Strongyloides stercoralis infection in patients presenting with lower respiratory tract infections: a retrospective case series from a tertiary hospital in Vietnam

Abstract

Background

Strongyloides stercoralis infection is frequently overlooked in patients presenting with lower respiratory tract infections (LRTIs) because of nonspecific clinical manifestations, despite its potential to cause severe pulmonary involvement, particularly in endemic regions.

Methods

We conducted a retrospective case series at Bach Mai Hospital, a tertiary referral center in northern Vietnam. Adult patients hospitalized between January 2024 and July 2025 with LRTIs and confirmed S. stercoralis infection were included. Patients were classified as having chronic strongyloidiasis (including parasitologically confirmed and serology-only probable cases) or hyperinfection syndrome. Clinical, laboratory, radiological, microbiological, and diagnostic data were analyzed.

Results

A total of 42 patients were included (mean age 73.3 ± 10.2 years; 83.3% male). Among the 42 patients, 59.5% were classified as chronic strongyloidiasis, including both parasitologically confirmed and serology-only probable cases, whereas 40.5% had hyperinfection syndrome. In the hyperinfection group, respiratory involvement was more frequent than gastrointestinal involvement (58.8% vs. 41.2%). Pneumonia and acute exacerbations of chronic obstructive pulmonary disease were the predominant clinical presentations. Laboratory abnormalities were common, including hypoalbuminemia (88.1%), anemia (64.3%), and hyponatremia (50.0%), whereas eosinophilia was observed in only 23.8% of patients. Serological testing using ELISA showed the highest diagnostic yield (90.5%), while direct parasitological methods demonstrated limited sensitivity. Gram-negative enteric bacteria predominated among microbiological isolates.

Conclusions

This retrospective case series characterizes the clinical and laboratory features of LRTIs in patients with concomitant S. stercoralis infection. Pneumonia and COPD exacerbations were predominant presentations, often with marked laboratory abnormalities despite infrequent eosinophilia. Although hyperinfection was frequent and direct parasitological tests showed limited sensitivity, no inference can be made regarding the frequency or strength of this association due to the descriptive design and lack of a comparator group. These findings may help clinicians consider strongyloidiasis in selected patients with unexplained or refractory LRTIs in endemic settings.

Introduction

Lower respiratory tract infections (LRTIs), including pneumonia, acute bronchitis, exacerbations of chronic obstructive pulmonary disease (COPD), and bronchiectasis, are a major global health burden. They are among the leading causes of morbidity and mortality worldwide and were responsible for over 2.4 million deaths in 2019, making LRTIs the fourth leading cause of death globally [1]. While most LRTIs are thought to be caused by bacterial or viral pathogens, parasitic infections are increasingly recognized as underdiagnosed but potentially serious causes of lower respiratory tract problems, particularly in endemic areas.

Vietnam is considered an endemic setting for Strongyloides stercoralis. Population-based serological data suggest a substantial burden, with one multi-region Vietnamese serum bank study reporting seroreactivity in 29.1% of adults and 5.5% of children, and markedly higher seroprevalence in rural areas (up to 42.4% in the rural Central Highlands). In northern Vietnam, a large hospital-based survey across 27 provinces also reported high seroprevalence (approximately 20% overall), with higher rates in rural versus urban residents. These data underscore that a considerable proportion of adults—particularly those with rural exposure—may harbor chronic infection and therefore be at risk of severe disease when immunosuppressed, supporting the clinical relevance of recognizing strongyloidiasis in patients presenting with lower respiratory tract infections [2, 3].

S. stercoralis is a soil-transmitted helminth endemic in tropical and subtropical regions. Its life cycle allows autoinfection, enabling chronic infection that may persist for decades with minimal or nonspecific symptoms. After skin penetration, larvae migrate through the bloodstream to the lungs, ascend the tracheobronchial tree, and are swallowed to reach the gastrointestinal tract where they mature into adult females. The eggs hatch in the intestinal mucosa and release non-infective rhabditiform larvae, which are usually passed in the stool. Under certain conditions, some larvae may transform into infective filariform larvae within the host and penetrate the intestinal mucosa or perianal skin, resulting in autoinfection [4, 5].

In immunocompromised patients—particularly those receiving corticosteroids or with chronic lung disease, malignancy, diabetes, organ transplantation, or other immunosuppressive conditions— S. stercoralis may progress to hyperinfection syndrome or disseminated disease. These severe forms involve accelerated larval proliferation, extensive pulmonary damage, and complications such as diffuse alveolar hemorrhage, ARDS, and secondary sepsis, frequently involving Gram-negative enteric bacteria; translocation of bacteria or fungi along migrating larvae may further worsen outcomes and contribute to high mortality [6–8].

