Human strongyloidiasis: complexities and pathways forward

SUMMARY

Strongyloidiasis is a World Health Organization neglected tropical disease usually caused by Strongyloides stercoralis, a parasitic worm with a complex life cycle. Globally, 300–600 million people are infected through contact with fecally contaminated soil. An autoinfective component of the life cycle can lead to chronic infection that may be asymptomatic or cause long-term symptoms, including malnourishment in children. Low larval output can limit the sensitivity of detection in stool, with serology being effective but less sensitive in immunocompromise. Host immunosuppression can trigger catastrophic, fatal hyperinfection/dissemination, where large numbers of larvae pierce the bowel wall and disseminate throughout the organs. Stable disease is effectively treated by single-dose ivermectin, with disease in immunocompromised patients treated with multiple doses. Strategies for management include raising awareness, clarifying zoonotic potential, the development and use of effective diagnostic tests for epidemiological studies and individual diagnosis, and the implementation of treatment programs with research into therapeutic alternatives and medication safety.

INTRODUCTION

Strongyloidiasis is a human parasitic infection that is most commonly caused by the nematode Strongyloides stercoralis, with other species (Strongyloides fuelleborni fuelleborni and Strongyloides fuelleborni kellyi) being less frequent pathogens (1). Exposure to environments contaminated with feces is the main risk factor for acquisition, and the autoinfective life cycle can result in decades-long chronic infection that, if untreated, can persist in populations long after improvements in sanitation (2). While chronic infections may be asymptomatic, about 50% of cases have long-term symptoms (3). Malnourishment in children is a significant morbidity, and immunosuppressive treatments can lead to a life-threatening flare in disease severity, with the need for screening based on risk factors (4–8). In terms of prevalence, it is estimated that globally 300 to 600 million people are infected, with most living in disadvantaged areas (9, 10). The World Health Organization (WHO) has consequently recognized strongyloidiasis as a neglected tropical disease (NTD) with defined management targets (11).

In this review, we aim to navigate the complexities of this infection and highlight strategies developed to improve disease management on a population and individual level. We will discuss the unusual biology and host-parasite interaction of S. stercoralis and how this has impacted diagnostics, available epidemiological data, and management of the disease. Box 1 outlines the search strategy and reference selection criteria, and Box 2 lists the key points.

Box 1. Search strategy and selection criteria.

References for this review were identified through searches of PubMed and Embase for articles published up to October 2022, by use of the following key terms: ("Strongyloides stercorali*" OR Strongyloidiasis) OR ("Strongyloides stercorali*" OR Strongyloidiasis AND treatment*). Articles resulting from these searches and relevant references cited in those articles were reviewed to verify their relevance for this work.

Box 2. Strongyloidiasis—Key Points.

1. The autoinfective cycle leads to chronic infection, unlike the other soil-transmitted helminthiases. If not treated, the infection can persist for life.

2. Chronic infection may be asymptomatic in about half of cases, and larval output in the stool may be low, reducing detection with stool microscopy.

3. Immunosuppression can lead to an increase in larval production and result in potentially fatal hyperinfection/dissemination.

4. Treatment recommendations vary according to disease severity, parasite burden, and host immunity.

5. Based on current evidence, a single dose of 200 µg/kg ivermectin could be used as an effective treatment for strongyloidiasis in population-based soil-transmitted helminthiases control programs.

BIOLOGY AND PATHOGENESIS

The three life cycles of Strongyloides stercoralis

Strongyloides stercoralis has the most complex life cycle of any human helminth, with the capacity for environmental (homogonic and heterogonic) cycles and an autoinfective cycle (Fig. 1) that include a number of developmental stages (12). Parasitic adults are female and are embedded in the mucosa of the duodenal crypts, sometimes the antrum, and rarely the fundus, of the stomach (13). From animal studies, these parthenogenetic females have between 10 and 12 eggs in utero and are estimated to pass a maximum of 15 eggs per day (14). Eggs in the intestine develop and hatch as L1r (rhabditiform) larvae in the mucosal epithelium or intestinal crypts of Lieberkűhn (2). Most larvae passed in the feces are L1r, with some molting to other stages within the gut, including L3a (autoinfective larvae) or L3i (filariform) larvae (14, 15). If passed onto soil, L1r larvae have been observed to molt to the larger L2r (rhabditiform) stage within 10–15 hours, and some or all transform to infective L3i (filariform) larvae within 30–40 hours of passage (16). Under favorable environmental conditions, these L3i larvae will survive for 3 to 4 weeks until they must find a host or die (17, 18). This is known as the direct (homogonic) life cycle (Fig. 1).

FIG 1.

Life cycles of Strongyloides stercoralis. (a) Homogonic life cycle. The L1r larvae are passed per rectum in feces ① and then mature in the soil to longer L2r ②, followed by molting further to become L3 filariform infective (L3i) larvae ③. These L3i larvae must find a host and infect it via transdermal penetration. These larvae randomly migrate (light blue arrows) or via the cardio-pulmonary-esophagus route (red arrows) to reach the small intestine. Once in the small intestine, they further mature to become new parasitic females. (b) Heterogonic life cycle. The L1r larvae are passed per rectum in feces ① and then mature in the soil to L2r larvae ②, followed by further molting again into morphologically identical but larger L3r larvae ③, where sexual differentiation begins, and then to the final L4r stage ④. The L4r larvae further mature and differentiate into free-living adult male ⑤ and female ⑥ worms, which mate and produce eggs ⑦. These eggs hatch in the soil into a new generation of L1r larvae ⑧, which molt into L2r larvae ⑨ and are then all transformed into L3i larvae ⑩, which must find a host to infect via transdermal penetration or perish. Once within the host, these larvae migrate and develop into new parasitic females as per the homogonic life cycle (blue and red arrows). (c) Autoinfective life cycle. The parasitic female S. stercoralis ① passes eggs ②, which typically develop in the intestinal crypts of Lieberkűhn ③ before hatching into L1 rhabditiform (L1r) larvae ④. These molt within the intestine to L2 rhabditiform (L2r) larvae ⑤ and further molt into L3 filariform autoinfective (L3a) larvae ⑥. These larvae randomly migrate (light blue arrows) or via the cardio-pulmonary-esophagus route (red arrows) to reach the small intestine. Once in the small intestine, they further mature to become new parasitic females.

