strongyloidiasis is Found
• How the Infection Spreads
• Signs and Symptoms
• Who is at Risk
• Serious Complications
• Diagnosing the Infection
• Treatment Options
• Prevention Strategies

What is Strongyloidiasis

Strongyloidiasis is a disease caused by infection with Strongyloides stercoralis, a type of parasitic roundworm that lives in contaminated soil^[1]. The worm gets into your body through your skin and makes its way to your lungs and intestines, where it matures and reproduces^[11]. This infection affects an estimated 30 to 100 million people worldwide, though the true number may be higher since many infected individuals have no symptoms^[2][11].

What makes strongyloidiasis unique among parasitic infections is its ability to persist for many years—even decades—inside the human body^[4]. This happens because the worm can complete its entire life cycle within a person through a process called autoinfection, where new infectious larvae develop inside the body and reinfect the same person without the need for contact with contaminated soil^[1][11].

threadworm, Anguilluliasis, Anguillulosis

B78.0; B78.9; B78.1; B78.7

D013322

10042254

The Causative Agent and Related Species

The main cause of strongyloidiasis in humans is Strongyloides stercoralis, a parasitic roundworm belonging to a group called nematodes^[1]. These worms are sometimes called “threadworms,” though in some countries this name refers to a different parasite^[1].

While Strongyloides stercoralis primarily infects humans, it can also naturally infect domestic dogs, cats, and primates^[1]. There are other species of Strongyloides that can occasionally infect humans. Strongyloides fuelleborni is a rarer species that normally infects chimpanzees and baboons but can cause limited infections in humans^[1][15]. There are also animal-associated species, such as those found in nutria and raccoons, that may produce mild, short-lived skin infections in humans but do not cause true strongyloidiasis^[1].

• Skin
• Lungs
• Small intestine
• Bloodstream

How the Infection Develops

Understanding how strongyloidiasis develops requires following the complex life cycle of the parasite. The infection process begins when infectious larvae in contaminated soil penetrate human skin when it comes into contact with the ground—you don’t need to have a cut or break in your skin for this to happen^[1][11].

Once inside your body, the tiny worms enter your bloodstream and travel to your lungs^[1]. In the lungs, they grow into adults and may reproduce, sometimes causing a cough or scratchy throat^[11]. The larvae are eventually coughed up and swallowed, traveling down into your stomach and small intestine^[1].

In the small intestine, the worms complete their development into adult female worms that live embedded in the wall of the intestine^[1]. These female worms reproduce without males through a process called parthenogenesis, producing eggs that hatch into larvae while still inside the body^[1]. This can cause digestive symptoms such as nausea, diarrhea, or stomach pain^[11].

Most larvae are passed out of the body in stool, where they can continue their life cycle in the soil^[1]. However, some larvae can develop into an infectious form while still inside the intestine or on the skin near the anus, penetrating back into the body to start the cycle again—this is the autoinfection process that allows the infection to persist for years without new exposure to contaminated soil^[1][4].

Where Strongyloidiasis is Found

Strongyloidiasis is found throughout the world, but it is most common in tropical and subtropical regions, particularly in Southeast Asia and the Western Pacific^[3][17]. The infection is more frequently found in warm, wet, rural areas and places with inadequate sanitation^[11][17].

In the United States, strongyloidiasis has been documented in rural areas of the South and Southeast, especially in Appalachia^[11]. However, most cases in the United States occur in people who were infected in other parts of the world before moving to the country^[11].

The infection is endemic—meaning it regularly occurs—in parts of sub-Saharan Africa, the West Indies, Central and South America, the Indian Ocean region, and South East Asia^[7]. Between 30 and 60 million people are estimated to be infected worldwide, though the true global burden remains largely unknown due to limited surveillance and the fact that many infected people have no symptoms^[2][7].

How the Infection Spreads

The primary way strongyloidiasis spreads is through direct contact between bare skin and soil contaminated with human feces containing the parasitic larvae^[1][17]. Walking barefoot on contaminated ground is a common way people become infected. The infectious larvae are too small to see without a microscope^[17].

Once the larvae penetrate the skin, they spread through the body and eventually settle in the small intestine where they lay eggs^[17]. Most roundworms pass out of the body in feces, but some mature and can reinfect the same person by penetrating through the walls of the intestine or the skin around the anus^[17].