Despite potentially life-threatening complications, probable chronic strongyloidiasis and its severe forms of infection are often overlooked in clinical practice due to nonspecific clinical manifestations and low levels of clinical suspicion. Delayed diagnosis can lead to delays in antiparasitic treatment, particularly ivermectin, thus significantly increasing morbidity and mortality. This is especially important in endemic countries such as Vietnam, where S. stercoralis infection is common, but systematic screening of patients presenting with lower respiratory tract infections (LRTI) is not routinely performed. Consequently, comprehensive clinical data describing LRTI in patients with concomitant strongyloidiasis remain limited.

Pulmonary involvement plays a central role in the clinical manifestations of both probable chronic strongyloidiasis and hyperinfection syndrome. Respiratory manifestations associated with S. stercoralis infection are varied, ranging from chronic productive cough, hemoptysis, wheezing and dyspnea with bronchospasm and respiratory failure, to recurrent bronchitis or widespread pneumonia [9–11]. These clinical features closely resemble bacterial pneumonia or exacerbations of chronic lung diseases, particularly in patients with COPD or underlying lung structural disorders, and are therefore often misdiagnosed.

Diagnosis of S. stercoralis infection remains challenging, particularly in patients with respiratory presentations, because direct parasitological methods often have limited sensitivity and imaging findings are frequently nonspecific. Larvae may be detected in sputum, bronchoalveolar lavage fluid, gastric or duodenal aspirates, or tissue biopsy samples [11]. Chest radiography and computed tomography typically reveal nonspecific abnormalities, such as micronodules, interstitial infiltrates, lobar consolidation, pulmonary fibrosis, lung abscesses, or alveolar hemorrhage [12–14]. In hyperinfection syndrome, pulmonary involvement is often extensive. Marcos et al. reported pneumonia as the most frequent manifestation, occurring in up to 83% of cases, frequently associated with secondary Gram-negative enteric bacterial infections and increased mortality [15].

Previous reports have described concomitant S. stercoralis infection in patients presenting with chronic respiratory disease exacerbations. Luvira et al. showed that pneumonia or acute exacerbations of COPD can represent the initial presentation of hyperinfection syndrome [11]. Case reports from the United Kingdom and France further described persistent or antibiotic-refractory pneumonia in which delayed recognition of strongyloidiasis resulted in poor clinical outcomes [16, 17].

Regarding treatment, ivermectin has consistently demonstrated superior efficacy compared with albendazole or thiabendazole, including in hyperinfection syndrome [18, 19]. Repeated or prolonged ivermectin regimens have been recommended to achieve adequate parasite clearance in patients with severe or disseminated disease [20].

In Vietnam, available studies have largely focused on community-based epidemiology, while the relationship between lower respiratory tract infections and S. stercoralis infection remains insufficiently characterized. Diagnosis is often delayed and incidental, underscoring the need for clinical studies addressing the manifestations, paraclinical features, and treatment outcomes of hospitalized patients with concomitant LRTIs and S. stercoralis infection. However, the spectrum of respiratory presentations associated with concomitant S. stercoralis infection in hospitalized patients remains poorly characterized, particularly in endemic settings. A clearer description of these clinical, laboratory, radiological, and microbiological features may help clinicians identify strongyloidiasis earlier in patients with unexplained or treatment-refractory LRTIs. Accordingly, this study aimed to describe the clinical and paraclinical characteristics of lower respiratory tract infections in patients infected with S. stercoralis treated at Bach Mai Hospital, one of the largest tertiary referral centers in northern Vietnam.

Methods

Study design and setting

This retrospective descriptive case-series study was conducted at the Respiratory Center of Bach Mai Hospital, a tertiary referral hospital in northern Vietnam. Medical records of eligible patients admitted between January 2024 and July 2025 were reviewed.

Study population

Adult patients (≥ 18 years) hospitalized with a diagnosis of lower respiratory tract infection (LRTI) were included if they had microbiological or serological evidence of S. stercoralis infection, according to the Vietnamese Ministry of Health (MoH) 2022 guidelines. LRTIs included community-acquired pneumonia (CAP), acute bronchitis, acute exacerbations of chronic obstructive pulmonary disease (AECOPD), and acute exacerbations of bronchiectasis, defined according to international and national guidelines (ATS/IDSA 2019; MoH 2020; GOLD 2022; BTS 2019).