Depending on environmental conditions and possibly genotype, all or some of the L2r larvae in the environment may choose a different developmental pathway, transforming into an L3r (rhabditiform) life stage and then differentiating into male and female free-living adult worms (2, 16). Free-living adults undergo sexual reproduction (19). Only one generation of the free-living adult stage has been observed, as all offspring must transform into infective L3i larvae and find a host or perish (20). This entire life cycle takes approximately 6 days at 25°C–28°C and is known as the indirect (heterogonic) life cycle (Fig. 1) (21). Environmental L3i larvae from the homogonic or heterogonic cycles usually infect humans via transdermal penetration and actively target hosts, being attracted to heat, sweat, and odorants found in human skin, sweat, and skin microbiota (22). Oral ingestion of larvae may be another potential source of infection, but the significance is uncertain (23). While larvae of S. fuelleborni have been demonstrated in the breastmilk of nursing mothers in Africa, transmammary transmission of S. stercoralis in humans has not been proven (24).

In the third, autoinfective life cycle of S. stercoralis, L3a larvae enter the host tissue through the wall of the lower gut or via the skin, usually in the perianal region (Fig. 1) (14). In the latter case, external subdermal migration may cause the pathognomonic larva currens (see Clinical Features) (25). Untreated strongyloidiasis can be a chronic, lifelong condition, with infection documented 75 years post-exposure (26). It is unclear if spontaneous cure may occur in some patients (27).

Once larvae enter the host, they migrate to the stomach and small intestine via two possible routes. Some will undertake this journey via the blood or lymphatic circulation to be transported into the alveoli of the lungs, thereafter migrating up the respiratory tree to be swallowed and then enter the gut (14). Others may randomly migrate through the organs to reach the small intestine (28). At this site, they will molt twice, develop into parasitic adult females, burrow into the intestinal wall, and produce eggs.

Parasite morphology

Morphological appearance is the classical reference standard for identification and is used to differentiate the various life stages of S. stercoralis (Fig. 2). The adult parasitic female (Fig. 3) has a transparent and thread-like appearance and is large enough to be seen with the naked eye (Table 1). The posterior region is wider than the anterior region, tapering to a cone-shaped, bluntly rounded tail (29). The reflected ovary is straight, and the stoma is hexagonal in shape (19, 29, 30). S. fuelleborni can be differentiated from S. stercoralis as it is longer, with an x-shaped stoma and spiral ovaries (29).

FIG 2.

Major life stages of Strongyloides stercoralis—light micrographs. (a) Rhabditiform larva; (b) autoinfective filariform larva; (c) infective filariform larva [reproduced from reference (12)]; (d) parasitic female; (e) mating free-living female and male.

FIG 3.

Anatomy of Strongyloides stercoralis adult life stages. (A) Parasitic adult; (B) free-living adult female; (C) free-living adult male. An, anus; Eg, eggs; Es, esophagus; Gu, gubernaculum; In, intestine; Nr, nerve ring; Rt, reproductive tubule; Sp, spicule; Te, testis; Ut, uterus; Vo, vaginal opening. (Image licensed under the terms of the Creative Commons Attribution 4.0 International license.)

TABLE 1.

Measurements of the major life stages of Strongyloides stercoralis (19, 30–32)

Life stage	Anatomical feature (length)	Number measured (n)	Mean lengthb (range) (µm)
Parasitic female	Total body	25	2,415 (2,000–2,800)
	Widtha	25	37 (37–40)
	Esophagus	25	575 (480–670)
	Mouth to vulva	25	1,670 (1,400–1,900)
	Anus to tail	25	54 (40–70)
Free-living female	Total body	31	1,140 (920–1,700)
	Widtha	31	62 (52–85)
	Esophagus	31	144 (125–150)
	Mouth to vulva	31	580 (470–820)
	Anus to tail	31	110 (80–170)
Free-living male	Total body	21	900 (810–1,000)
	Widtha	21	43 (40–50)
	Esophagus	21	118 (110–125)
	Anus to tail	21	74 (55–95)
	Spicule	21	37 (35–40)
Rhabditiform (L1) larvae	Total body	ns	210 (180–240)
	Widtha	ns	15 (ns)
	Esophagus	ns	ns (33–38)
Infective filariform (L3i) larvae	Total body	31	563 (490–630)
	Widtha	31	15.8 (15–16)
	Esophagus	31	242 (220–270)
	Anus to tail	31	69 (60–80)
Autoinfective filariform (L3a) larvae	Total body	ns	269 (234–317)
	Widtha	ns	11 (10–13)
	Esophagus	ns	ns (39–73)
Eggs	Diameter	ns	ns (74–104)

^a At widest point.

^b ns, number not specified.

S. stercoralis rhabditiform larvae have a rhabditoid esophagus with a single posterior bulb (Fig. 4) (19). The four stages are all slug-like in appearance and morphologically similar, differentiated primarily by an increase in size following each molt (Fig. 5), while other subtle morphological changes do occur (19). The rhabditiform-stage larvae of all Strongyloides spp. are essentially indistinguishable, but in human feces, care must be taken to differentiate them from other superficially similar larvae such as those of the hookworms, Oesophagostomum spp., Rhabditis spp., Ternidens deminutus, and Trichostrongylus spp. (Fig. 6) (33). Distinguishing features of Strongyloides larvae are the very short buccal cavity, straight gut, and lateral rhomboid genital primordium halfway down the larval body (33).

FIG 4.

Anatomy of Strongyloides stercoralis larval life stages. (A) Larvated egg; (B) L1 rhabditiform larva; (C) L3 autoinfective filariform larvae; (D) L3 infective filariform larvae. An, anus; Bc, buccal cavity; Eg, egg; Es/In, esophageal-intestinal junction; Gp, genital primordium; Nr, nerve ring. (Image licensed under the terms of the Creative Commons Attribution 4.0 International license.)

FIG 5.

Development of the four stages of rhabditiform larvae of Strongyloides stercoralis in the heterogonic (free-living) life cycle. (Adapted from original drawings provided by Emeritus Professor Richard “Rick” Speare.)

FIG 6.

Morphological comparison of rhabditiform larvae that may be recovered from human fecal samples. (Reproduced from reference 31.)