In rare cases, strongyloidiasis can spread from person to person through organ transplants from infected donors^[15][17]. Transmission has also been documented in facilities for people with cognitive disabilities who need help with daily activities and personal hygiene, long-term care facilities, and daycare centers^[17].

For one subspecies called Strongyloides fuelleborni kellyi, transmission to infants through breastfeeding has been reported^[1].

Signs and Symptoms

Many people infected with Strongyloides have no symptoms at all—approximately 50 percent of cases are asymptomatic^[7][17]. When symptoms do occur, they can vary depending on which stage of infection the person is experiencing and which part of the body is affected.

During the early infection when larvae first penetrate the skin, people may notice an itchy rash at the site where the worm entered the body^[11]. A characteristic skin finding called larva currens (meaning “racing larva”) may appear, which shows as red hives or a moving, snake-like rash near the anus^[17].

When the larvae reach the lungs, respiratory symptoms can develop, including a dry cough, wheezing, scratchy throat, or difficulty breathing^[7][11][17].

Once the worms establish themselves in the intestines, digestive symptoms may appear. These include upper abdominal burning or pain, bloating, heartburn, nausea, diarrhea alternating with constipation, and weight loss^[7][17]. However, over half of infected individuals remain without any noticeable symptoms^[7].

In chronic infections that have lasted for years, people may experience on-and-off digestive or skin symptoms, allergic reactions, or unexplained eosinophilia (an increase in a type of white blood cell)^[13]. In rare cases, chronic strongyloidiasis can lead to arthritis, kidney problems, or heart conditions^[17].

Who is at Risk

Anyone can become infected with Strongyloides, but certain factors increase the likelihood of infection. People who travel to or live in tropical and subtropical regions, particularly Southeast Asia and the South Pacific, are at higher risk^[17]. Living in or visiting rural areas with warm, wet climates also increases risk^[17].

Walking barefoot on soil, especially in areas with poor sanitation, is a major risk factor^[11][17]. People who work in agricultural settings or occupations such as farming or coal mining that involve contact with potentially contaminated soil face increased exposure^[11].

Living in areas with inadequate sanitation facilities—where there is no proper separation of human waste from human contact—significantly raises the risk of infection^[11][17]. People in long-term care facilities may also be at increased risk^[17].

Certain medical conditions greatly increase the risk of developing severe disease. People with weakened immune systems are particularly vulnerable, especially those taking corticosteroid medications or other drugs that suppress the immune system^[9][11]. Infection with human T-cell lymphotropic virus type 1 (HTLV-1) is another major risk factor for severe strongyloidiasis^[2][9][11].

Other conditions that increase vulnerability to severe infection include HIV infection, blood cancers such as leukemias and lymphomas, having received or being considered for organ transplantation, and chronic alcoholism^[9][13].

Serious Complications

While most people with strongyloidiasis have no symptoms or only mild illness, the infection can become extremely dangerous in people with weakened immune systems. Two life-threatening complications can occur: hyperinfection syndrome and disseminated strongyloidiasis^[4][7].

Hyperinfection syndrome occurs when the autoinfection process accelerates dramatically, leading to a massive increase in the number of larvae in the body^[7][13]. The defining feature of hyperinfection syndrome is the severity of organ failure that often requires intensive care^[7]. Respiratory symptoms, severe gastrointestinal problems, skin manifestations, and neurologic symptoms can all occur^[7].

Disseminated strongyloidiasis happens when large numbers of larvae break through the intestinal wall and spread throughout the body to organs beyond the intestines and lungs, including the central nervous system^[11][15]. This can cause severe inflammation of multiple organs^[11].

When larvae pierce the bowel wall during hyperinfection or dissemination, bacteria from the intestines can enter the bloodstream, causing bacterial infections of the blood (bacteremia) and brain coverings (meningitis)^[15]. Patients often develop complications including sepsis (a life-threatening response to infection), shock, and acute respiratory distress syndrome^[7][10].

These severe complications are fatal in 60 to 70 percent of cases^[7]. The most common trigger for hyperinfection syndrome is treatment with corticosteroid medications^[2][13]. Infection with HTLV-1 is another major risk factor, as are HIV infection and conditions requiring immunosuppressive therapy^[2].

Diagnosing the Infection

Diagnosing strongyloidiasis can be challenging because symptoms are often nonspecific or absent entirely, and the number of larvae shed in stool can be very low^[4]. Healthcare providers should consider strongyloidiasis in anyone with a travel history to or residence in areas where the infection is common, especially if the person has unexplained digestive, respiratory, or skin symptoms, or shows increased eosinophils in blood tests^[9].