Patients with S. stercoralis infection without lower respiratory tract involvement or with insufficient clinical or paraclinical data were excluded.

Definitions and classification

S. stercoralis infection was categorized into the following clinical forms:

• Parasitologically confirmed chronic strongyloidiasis: defined by detection of S. stercoralis larvae on direct stool microscopy.

• Probable chronic strongyloidiasis: defined by positive S. stercoralis serology (ELISA) without direct parasitological confirmation, in accordance with the Vietnamese Ministry of Health 2022 guideline.

• Hyperinfection or disseminated strongyloidiasis, according to Grover (1996), defined by detection of larvae in respiratory specimens (sputum or bronchoalveolar lavage), gastric fluid, or extraintestinal organs, indicating accelerated autoinfection and/or dissemination.

For descriptive analyses, patients were grouped into chronic strongyloidiasis (including both parasitologically confirmed and serology-only probable cases) or hyperinfection syndrome.

Testing strategy

Strongyloides testing was typically considered in LRTI patients whose radiological severity appeared disproportionate to the inflammatory biomarker profile and/or who showed an unusually slow clinical response to appropriate antibiotic therapy compared with typical bacterial pneumonia, particularly in the presence of predisposing factors (e.g., corticosteroid exposure, chronic lung disease, advanced age, or gastrointestinal symptoms).

Data collection

Data were extracted retrospectively using a standardized case-report form and included demographic characteristics, comorbidities and immunosuppressive risk factors, clinical manifestations, laboratory and imaging findings, microbiological and parasitological results, treatment regimens, and in-hospital outcomes.

Microbiological investigations

All patients were evaluated for common etiologies of LRTIs according to routine hospital practice. Microbiological workup included sputum culture, blood culture, and bronchial aspirate/bronchoalveolar lavage (BAL) culture when clinically indicated. Fungal investigations were performed when clinically suspected. Microbiological results were recorded to characterize co-infections in this cohort.

Respiratory sampling approach

In routine clinical practice, etiologic investigations for LRTIs were performed stepwise. Sputum examinations were performed at least three times for each patient; if these sputum tests remained negative and the patient’s clinical condition allowed, bronchoscopy with bronchial aspirate/bronchoalveolar lavage (BAL) was performed for further evaluation.

Parasitological examinations

Stool specimens were examined using direct microscopic examination according to routine parasitological laboratory procedures at Bach Mai Hospital. When clinically indicated, repeated stool examinations were performed to improve larval detection. Respiratory specimens, including sputum and bronchoalveolar lavage fluid, were also examined microscopically for larval detection in suspected cases of strongyloidiasis or hyperinfection syndrome.

Serology

S.stercoralis–specific IgG was measured using a commercially available serum ELISA assay as part of the routine diagnostic workup in the hospital parasitology laboratory. Test interpretation followed the manufacturer’s instructions and institutional laboratory practice [21].

Statistical analysis

Data were analyzed using SPSS version 26.0 (IBM Corp., Armonk, NY, USA). Continuous variables were summarized as mean ± standard deviation or median (interquartile range), and categorical variables as frequencies and percentages. Given the descriptive retrospective case-series design and absence of a comparator group, analyses were limited to descriptive statistics without formal hypothesis testing or causal inference.

Ethical considerations

This retrospective study involved no therapeutic intervention. Patient confidentiality was strictly maintained in accordance with institutional and national ethical standards.

Results

The study included 42 patients with lower respiratory tract infections and concomitant S. stercoralis infection. The mean age was 73.3 ± 10.2 years, with the majority of patients aged ≥ 60 years (92.9%). Male patients predominated (83.3%), with a male-to-female ratio of approximately 5:1. Most patients resided in rural or mountainous areas (71.4%). Common comorbidities included hypertension (50.0%), COPD or asthma (38.1%), and diabetes mellitus (23.8%). Nearly half of the patients (47.6%) had a history of corticosteroid use prior to admission (Table 1). Gastrointestinal symptoms were also common, occurring in 17/42 patients (40.5%). Abdominal pain was the predominant gastrointestinal complaint (15/42, 35.7%), whereas abdominal distension/bloating and diarrhea were each reported in 5/42 patients (11.9%). Nausea or vomiting was noted in 4/42 patients (9.5%). The symptom frequencies shown in Table 2 reflect the initial clinical presentation at admission. While all patients met the study definition of LRTI, a subset presented primarily with fever and gastrointestinal symptoms and did not initially report prominent respiratory symptoms such as cough or dyspnea. In the cohort of 42 patients with lower respiratory tract infections and concomitant S. stercoralis infection, chronic strongyloidiasis, including both parasitologically confirmed and serology-only probable cases, accounted for 59.5% of the cohort, whereas hyperinfection syndrome was identified in 40.5%. Among the 17 patients with hyperinfection syndrome, the predominant initial clinical presentation was respiratory in 10 patients (58.8%) and gastrointestinal in 7 patients (41.2%). Within respiratory manifestations (n = 10), pneumonia was the most common presentation (54.5%), followed by AECOPD in 45.5%. Gastrointestinal manifestations (n = 7) were predominantly gastroenteritis (66.7%), while paralytic ileus and intestinal obstruction were less frequent, each occurring in 16.7% of cases (Fig. 1).