Two filariform stages of Strongyloides have been reported. The more commonly observed L3i stage is found in both external life cycles. These larvae are quite distinct from other filariform larvae that may be found in human stool (Fig. 7). Differentiating features are the smaller size of Strongyloides filariform larvae and their far more slender and serpentine appearance. The intestine-to-esophagus ratio is 1:1, whereas it is 1:2 in other human-infecting filariform larvae (Fig. 7). The intestine is straight, and the tail tapers to a point with a distinct notch. The L3a stage is very similar in appearance to L3i larvae; it is shorter and has a more bluntly rounded anterior than L3i (31, 34).

FIG 7.

Morphological comparison of filariform larvae that may be cultured from human fecal samples. (Reproduced from reference 31.)

The free-living female stage (Fig. 3) has a rhabditoid esophagus, a cylindrical procorpus, a narrow isthmus, and a large posterior bulb (19). These stages are relatively large (Table 1), allowing detection with the naked eye on agar plate cultures (Fig. 7). The females are spindle-shaped and have a tapering anterior and posterior, with marked central widening to allow for the egg-filled uterus, which takes up most of the body. The tail gently tapers to a point (30).

The anterior of the free-living male is essentially the same as the female (Fig. 3), but males are much shorter and smaller (Table 1). The posterior curls more than 90° and rapidly tapers to a point. The reproductive system is a straight tube; at the anterior, there is a single testis, which transforms without clear demarcation into the vas deferens and seminal vesicle. There are two small, sickle-shaped spicules and a single gubernaculum (19, 30, 32).

Unlike S. fuelleborni, the eggs of S. stercoralis (Fig. 4) are not passed in feces, except in cases of hyperinfection or systemic disease (19). Eggs are produced by both adult parasitic females and free-living females, and the latter may be observed on agar plate cultures if they are incubated for sufficient time (~5 days). Strongyloides stercoralis eggs are thin-shelled, round to oval, and contain partially developed L1r larvae on passage (19).

Innate and adaptive immune responses

Most information about the pathogenesis of Strongyloides infection comes from studies carried out in gerbils (infected with S. stercoralis) or rats (infected with either Strongyloides ratti or Strongyloides venezuelensis), which do not develop autoinfection unless immunosuppression is induced (35).

In line with other helminth infections, eosinophils play a prominent role in the innate immune response to S. stercoralis. This explains why eosinophilia is frequently observed in people with chronic infection, while eosinophil depletion is common in individuals with hyperinfection and associated immunocompromise (36). As a first line of defense against the parasite, eosinophils, along with neutrophils and complement activation, entrap and kill larvae intercepted in the skin through the release of toxic molecules (innate immunity) (35). In addition, eosinophils elicit the adaptive immune response by acting as antigen-presenting cells, thus inducing a T-helper 2 response with the consequent production of cytokines and specific antibodies (both IgM and IgG) against the worm (35). The cytokines interleukin (IL)-4 and IL-5 are relevant for modulating the cellular and humoral responses, respectively. IL-4 induces the development of alternatively activated macrophages, which is associated with larval killing, whereas IL-5 is required to stimulate B-cells to produce IgM (35).

Host-parasite balance, the autoinfective cycle, and hyperinfection/dissemination

Helminths have the capacity to dampen the immune response, and in immunocompetent individuals, the interaction between Strongyloides and the host reaches a steady state that leads to the persistence of the infection, with sometimes nil, relatively mild, or transient clinical manifestations, where eosinophilia may be absent or the only diagnostic clue (1, 37, 38). Animal studies have indicated that in chronic infections, many adult females enter a life stage where they cease producing eggs (39). This would explain the low larval output seen in chronic infections, which does not necessarily reflect low numbers of parasitic adults in the intestine. The number of parasitic females in the host and their fertility are controlled by poorly understood factors involving interactions with host immunity and possibly interactions between crowded adult parasites via chemical signaling (14, 39, 40). If host immunity is suppressed, the proportion of egg-producing females and L3a larvae can increase, resulting in an autoinfective burst (39). This accelerated autoinfection leads to an amplification of the parasite burden that can result in the potentially fatal consequence of hyperinfection (7). This severe illness is characterized as disseminated strongyloidiasis since large numbers of larvae can be readily located throughout the internal organs of the body, with adult females and ova occasionally found in the lung (7, 41).

Hyperinfection/disseminated disease is associated with specific types of immunosuppression, most commonly corticosteroid exposure, transplantation with the associated immunosuppression, chemotherapy, malignancy, including some leukemias and lymphomas, human T-lymphotropic virus type 1 (HTLV-1) infection, diabetes mellitus, liver cirrhosis, alcoholism, and biological and small molecule immune-modulating therapies, e.g., tumor necrosis factor-α inhibitors (6–8, 42–45). Importantly, corticosteroids act directly upon the L1 larvae in vivo, accelerating ecdysis (molting) and transformation into L3a, increasing autoinfection (46–50). There is some debate about the impact of HIV infection on strongyloidiasis; while initially disseminated strongyloidiasis was considered an AIDS-defining illness, it was then removed from the list (51). The immunological impairment caused by HIV does not cause a depletion of T-helper 2 lymphocytes, which induce the activation of eosinophils and the production of IgE and cytokines (e.g., IL-4, IL-5), essential for immunity to helminth infections (52). Conversely, human T-lymphotropic virus type 1 (HTLV-1) decreases the type 2 response and reduces immunity to helminth infections (53). This virus is clearly associated with a high risk of hyperinfection/disseminated strongyloidiasis [odds ratio = 59.9, 95% confidence interval (CI) 18.1–199] and a poor treatment response (53). Moreover, Strongyloides and HTLV-1 co-infection seem to worsen the viral infection (53). In tuberculosis, S. stercoralis co-infection leads to significantly lower levels of type 1 cytokines and higher levels of type 2 cytokines (54, 55). This immune modulation was associated with higher bacillary numbers in cases of tuberculous lymphadenitis (55). Interestingly, in tuberculous meningitis, the reduction in pro-inflammatory cytokines (interferon-γ, IL-2, tumor necrosis factor-α) associated with strongyloidiasis led to improved clinical outcomes (56). Finally, co-infection with strongyloidiasis and severe acute respiratory syndrome coronavirus 2 was relevant due to the widespread use of corticosteroids and other immune modulatory therapies to manage coronavirus disease 2019 (57–59). This highlighted the importance of systematic screening for strongyloidiasis in patients with epidemiological risk factors and immunocompromise.