Several methods can be used to diagnose the infection. The traditional approach involves examining stool samples under a microscope to look for Strongyloides larvae^[4][17]. However, this method has low sensitivity—meaning it can miss infections—and requires a minimum of three stool samples to improve accuracy^[2][7]. Special techniques like agar plate culture can improve detection rates but are not widely available^[7].

Blood tests that detect antibodies against Strongyloides (serology) are more sensitive than stool examination for chronic infections^[4][17]. However, serology is less sensitive in people with weakened immune systems^[4]. A positive antibody test shows that a person has been exposed to Strongyloides at some point but doesn’t necessarily prove active current infection.

In severe cases of hyperinfection or disseminated disease, larvae may be found in samples of sputum (mucus coughed up from lungs), fluid from the airways obtained during bronchoscopy, or in biopsies of tissue^[9][10].

Newer testing methods using polymerase chain reaction (PCR) technology to detect the parasite’s genetic material are being developed and may improve diagnosis in the future^[4].

Healthcare providers should be especially careful to screen for Strongyloides in patients who are about to start corticosteroid therapy or other immunosuppressive medications, those with HTLV-1 infection, people with blood cancers, those being considered for organ transplantation, and anyone with unexplained eosinophilia who has a history of living in or traveling to areas where strongyloidiasis is common^[9][21].

Treatment Options

All people found to have Strongyloides infection should be treated, even if they have no symptoms, because of the risk of developing life-threatening hyperinfection syndrome in the future^[9][10]. Two medications are available in the United States to treat strongyloidiasis^[9][21].

The first-line, or preferred, treatment is ivermectin, taken as a single oral dose of 200 micrograms per kilogram of body weight for one to two days^[9][21]. This medication is highly effective and generally well tolerated. However, ivermectin should not be used in pregnant or breastfeeding women, persons weighing less than 15 kilograms, or people with confirmed or suspected infection with another parasite called Loa loa^[9][21].

The alternative treatment is albendazole, taken as 400 milligrams orally twice a day for seven days^[9][21]. This medication should be avoided in the first three months of pregnancy and in people with known allergy to benzimidazole compounds^[9][21].

After treatment, follow-up stool examinations should be performed two to four weeks later in patients who had positive stool tests and continue to have symptoms, to confirm that the infection has been cleared^[9][21]. If larvae reappear, retreatment is necessary^[9][21].

Treatment of hyperinfection syndrome or disseminated strongyloidiasis is more intensive. If possible, immunosuppressive medications like corticosteroids should be stopped or reduced^[9][21]. Ivermectin is given at 200 micrograms per kilogram per day orally until stool and/or sputum examinations are negative for two weeks^[9][21]. For patients who cannot take oral medications due to intestinal problems, the medication can sometimes be given rectally^[9][21].

Patients with severe disease often require intensive care and hospitalization to manage complications such as sepsis, shock, and respiratory failure^[7][10]. Any bacterial infections that develop must be treated aggressively with antibiotics^[10].

In pregnant women, the risks and benefits of treatment must be carefully weighed, as both ivermectin and albendazole are pregnancy category C medications^[9][10]. Some healthcare providers may choose to delay treatment until after the first trimester^[10].

Prevention Strategies

Preventing strongyloidiasis involves avoiding contact between bare skin and contaminated soil. The most important preventive measure is wearing shoes when walking on soil, especially in areas where the infection is common^[17][19].

Access to improved sanitation facilities is fundamental to preventing new cases^[6][17]. Using toilets or latrines that properly separate human waste from human contact helps break the transmission cycle. Clean water supplies and good personal hygiene practices, including thorough handwashing, also help prevent infection^[17].

Avoiding contact with fecal matter or sewage is important^[17]. People should clean up after dogs, which can also carry Strongyloides^[17].

For healthcare providers, an important prevention strategy is screening high-risk patients before they start taking immunosuppressive medications like corticosteroids^[9][13]. This includes people with a history of living in or traveling to areas where strongyloidiasis is common, those with HTLV-1 infection, patients being considered for organ transplantation, and anyone with unexplained eosinophilia^[9][21]. Treating any detected infections before starting immunosuppression can prevent life-threatening complications.

In healthcare settings, standard precautions should be observed for hospitalized patients with strongyloidiasis, including wearing gloves and gowns, practicing good hygiene, and careful handwashing when coming into contact with the patient’s feces^[9][21].