Table 1.

Baseline demographic characteristics and risk factors of patients with lower respiratory tract infections associated with Strongyloides stercoralis (n = 42)

Category	Variable	Value
Age	Mean ± SD, years	73.3 ± 10.2
	Range, years	53–93
	Age ≥ 60 years	39 (92.9)
Sex	Male	35 (83.3)
	Female	7 (16.7)
Residence	Rural or mountainous areas	30 (71.4)
	Urban areas	12 (28.6)
Comorbidities	Hypertension	21 (50.0)
	COPD / asthma	16 (38.1)
	Diabetes mellitus	10 (23.8)
Risk factors	Prior corticosteroid use	20 (47.6)
	Alcohol abuse	3 (7.1)

Data are presented as number (percentage) or mean ± standard deviation

COPD: Chronic obstructive pulmonary disease

Table 2.

Initial presenting symptoms at admission in patients with LRTIs associated with Strongyloides stercoralis (n = 42)

Manifestations	Patients with symptoms (n, %)	Symptom	n	%
Gastrointestinal	17 (40.5)	Abdominal pain	15	35.7
		Abdominal distension/bloating	5	11.9
		Diarrhea	5	11.9
		Nausea/vomiting	4	9.5
Respiratory	37/42 (88.1)	Productive cough (sputum)	37	88.1
		Dyspnea	32	76.2
		Hemoptysis	3	7.1
Other signs/symptoms	5/42 (19.1)	Weight loss	4	9.5
		Cutaneous larva migrans / “larvae crawling under the skin”	2	4.8
		Fever	31	73.8

Fig. 1.

Distribution of probable chronic strongyloidiasis and hyperinfection syndrome, and clinical subtypes of hyperinfection among patients with lower respiratory tract infections (n = 42)

Productive cough was the most frequent symptom in both groups, reported in 88.2% of patients with hyperinfection syndrome and 88.0% of those with chronic infection. Dyspnea and fever were common in both groups but occurred more frequently in patients with hyperinfection. Gastrointestinal manifestations, particularly abdominal pain, were observed more frequently in the hyperinfection group (47.1% vs. 28.0%). In contrast, weight loss was observed exclusively in patients with probable chronic strongyloidiasis (Fig. 2).

Fig. 2.

Comparison of clinical manifestations between probable chronic strongyloidiasis and hyperinfection syndrome (n = 42)

Laboratory abnormalities were common in this cohort. Anemia was observed in 64.3% of patients, predominantly mild anemia with hemoglobin levels between 100 and 120 g/L (45.2%). Moderate anemia occurred in 14.3% of cases, while severe anemia was uncommon (4.8%), and no patient presented with very severe anemia (hemoglobin < 60 g/L). Eosinophilia, defined as an absolute eosinophil count > 0.5 G/L, was detected in 23.8% of patients. Hyponatremia (serum sodium < 135 mmol/L) was present in half of the cohort (50.0%). Hypoalbuminemia was highly prevalent, affecting 88.1% of patients. The most frequent patterns were moderate hypoalbuminemia (25–29.9 g/L) in 40.5% and severe hypoalbuminemia (< 25 g/L) in 33.3%, whereas only 11.9% of patients had normal serum albumin levels. Chest CT imaging most commonly demonstrated diffuse patchy opacities (64.3%), consistent with infectious or inflammatory lung involvement. Interstitial pneumonia patterns (19.0%) and lobar pneumonia (9.5%) were less frequent, while other radiological patterns accounted for 7.2% of cases (Table 3).

Table 3.