Is Strongyloides stercoralis a zoonosis?

S. stercoralis can infect other mammals, including dogs, cats, and nonhuman primates (60–63). The cox1 lineage A genotype has been reported in all hosts, while the lineage B genotype of S. stercoralis appears restricted to dogs (60–63). It remains unclear at this time if S. stercoralis may be acquired by humans as a zoonosis from animals or by companion animals as a reverse zoonosis from humans. While similarities in genotypes indicate cross-species transmission (64, 65), the sparsity of reports of human cases acquired from dogs (66), the absence of reported cross-infections between dogs and owners, and the difficulty often encountered in establishing lasting cross-species infection under experimental conditions without immunosuppression indicate otherwise (16, 18, 67–74). Further investigation using long-read sequencing of dog and human isolates taken from the same community may clarify the question of zoonotic transmission (64).

Omics and bioassays

Advanced genomic, transcriptomic, and proteomic approaches, as well as bioassays, often previously trialed in the similar nematode Caenorhabditis elegans, have been applied to S. stercoralis, S. ratti, and other Strongyloides spp. (75, 76). This knowledge about the fundamental biology of Strongyloides may be translated into improved diagnostics, control strategies, and therapeutics.

The S. stercoralis genome is 42.6 MB in length and AT-rich, with a GC content of 22% (77). Multiple whole genome sequences of isolates from dogs and humans from various geographical locations are now available. A comparison of isolates from humans in Myanmar and Japan with the reference strain from a dog in the USA demonstrated significant population variation, especially in samples from Myanmar (78). The publication of high-quality genomes of several Strongyloides species, along with the genomes of related parasitic and free-living nematode genera, has allowed investigation into the genetic basis of parasitism (76). This work included the determination of the transcriptomes of parasitic and free-living females and the L3i stages of S. stercoralis (76). Notably, excretory/secretory (E/S) proteins, which are upregulated in the parasitic female stages of S. ratti, are likely essential to parasitism and may present effective targets for drug therapies and vaccines (76, 77). The proteomes of multiple L3i stage larvae have been published by several authors, providing insights into biology and novel targets for both serological markers and stool antigen detection assays (79–83).

Bioassays have shown that in free-living S. stercoralis, the nuclear hormone receptor DAF-12 is required for the developmental arrest that results in L3i larvae rather than adult worms (75, 84). This was also dependent on a co-activator, DAF-12-interacting protein 1 (84). Such insights might be used to develop environmental control strategies or treatments that slow or arrest human infection (75). Investigations into the effects of temperature, metalloproteases, and other interventions on the skin penetration rate of S. ratti may translate into improved post-exposure prophylaxis protocols for laboratory staff working with S. stercoralis L3i (75). Finally, data from bioassays evaluating Strongyloides infective processes and circadian rhythms may lead to improvements in the timing and dosage of anthelmintic therapies (75).

EPIDEMIOLOGY

In the past, many field studies with the aim of assessing the prevalence of soil-transmitted helminthiases (STH) were based on Kato-Katz and/or other diagnostic methods that have poor sensitivity for the detection of S. stercoralis (see Diagnostics). This impacted attempts to assess the global prevalence, which was previously largely underestimated, further contributing to the neglect of the infection (85). Recent studies have revised the estimates of prevalence. They either compared the distribution of S. stercoralis with that of hookworm, or used a statistical modeling approach that, based on the characteristics of the settings and populations where the infection was found, enabled the extension of estimates to areas with unknown data (9, 10). According to these novel estimates, the global prevalence should be between 386 (95% CI 324–449) and 613.9 (95% CI 313.1–910.1) million people, significantly higher than the previous estimate of 30–100 million individuals at risk of infection (9, 10, 86). Given the transmission route, the increased risk of infection in rural areas is not surprising, where there is often a lack of access to proper latrines and where barefoot walking is common (9, 87–92). Moreover, prevalence tends to be higher in adults than in children, reflecting the chronic nature of infection and increased opportunities for exposure over time (9, 88, 90–92). Other risk factors for acquisition include institutionalization, household contact, laboratory exposure, and solid organ donor-derived infection, e.g., following kidney or kidney-pancreas transplantation (1, 93, 94).

Although the large majority of infections occur in the poorest areas of Southeast Asia, Africa, and the Western Pacific Region, localized foci of low-level transmission have been identified in high-income countries and areas of temperate climate (9). In the United States of America (USA), Strongyloides transmission was documented mainly in southern states and in rural Appalachia (95). Infection might be endemic in 10 additional states of the USA, as indicated by a study based on a species distribution model (95). In nonendemic areas, infection was found in migrants from areas of endemicity (96). In any case, strongyloidiasis tends to be more frequently diagnosed in people living in disadvantaged conditions, as pointed out by an analysis of hospitalizations throughout the USA, which found an increased risk of infection associated with low socioeconomic status, along with nonwhite ethnicity, male sex, and older age (96).

In Europe, cases of recent local transmission are occasionally reported, mostly in countries along the Mediterranean basin and in Eastern states (97, 98). In fact, strongyloidiasis is generally diagnosed in immigrants from endemic areas and, in some southern countries (epidemiological studies were mostly carried out in Italy and Spain), in elderly people who never traveled abroad, frequently had a history of agricultural work, and who presumably acquired the infection in their youth (99–103). Similarly, in Japan, the infection is diagnosed mostly in older people, and local transmission is probably very rare or absent as a result of improved sanitation and the ban on using human feces as fertilizer (104, 105). In Northern and Central Australia, ongoing local transmission is still an issue in remote Aboriginal communities, with up to 59% of people tested with serology found positive in some parts of the Northern Territory (106–108). Like in the USA and Europe, cases in metropolitan areas are mostly due to immigration and travel to endemic areas (108).