Hematological, biochemical, and chest CT characteristics of patients with lower respiratory tract infections and Strongyloides stercoralis infection (n = 42)

Domain	Variable	n (%)
Hemoglobin level	Hb > 120 g/L	15 (35.7)
	Hb 100–120 g/L (mild anemia)	19 (45.2)
	Hb 80–100 g/L (moderate anemia)	6 (14.3)
	Hb 60–80 g/L (severe anemia)	2 (4.8)
	Hb < 60 g/L (very severe anemia)	0 (0.0)
Eosinophil count	Eosinophilia (> 0.5 G/L)	10 (23.8)
Serum sodium	Hyponatremia (< 135 mmol/L)	21 (50)
Serum albumin level	Normal (≥ 35 g/L)	5 (11.9)
	Mild hypoalbuminemia (30–34.9 g/L)	6 (14.3)
	Moderate hypoalbuminemia (25–29.9 g/L)	17 (40.5)
	Severe hypoalbuminemia (< 25 g/L)	14 (33.3)
Chest CT findings	Diffuse patchy opacities	27 (64.3)
	Interstitial pneumonia pattern	8 (19.0)
	Lobar pneumonia	4 (9.5)
	Other patterns	3 (7.2)

Serological testing using ELISA for S. stercoralis antibodies yielded the highest positivity rate in the overall cohort (90.5%). In contrast, direct microscopic detection methods showed substantially lower detection rates, including bronchoalveolar lavage or sputum examination (26.2%), stool microscopy (23.8%), and gastric fluid examination (16.7%). In patients with hyperinfection syndrome, ELISA remained the most sensitive diagnostic method, with a positivity rate of 88.2%. Direct detection in bronchoalveolar lavage fluid was positive in 64.7% of cases, while gastric fluid examination was positive in 41.2%; in contrast, stool microscopy failed to identify any positive cases. One patient demonstrated simultaneous positivity in both bronchoalveolar lavage and gastric fluid samples (Fig. 3).

Fig. 3.

Diagnostic yield of Strongyloides stercoralis detection methods. (A) Positive rates of different diagnostic tests in the overall cohort (n = 42). (B) Detection rates of Strongyloides stercoralis according to diagnostic methods in patients with hyperinfection syndrome (n = 17)

Microbiological cultures predominantly yielded Gram-negative bacteria, accounting for 63.2% of all isolates, mainly Escherichia coli, Pseudomonas spp., and Acinetobacter baumannii. Gram-positive organisms represented 21.1% of isolates, whereas fungal isolates, exclusively Aspergillus fumigatus, accounted for 15.8%; however, the clinical significance of these isolates could not be definitively determined in this retrospective study (Table 4).

Table 4.

Distribution of microbial categories isolated in patients with lower respiratory tract infections and Strongyloides stercoralis infection (Based on total number of isolates, n = 19)

Microbial category	Identified pathogens	Number of isolates	Percentage (%)
Gram-negative bacteria	Klebsiella pneumoniae, Escherichia coli, Pseudomonas species, Acinetobacter baumannii	12	63.2
Gram-positive bacteria	Enterococcus faecium, Enterococcus faecalis, Staphylococcus aureus	4	21.1
Fungi	Aspergillus fumigatus	3	15.8

Note: Percentages were calculated based on the total number of microbial isolates. Pathogens were isolated from blood, bronchoalveolar lavage/bronchial fluid, and sputum samples

Discussion

Clinical characteristics of patients with S. stercoralis infection and lower respiratory tract infections

In this study, patients with concomitant S. stercoralis infection and lower respiratory tract infection were predominantly elderly, with a mean age of 73.3 ± 10.2 years, and the majority were aged ≥ 60 years. This mean age is substantially higher than that reported in an 11-year retrospective study from Thailand, in which the mean age was 54.4 ± 15.7 years [22]. This difference may reflect differences in study design and patient populations, as our cohort included only hospitalized patients with lower respiratory tract infections, a clinical scenario more frequently encountered in older adults with multiple comorbidities and varying degrees of immunosuppression. Our findings are comparable to those reported in Spain (mean age 68.3 years) [23] and remain higher than data from the National Hospital for Tropical Diseases in Vietnam, where the median age was 64.5 years [24].

A marked male predominance was observed, with men accounting for 83.3% of cases, consistent with reports from New Zealand, where 72% of patients were male [25]. This sex distribution may be related to differences in occupational exposure to contaminated soil, higher prevalence of smoking, alcohol consumption, and chronic lung disease among men, as well as delayed healthcare-seeking behavior, which may contribute to more advanced disease at the time of diagnosis. Most patients originated from rural or mountainous areas (71.4%), in line with the established epidemiology of strongyloidiasis. Nevertheless, nearly one-third of patients resided in urban areas, likely reflecting prior exposure in rural environments followed by migration, a pattern also described in previous studies [20].

The burden of comorbidities in this cohort was substantial. Hypertension, chronic obstructive pulmonary disease (COPD) or asthma, diabetes mellitus, and alcohol use disorder were common, and nearly half of the patients had a history of corticosteroid use. These conditions have previously been reported in association with hyperinfection syndrome. Previous reports have documented patients with COPD or asthma receiving long-term corticosteroid therapy who presented with apparent respiratory exacerbations and were subsequently diagnosed with hyperinfection syndrome [23, 24]. Corticosteroid exposure has been frequently reported in patients with hyperinfection syndrome and severe strongyloidiasis [20, 26].

Overall, more than three-quarters of patients had multiple comorbidities. Advanced age, chronic underlying diseases, and corticosteroid exposure were common characteristics in this cohort and have been frequently described in previous reports of severe strongyloidiasis [20].

Paraclinical abnormalities were highly prevalent in this cohort, reflecting severe and often prolonged systemic illness

The three most prominent disturbances were anemia, hypoalbuminemia, and hyponatremia, all of which have been reported in previous studies of severe strongyloidiasis and hospitalized infections.

Anemia was observed in 64.3% of patients, predominantly of mild severity, while moderate-to-severe anemia accounted for approximately one-fifth of cases. This distribution suggests a chronic, insidious disease process that may remain unrecognized in the absence of routine laboratory evaluation. Anemia has been frequently reported in strongyloidiasis [15] and appears to be more prevalent among hospitalized or critically ill patients. Potential mechanisms include chronic inflammation, malnutrition, impaired iron absorption, and occult gastrointestinal blood loss due to intestinal mucosal damage.

Hypoalbuminemia was the most frequently observed laboratory abnormality, affecting 88.1% of patients, with the majority exhibiting moderate to severe reductions. This finding has consistently been associated with disease severity and increased mortality [20, 27]. Mechanistically, low serum albumin levels may reflect systemic inflammation, reduced hepatic synthesis, increased vascular permeability, and protein loss through damaged intestinal mucosa. Hypoalbuminemia has been associated with edema, pleural effusion, and respiratory compromise in previous reports.

Hyponatremia was detected in 50% of patients and appeared more frequent among patients with hyperinfection syndrome. Similar associations between hyponatremia and mortality in severe strongyloidiasis have been reported previously [15, 20, 27]. The underlying mechanisms are likely multifactorial, including gastrointestinal fluid losses, inappropriate antidiuretic hormone secretion during severe infection, malnutrition, and hypoalbuminemia. These laboratory abnormalities are common in older hospitalized patients and are not specific to strongyloidiasis; therefore, they should be interpreted as accompanying features in confirmed cases rather than prognostic or diagnostic markers. Future studies with age-matched Strongyloides-negative LRTI comparators are needed to quantify their incremental predictive value. In addition, diagnostic suspicion bias should be considered when interpreting these findings. In routine clinical practice, Strongyloides testing was more likely to be performed in patients with atypical, severe, or treatment-refractory respiratory presentations, particularly in the presence of corticosteroid exposure, chronic lung disease, gastrointestinal symptoms, or unexplained laboratory abnormalities. As a result, the study cohort may overrepresent clinically severe or diagnostically complex cases, which could influence the observed frequency of hyperinfection syndrome and associated laboratory abnormalities.

Eosinophilia, defined as an absolute eosinophil count ≥ 500 cells/µL, was observed in only 23.8% of patients and was markedly less frequent in hyperinfection syndrome than in probable chronic strongyloidiasis. This finding is consistent with previous reports indicating that eosinophilia may be present during chronic infection but is often absent in severe strongyloidiasis, particularly in the setting of corticosteroid exposure, immunosuppression, severe sepsis, or malnutrition [15, 20]. Corticosteroid exposure, immunosuppression, severe sepsis, and malnutrition may suppress eosinophil production and function in hyperinfection syndrome.

Regarding imaging, chest CT demonstrated heterogeneous and largely nonspecific abnormalities. The most common pattern was diffuse patchy opacities (64.3%), followed by interstitial involvement (19.0%) and lobar pneumonia (9.5%), while other findings such as pleural effusion or pneumothorax were uncommon. These imaging features may overlap with those of community-acquired pneumonia, acute exacerbations of COPD, or infected bronchiectasis, often leading to an initial diagnosis favoring bacterial etiologies. International reports indicate that, in strongyloidiasis—particularly in hyperinfection syndrome—pulmonary lesions are often diffuse, non-segmental, and may change rapidly over time, further complicating early recognition and differentiation from other causes of lower respiratory tract infection [14, 15, 17, 28].