Traveling abroad does not seem to represent a major risk of infection, particularly when trips are of short duration. Extended stays lead to an increased cumulative risk of infection (109, 110). Conversely, migrants arriving in areas of low endemicity from highly endemic areas have a greater risk of infection, which should be considered for screening policies (101, 111, 112). In a systematic review with meta-analysis, the pooled prevalence of strongyloidiasis in migrants was estimated at 12.2% (95% CI 9.0–15.9) with serology and 1.8% (95% CI 1.2–2.6%; 98%) with stool tests (112). The seroprevalence was higher in people originating from East Asia and the Pacific (17.3%, 95% CI 4.1–37.0), followed by sub-Saharan Africa (14.6%, 95% CI 7.1–24.2), then Latin America and the Caribbean (11.4%, 95% CI 7.8–15.7).

CLINICAL FEATURES

Acute infection has been reported rarely in travelers to endemic areas (113). Skin rash may be seen at the site of skin penetration by the infective larvae, and urticaria, pruritus, and cough (Loëffler syndrome) may appear as early as 7 to 10 days after skin penetration of larvae, while gastrointestinal symptoms (diarrhea, abdominal pain) occur after 3 to 4 weeks, when larvae start to be shed in feces (114, 115). Eosinophilia can be a diagnostic clue in the earlier phases, though it is frequently present in other parasitic infections (114).

In chronic strongyloidiasis, according to a systematic review with meta-analysis, about half of the populations surveyed were asymptomatic, and about half were symptomatic, with 50.4% (95% CI 47.6–53.1) reporting at least one symptom (3). In those infected, the rate of eosinophilia was 70% (3). The remaining 30% with no symptoms and no indicative eosinophilia are reliant on the completeness of the medical history and clinician awareness to ensure screening prior to immunosuppressive therapies (116).

Where symptoms are present, they most frequently involve the skin, intestine, and respiratory tract and can range from mild to severe (3). Some studies have also reported systemic manifestations, including malnourishment and stunting in children (4, 5). Larva currens is a pathognomonic, subcutaneous larval migration of 5–10 cm/hour that is visible as urticarial serpiginous tracks on the skin of the buttocks, thighs, or abdomen (25, 117). Other symptoms are nonspecific, with features in common with other intestinal infections (e.g., diarrhea, abdominal pain, pruritus) or chronic respiratory conditions (e.g., asthma-like features, cough) (118). In a systematic review with meta-analysis, urticaria showed a clear association with strongyloidiasis, being reported by 28% of infected individuals, while diarrhea, abdominal pain, or respiratory symptoms showed a weaker association (118). In general, data from studies adequately designed with proper diagnostic tests are scarce, and this has limited the evaluation of the burden of strongyloidiasis (118).

As discussed in the Biology and Pathogenesis, immunocompromise can trigger the development of life-threatening strongyloidiasis with a dramatic increase in larval load (7). In this circumstance, hyperinfection and dissemination are probably better identified as different descriptions of the same clinical condition, characterized by severe symptoms (e.g., paralytic ileus, respiratory impairment, altered state of consciousness), sometimes associated with sepsis and/or meningitis by intestinal bacteria. Of note, in a systematic review of the literature describing cases of Streptococcus gallolyticus meningitis, a large proportion of cases retrieved (41%) were associated with S. stercoralis infection (119).

DIAGNOSTICS

Body fluid microscopy and histopathology

While stool and serum are the usual diagnostic specimens, Strongyloides larvae may be identified on microscopy of respiratory secretions, ascitic fluid, cerebrospinal fluid (CSF), and rarely the urine, and are typically associated with immunosuppression and dissemination/hyperinfection (7, 41, 120, 121). There are case reports of larvae being identified in cervical smears and a single report of male infertility associated with larvae in the semen (41, 122, 123). Where there is a higher burden of disease, parasites may also be visualized in histopathological specimens from tissue biopsies or at autopsy (41).

In the small intestine (Fig. 8), disease may range from congestion and mild inflammation to severe ulcerative enteritis, where ulcers may extend to the muscularis layer, the epithelium is atrophic, and large numbers of parasites are seen, including invasive filariform larvae (41). Granulation tissue and fibrosis may be present, and there may be damage to the myenteric plexus, increasing transit time and contributing to ileus (41). In the large intestine, there may be significant edema of the lamina propria, associated with penetration of the larvae, patchy ulceration, and granuloma formation. Eosinophils may not be prominent unless there are dead larvae, and fibrosis may develop from recurrent inflammation (41). Strongyloides may be present in the stomach and lower esophagus, with a range from an absent or mild reaction to widespread damage (41). In the liver, there may be granulomatous hepatitis (124).

FIG 8.

Various life stages of Strongyloides stercoralis in the histological section of the duodenum. Active chronic duodenitis. (a) eggs (Eg); (b) L1 rhabditiform larvae (Rh) developing within eggs; (c) parasitic adult female, sectioned at the midline and demonstrating the two branches of the reflected uterus (Ut) and the single intestinal tube (In); all 400× magnification; (d) larvated eggs (white darts) and parasitic females in transverse (black darts) and longitudinal (arrows) section; 100× magnification. (Photographs courtesy of K. Hart, reproduced with permission.)

Although strongyloidiasis is frequently associated with hemorrhagic bronchopneumonia in the lungs, relatively few larvae are present on histopathology (41, 125). Larvae may be found in the mesenteric arteries and the portal system and can invade the myocardium (41, 126). Bacterial meningitis may indicate larval migration to the central nervous system, and while infiltration of the brain is described, it is less common (41, 127). When larva currens is present, larvae can be identified on skin biopsy, and in hyperinfection, larvae may also be found in purpuric lesions (41, 128, 129).

Conventional stool-based tests

The diagnosis of S. stercoralis infection in the stool can be complicated by variable and low larval output in chronic infections (130, 131). This affects the sensitivity of conventional stool-based tests, where morphological identification is used as a reference standard for molecular and serological diagnostics (33). An understanding of the limitations of these reference tests leads to a clearer interpretation of individual patient results, assay validations, epidemiological studies, and drug trials (33, 131, 132).

Routine direct-smear fecal microscopy and Kato-Katz microscopy have a low sensitivity of 0%–18% (Fig. 9) (33). While there are only limited data on the efficacy of mini-FLOTAC for the detection of S. stercoralis larvae, it suggests a sensitivity equal to, or below, direct smear microscopy (133–136). Filtration-based stool concentration can also have a low sensitivity, presumably due to larvae retained by the filter (33). Concentration by formalin-ether sedimentation or subsequent modifications of the technique have published sensitivities ranging from 6% to 60% when compared to more sensitive fecal diagnostic methods (Fig. 9) (33). In the hands of an experienced morphologist, microscopy is highly specific, but poorly trained operators may confuse other rhabditiform larvae found infecting human feces (see Parasite Morphology) (33).