Diagnostic value of parasitological and serological tests

The diagnosis of S. stercoralis infection is challenging because intermittent larval shedding limits the sensitivity of conventional parasitological methods, whereas serological testing provides superior diagnostic performance in endemic settings. Our results are consistent with previous reports showing poor sensitivity of stool examination due to irregular larval shedding, even in chronic infections [20]. In contrast, ELISA allows detection of specific antibodies with substantially higher sensitivity and is particularly useful for screening high-risk populations. Mitchell et al. similarly demonstrated that serological testing detected significantly more infections than stool microscopy in refugee populations prior to resettlement [29]. Together, these findings are consistent with previous studies reporting higher sensitivity of ELISA compared with stool microscopy in endemic settings.

In patients with hyperinfection syndrome, ELISA positivity remained high (88.2%), whereas stool microscopy failed to detect any cases. Conversely, direct examination of bronchoalveolar lavage and gastric fluid showed substantially higher diagnostic yields (64.7% and 41.2%, respectively). This observation is in line with reports by Nabeya et al., who emphasized that in hyperinfection syndrome, larvae preferentially accumulate in the respiratory tract, in which larvae were more frequently detected in airway specimens than in stool samples [27].

In clinical practice, interpretation of these findings requires consideration of the overall clinical, radiological, and mycological context. Serological testing is well suited for screening and identifying occult infections when stool examinations are negative. However, because ELISA cannot reliably distinguish past from active infection and may show cross-reactivity with other helminths, parasitological confirmation using respiratory or gastrointestinal specimens remains essential in suspected hyperinfection syndrome, particularly in patients with severe respiratory manifestations or sepsis.

Microbiological co-infections in pulmonary strongyloidiasis

Microbiological cultures in this cohort predominantly yielded Gram-negative enteric bacteria, most commonly Klebsiella pneumoniae and Escherichia coli, along with other Gram-negative pathogens such as Pseudomonas spp. and Acinetobacter baumannii. Gram-positive bacteria, particularly Staphylococcus aureus, as well as opportunistic fungal pathogens, including Aspergillus fumigatus, were also identified. These co-infections were observed more frequently among patients with hyperinfection syndrome, consistent with previous reports describing concomitant secondary bacterial and fungal infections in severe strongyloidiasis. Similar findings have been consistently reported in the literature, where Gram-negative enteric bacteria predominate in cases of bacteremia and severe pulmonary involvement associated with strongyloidiasis hyperinfection [15, 20]. In addition, several studies have highlighted the role of Aspergillus spp. in severe pulmonary forms, with fungal co-infection contributing to increased mortality [27]. Therefore, antifungal treatment should be guided by the overall clinical, radiological, and mycological context rather than by culture positivity alone. Overall, these findings underscore the complex interplay between parasitic infection and secondary bacterial or fungal infections in pulmonary strongyloidiasis. Unrecognized concomitant strongyloidiasis may complicate the interpretation of persistent or treatment-refractory pulmonary infections.

While our findings primarily inform clinical recognition at the hospital level, they also highlight a broader gap in strongyloidiasis control in endemic settings. The diagnostic delay and severe presentations observed in older adults and patients receiving corticosteroids highlight the potential public health relevance of earlier recognition and screening strategies in endemic settings. In this context, the World Health Organization (WHO) released its first guideline on the public health control of strongyloidiasis, conditionally recommending annual mass drug administration with single-dose ivermectin (200 µg/kg) for individuals aged ≥ 5 years in endemic settings where local prevalence meets the WHO threshold (≥ 5%). In Vietnam—where public health interventions for other intestinal helminths are already implemented—integrating strongyloidiasis into existing neglected tropical disease platforms (surveillance, targeted screening of high-risk groups prior to immunosuppression, and consideration of preventive chemotherapy where epidemiologically appropriate) may help reduce missed diagnoses and severe complications [30].

Conclusion

In this hospital-based retrospective case series of patients with lower respiratory tract infections and S. stercoralis infection, systemic laboratory abnormalities were a prominent feature. Hypoalbuminemia was the most frequent finding, followed by anemia and hyponatremia, whereas eosinophilia was uncommon, particularly in hyperinfection syndrome. Chest CT abnormalities were heterogeneous and largely nonspecific. Microbial co-infections, mainly Gram-negative enteric bacteria and opportunistic fungi, were frequently documented, especially in clinically severe cases.

Strengths and limitations

Strengths

This study provides a comprehensive clinical and paraclinical characterization of lower respiratory tract infections associated with S. stercoralis in an endemic setting. By systematically distinguishing between probable chronic strongyloidiasis and hyperinfection syndrome, it provides descriptive clinical and diagnostic data regarding patients with concomitant lower respiratory tract infections and S. stercoralis infection in an endemic setting. The study was conducted at a large tertiary referral center, allowing detailed evaluation of severe and complex cases that are often underrepresented in community-based studies.

Limitations

This study has several limitations. First, its retrospective single-center case-series design limits causal inference and is vulnerable to incomplete or missing documentation. Second, because eligibility required evidence of S. stercoralis infection (per the Vietnamese MoH 2022 criteria and Grover 1996 definitions), we did not include a contemporaneous Strongyloides-negative LRTI comparator cohort; therefore, we could not estimate the prevalence of strongyloidiasis among all LRTI admissions or formally compare demographic, clinical, and laboratory characteristics between infected and uninfected patients. This introduces potential selection bias, as Strongyloides testing in routine practice may be more frequently pursued in patients with atypical/severe presentations or relevant risk factors (e.g., corticosteroid exposure). Third, key laboratory and microbiological procedural details were not consistently captured in the medical records (e.g., complete specifications of the ELISA assay, stool concentration/culture techniques, and systematic recording of larval stage), which may limit reproducibility and could contribute to misclassification of disease severity. Furthermore, because serological testing may not accurately differentiate between past and active infections, patients diagnosed solely by ELISA testing were classified as having probable chronic strongyloidiasis. Diagnostic suspicion bias was also likely present, as Strongyloides testing in routine practice was more frequently pursued in patients with atypical, severe, or treatment-refractory presentations, potentially enriching the cohort for hyperinfection syndrome and severe laboratory abnormalities. Incomplete mycological and radiological data additionally prevented reliable differentiation between Aspergillus colonization and true invasive pulmonary aspergillosis. Finally, treatment regimens and clinical outcomes were not evaluated in the present analysis and will be addressed in a separate report focusing on therapeutic strategies and patient outcomes.

Abbreviations

AECOPD

Acute Exacerbation of Chronic Obstructive Pulmonary Disease

ARDS

Acute Respiratory Distress Syndrome

ATS/IDSA

American Thoracic Society / Infectious Diseases Society of America

BAL

Bronchoalveolar Lavage

BTS

British Thoracic Society

CAP

Community-Acquired Pneumonia

COPD

Chronic Obstructive Pulmonary Disease

CT

Computed Tomography

ELISA

Enzyme-Linked Immunosorbent Assay

GOLD

Global Initiative for Chronic Obstructive Lung Disease

IgG

Immunoglobulin G

IQR

Interquartile Range

LRTIs

Lower Respiratory Tract Infections

MoH

Ministry of Health

SD

Standard Deviation

Author contributions

Vu Van Giap contributed to Conceptualization, Methodology, Formal analysis, Supervision, and Writing—review & editing. Dao Xuan Co contributed to Methodology, Supervision, and Writing—review & editing. Phan Thu Phuong contributed to Investigation, and Writing—review & editing. Pham Thi Quynh Huong contributed to Investigation, Data curation, Formal analysis and Writing—review & editing. Phan Thanh Thuy contributed to Conceptualization, Methodology, Formal analysis, Visualization, Writing—original draft, Writing—review & editing. All authors read and approved the final manuscript.

Funding

This research received no external funding.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

The study protocol was reviewed and approved by the Ethics Committee of Bach Mai Hospital, Hanoi, Vietnam. The study was conducted in accordance with the ethical standards of the institutional review board and the principles of the Declaration of Helsinki. Given the retrospective case series design and the use of anonymized clinical data, the requirement for written informed consent was waived by the Ethics Committee of Bach Mai Hospital in accordance with national regulations and institutional policies.

Consent for publication

Not applicable.

AI policy compliance

In accordance with the journal’s policy on the use of artificial intelligence, the authors declare that ChatGPT (OpenAI) was used solely to assist with English language editing, stylistic refinement, and standardization of tables and figures. All AI-assisted content was carefully reviewed, revised, and validated by the authors, who take full responsibility for the scientific accuracy, interpretation, and integrity of the manuscript. No AI tools were used for data collection, data analysis, result interpretation, or formulation of scientific conclusions.

Competing interests

The authors declare no competing interests.