FIG 9.

Sensitivity ranges of Strongyloides diagnostic tests. Limited studies have indicated that mini-FLOTAC and filtration stool concentration methods have sensitivities similar to direct smear. Formalin-ether concentration includes modified methods. Baermann sedimentation and agar plate culture require live larvae. PCR sensitivity is dependent on the amount of stool extracted and the effectiveness of the stool extraction method. Serology false-negative results may occur in immunosuppression/hyperinfection.

Methods that utilize the motility of live S. stercoralis larvae to separate them from larger volumes of feces are among the most sensitive approaches for the diagnosis of strongyloidiasis. Baermann sedimentation and agar plate culture (APC) have reported sensitivities of between 40%–80% and 60%–98%, respectively, when compared to composite reference standards (Fig. 9) (33). Like microscopy, these methods are highly specific in the hands of well-trained operators. Several variations of each technique have been proposed to increase larval yield (137–143). The analysis of multiple fecal collections further increases sensitivity (144). Ideally, three stool collections are recommended, and these need to be transported at ambient temperature as refrigeration lowers the percentage of viable larvae (33, 145, 146). Larvae should be identified morphologically in these assays to confirm their identity as S. stercoralis.

As these approaches yield highly motile, infective L3i, they present a significant laboratory infection risk (Video S1). All operators should wear two sets of gloves and cover exposed arms and legs when performing these assays (147). With the absence of data on the larvicidal effects of disinfectants on S. stercoralis L3i, it is recommended that larvae at least be treated with 70% ethanol or formalin prior to microscopy (147). Should an operator be exposed to L3i, the affected area should be flooded with 1% povidine iodine (147). Contaminated equipment should be cleaned with 70% ethanol and allowed to dry (147).

Molecular testing

Molecular approaches have a significant advantage over tests based on larval separation (e.g., APC and Baermann sedimentation) since they can be performed on preserved fecal samples without the need for viable larvae (33). This allows transportation and long-term storage of samples where required (33). Both PCR and loop-mediated isothermal amplification (LAMP) assays for the detection of S. stercoralis have been described (33). The most widely used and well-validated assay, a TaqMan real-time PCR developed by Verweij and colleagues, has a sensitivity of 30.9%–100% and a specificity of 65.4%–98.9% when compared to composite reference standards, including larval separation techniques (Fig. 9) (33, 148). This assay is the only S. stercoralis real-time PCR to have undergone thorough clinical validation in an endemic population setting (33, 149). The reported analytical sensitivity and specificity claimed for other assays may not reflect their efficacy when applied in an endemic context. Furthermore, variations in preanalytical variables, such as extraction methods, will affect the sensitivity of any chosen molecular test (33). In addition, almost all molecular assays for S. stercoralis will amplify other members of the genus, including other Strongyloides species infecting humans (viz. S. fuelleborni) and animals (61, 150). To provide ongoing quality assurance for nucleic acid tests, an external quality assessment scheme for S. stercoralis and other human helminths has recently been established in the Netherlands, with another program planned for introduction in Australia (151–153).

Next-generation sequencing and metagenomic analysis

Next-generation sequencing (NGS) with metagenomic analysis has been used to investigate the effects of strongyloidiasis on the gut microbiome and as a diagnostic tool (154, 155). Currently, the routine diagnosis of strongyloidiasis from stool based on NGS is limited by cost, laboratory infrastructure, the availability of sequencing instrumentation, and the need to manipulate large bioinformatic data sets, including DNA sequences from the host and microbiota (156, 157). Where the technology is available, metagenomic analysis may be useful for specimens from sterile sites, e.g., CSF, where Strongyloides meningitis may not be suspected or made with microscopy, culture, or routine molecular tests (155).

Serological testing

Serological assays have the advantage of being fast, relatively reproducible, and able to be performed in batches; using widely available technology platforms; and being able to be performed on samples such as serum or finger prick blood (158). The time to seroconversion in humans is unknown, but in animal studies, IgG antibodies were detectable within 2 weeks of infection and reached their highest titer 6 weeks post-infection (74). Sero-reversion following treatment (IgG antibodies dropping to a negative level titer) occurs within 6 months to 2 years after treatment (159), and while results may vary between assays, it does indicate that seropositivity represents a current or relatively recent infection. Whether residual seropositivity indicates treatment failure or residual antibody titers remains unclear.

Various commercial crude larval and recombinant antigen assays are available, with reported sensitivities of between 75.4% and 94.7% and specificities of between 42.1% and 95.4% when compared to parasitological tests (Fig. 9) (33, 160–164). A bead-based serological assay using the NIE recombinant antigen had a sensitivity of 93% and a specificity of 95% compared to a microscopy-based reference standard, and this technology has been applied to a multiplex assay to detect soil-transmitted helminths and Plasmodium falciparum (165, 166). Various rapid immunochromatographic serological assays have been developed using crude larval extracts or recombinant antigens (NIE and the S. stercoralis immunoreactive antigen, SsIR) (167–169). Only one of these has undergone clinical evaluation using frozen sera in a retrospective study (sensitivity 82.4%, specificity 73.8%, with a fecal test reference standard) and whole blood in a prospective study (sensitivity 79.4%, specificity 93.6%, with a Bayesian latent class analysis) (167, 170). An immunoblot and an enzyme-linked immunosorbent assay(ELISA) combining the recombinant antigens NIE and SsIR have recently been described, and diagnostic evaluations in a clinical setting have been performed for the latter only (161, 170, 171). A prospective evaluation of the NIE/SsIR ELISA assay performed in the field using dried blood spots was found to have a sensitivity of 83.5% (95% CI 73.8–91.8) and a specificity of 91.7% (95% CI 89.6–93.8) (170).

A coproantigen ELISA employing antibodies targeting the S. ratti E/S antigen has been developed but has not reached commercialization (172).

Diagnostic approach

The choice of diagnostic test will depend on the cohort to be tested (population-based versus individual), sensitivity and specificity, and laboratory capacity (Table 2) (33). Due to its convenient methodology and high sensitivity, serology is suited to population screening for strongyloidiasis, with a subgroup having stool testing for quality assurance purposes (33, 158). In routine laboratory use, serology also has a high diagnostic yield compared to stool-based tests, likely due to preanalytical issues affecting the latter, e.g., sampling error from low parasite numbers, larval mortality, and limitations of nucleic acid extraction methods (33, 173). However, not all people infected with S. stercoralis will have positive serology. Immunocompromised patients, at risk of severe disease, may be seronegative while fecal tests are positive (162, 174–176). For screening and diagnosis in asymptomatic individuals with no current or planned immunocompromise, serology testing alone would generally be adequate (33). If a patient is symptomatic or is being screened due to immunosuppression, and to maximize the opportunity for diagnosis, serological testing and testing of multiple stool samples would be recommended for diagnosis and monitoring of treatment efficacy (33, 132, 177). A high-sensitivity conventional method for stool testing, such as APC or Baermann sedimentation, can be performed where molecular testing is not available (33). In laboratories where molecular assays are used, separation tests may still play a role, as they can be positive when molecular tests are negative, possibly due to larger sample volumes (33).

TABLE 2.

Population-based and individual disease control

Disease control	Population-based	Individual
Immune status	Immunocompetent	Immunocompetent	Immunocompromiseda
Diagnostic tests	Serology + subset for stool testingb and follow-up testing to evaluate program efficacy	Serology + stool testing if symptoms are present	Serology + stool testing repeat for test of cure
Treatment	Ivermectin single dose	Ivermectin single dose or albendazolec	Multiple doses of ivermectin duration based on disease severity/larval burden, consider adding albendazole if there is severe infection

^a Or imminent immunosuppression.

^b Quality assurance for serology and test of cure.

^c If ivermectin contraindicated.

TREATMENT

Treatment for strongyloidiasis can be instituted on the basis of mass drug administration, for control in a population assumed to be immunocompetent, or in the context of individual therapy, where host immunity and illness severity are considered (Table 2) (132). In a healthy population with uncomplicated disease, first-line treatment is ivermectin, and randomized controlled trials have demonstrated that a single dose of 200 µg/kg is as effective as multiple doses (178–180). The efficacy of a single dose ranged from 76% to 100%, depending on the study setting and methods for assessing cure (178, 179). Although based on expert opinion, in cases of immunocompromise and particularly hyperinfection/dissemination, repeated doses of ivermectin are recommended where the length of treatment is guided by parasitological clearance (1, 132). Subcutaneous administration of parenteral ivermectin has been reported in severe cases with major obstacles to oral intake, e.g., in cases of unconsciousness or paralytic ileus (181). While parenteral formulations of the drug are only approved for veterinary usage, they have been effective, on occasion, for the management of severe disease (132, 181).

Ivermectin targets the glutamate-gated chloride channels of the helminths, causing their paralysis and consequent death (182). In humans, it induces the release of gamma-aminobutyric acid from neurons, which are protected from the drug by the blood-brain barrier (182). Hence, ivermectin has a high tolerability profile and can be administered during breastfeeding (the WHO recommends excluding women during the first week of lactation only) and in children over 5 years old/15 kg (183). In the second and third trimesters of pregnancy, the drug is presumably safe, but due to the lack of evidence of high quality, it cannot be recommended (184). A thorough risk-benefit evaluation should be done for single cases when the drug is not approved.

Albendazole has lower efficacy than ivermectin, so usage would usually be limited to treatment where ivermectin is not available or not currently recommended, e.g., in the second or third trimesters of pregnancy and in children >1 year old, with post-treatment follow-up to confirm cure (132, 180). The recommended duration of albendazole therapy for uncomplicated strongyloidiasis is 3 or 7 days, while single dosing as part of a helminth control strategy demonstrated no significant reduction in infection (132, 146). Longer courses of albendazole have been used in combination with ivermectin to manage hyperinfection, where benefits may include the treatment of larvae disseminated to the central nervous system (132, 185, 186). Thiabendazole has demonstrated high efficacy for the treatment of strongyloidiasis, similar to ivermectin, but it is generally not recommended due to its low tolerability (180).

A possible alternative treatment in the future may be moxidectin, a macrocyclic lactone that acts on glutamate-gated chloride channels like ivermectin (187). The main advantage of moxidectin is its fixed-dose (rather than weight-based) administration, which would be useful for control programs in endemic areas based on preventive chemotherapy (187). Another benefit is that the resistance patterns of the two drugs are different; hence, moxidectin might be used in the event of the emergence of resistance to ivermectin (188). However, approval for use in individuals under 12 years old is still needed, and the drug still needs to demonstrate at least noninferiority to ivermectin for the treatment of strongyloidiasis (187).

CONCLUSIONS

In this review, we illustrate the main characteristics of this unique parasite and summarize management strategies, including opportunities for future research (Table 3). Some of the main consequences for individual and public health are due to the peculiar and complex nature of the life cycle. In the absence of adequate treatment, the continuous process of autoinfection guarantees a very long presence in the individual host and at the population level.

TABLE 3.

Characteristics of strongyloidiasis and strategies to improve disease management

Characteristic	Strategy
Potential zoonotic transmission	Studies that characterize subpopulations of Strongyloides stercoralis in humans and animal hosts
Extent of disease distribution and population infected	Epidemiological studies and mapping that take into consideration the limitations of diagnostic tests—overestimates vs underestimates Development of disease registries
Limited awareness about chronic asymptomatic infections or nonspecific symptoms	Health care worker education regarding clinical features, risk factors for acquisition, and the autoinfective cycle
Lack of a single, high-sensitivity reference standard diagnostic test	Use of composite reference standards and/or latent class analysis for test validation and strategic use of the most effective diagnostics, with awareness of test limitations
Optimal diagnostics for use in mass drug administration programs versus individual diagnosis	Use of simple testing strategies (e.g., serology) for mass screening and more intensive strategies (serology and stool-based testing) for test-of-cure and individual diagnostics
Treatment of a spectrum of disease severity	Effective regimens for mass drug administration: chronic asymptomatic infection in immunocompetent persons versus individualized therapy based on disease burden and host immune status
Treatment of special groups	Further research into safe and effective regimens for pregnant/breastfeeding women and young children

While the prevalence of the other STH (Ascaris lumbricoides, Trichuris trichiura, and hookworm) is declining in several endemic countries due to community treatment and possibly improvements in hygiene and sanitation, for Strongyloides, the current community treatment programs (largely based on regimens of albendazole that are ineffective for this parasite) are inadequate. Although improved hygiene and sanitation would potentially impact transmission, it would not prevent the long-term persistence of the parasite in the infected population, potentially mirroring what happened in temperate, high-income countries such as Italy, Spain, or Portugal (99, 101–103, 142). The fact that this parasite has now been formally included in the WHO control targets is good news, and the inclusion of ivermectin in STH control programs will have an impact on global prevalence (11, 189). The first WHO guidelines for the control of strongyloidiasis are currently under development, and they will include specific recommendations about the target population (school-age children or the community), diagnostic tools to estimate local prevalence, and thresholds of prevalence above which intervention is indicated.

The real clinical burden of strongyloidiasis is still poorly understood because most reported case series did not have a control group, although this gap has been partially filled by recent studies (5, 118). In particular, the true burden of severe strongyloidiasis and consequent mortality is still a gray area. Published cases are the tip of the iceberg of those diagnosed, and the latter are a tiny fraction of the real ones, as the severe clinical presentations are far from specific and the underlying infection is unlikely to be diagnosed if not specifically suspected (42).

Moreover, although the risk factors for severe complications are well known, the extent of this risk, that is, the likelihood that a patient with an undiagnosed chronic S. stercoralis infection will complicate following treatment with corticosteroids or other immunosuppressive treatments, is unknown. A case registry would be an important step toward a better understanding of this serious syndrome.

Indeed, the lack of a high-sensitivity diagnostic gold standard has played a major role in the neglect of this parasite, causing underestimation of its global prevalence and misdiagnoses at the individual level. The relative absence of appropriate and comparable validation data for all tests in the currently available literature makes it difficult to select and develop efficient diagnostic protocols, both in the context of population screening and individual diagnostics. In recent years, more attention has been given to the proper evaluation of novel diagnostic tests, i.e., using adequate composite reference standards and/or latent class analysis, which are recommended methods in the absence of a single gold standard. However, choosing the appropriate test can still be challenging. It is imperative that clinicians ordering, performing, and interpreting diagnostic tests for strongyloidiasis are aware of the limitations and advantages of each type of test. A “one size fits all” approach will not yield the best diagnostic outcomes and may cause fatal errors.

Another gray area concerns the zoonotic potential of this parasite. Two genetically distinct populations of S. stercoralis exist in dogs, one of which is shared with humans (60). However, the epidemiological relevance of this finding for human or cross-species transmission remains unknown.

Knowledge and research into strongyloidiasis continue to grow as awareness of its importance in public health and as an individual disease increases. Despite many remaining gaps, our current understanding reinforces the gravity of this disease, which until very recently was so neglected that it was not even formally recognized as an NTD.

Biographies

Dora Buonfrate, MD, DTM&H, FFTM RCPS (Glasg), PhD, is an infectious diseases specialist, leading the clinical research in tropical diseases at the research hospital IRCCS Sacro Cuore Don Calabria in Negrar, Verona, Italy. She is co-heading the WHO collaborating center on strongyloidiasis and other neglected tropical diseases since 2014, and currently is a member of the WHO Guideline Development Group (GDG) on “Public health measures for the control of strongyloidiasis”. Her main research interest is strongyloidiasis, including control in endemic areas, diagnosis and treatment. Her research over the last 10 years has also focused on other neglected tropical diseases (mainly schistosomiasis and Chagas disease), and migrant’s health.

Richard Bradbury is a Senior Lecturer in Microbiology at Federation University in Australia. He began his career as a Medical Laboratory Scientist in Australia, often seeing Strongyloides stercoralis cases from Australian born and immigrant patients. His career has involved both academic and clinical laboratory positions in Australia, West Africa, Slovakia, and the United States of America. His areas of research include diagnostics and epidemiology in medical parasitology, with a particular interest in strongyloidiasis and in maintaining knowledge for diagnostic morphological parasitology. He is the former Team Lead of the Parasitic Diseases Diagnostic Reference Laboratory at the CDC in the United States and a member of the World Health Organisation (WHO) Diagnostic Technical Advisory sub-groups for surveillance of neglected tropical diseases and strongyloidiasis. He was the 2020 Australian Society for Microbiology Lyn Gilbert award winner for major contributions in any area of diagnostic laboratory microbiology nationally or internationally.

Matthew R. Watts has a PhD in parasitology and is a Clinical Microbiologist and Infectious Diseases Physician, with fellowships of the Royal College of Pathologists of Australasia and the Royal Australasian College of Physicians. He is an Associate Professor at the Western Clinical School, University of Sydney. He is a senior staff specialist based at the Centre for Infectious Diseases and Microbiology, Institute for Clinical Pathology and Medical Research – New South Wales Health Pathology (NSWHP), Westmead Hospital, Sydney. This is a government reference laboratory providing services to the state and more locally to Western Sydney and regional NSW, which includes clients from first nations communities and culturally and linguistically diverse backgrounds. More broadly, Dr. Watts is an Associate Director of Clinical Operations for NSWHP and a Local Pathology Director for Southern New South Wales. He has a strong commitment to research and education, improving diagnostic capacity and high-quality service provision.

Zeno Bisoffi, MD, DTM&H, PhD in Clinical Sciences, is former Director of the Department of Infectious, Tropical Diseases and Microbiology, IRCCS Sacro Cuore Don Calabria, Negrar di Valpolicella, Italy, currently Scientific Director of the IRCCS (Clinical and Research Institute), a major research institute of the Ministry of Health on Infectious and Tropical Diseases in Italy. Former co-director of the WHO Collaborating Center on Strongyloidiasis and Other Neglected Tropical Diseases, his main research interests are in tropical and parasitic diseases, particularly NTDs, with a focus on strongyloidiasis.

SUPPLEMENTAL MATERIAL

The following material is available online at https://doi.org/10.1128/cmr.00033-23.

Legend of video. cmr.00033-23-s0001.docx.

Description of Strongyloides larvae video.

Video: Strongyloides larvae. cmr.00033-23-s0002.avi.

Highly motile Strongyloides stercoralis larvae on the surface of an agar plate culture.

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