Isolated colonic strongyloidiasis presenting as iron deficiency anemia with negative duodenal biopsies and stool studies

Abstract

Background

Strongyloides stercoralis is a parasitic nematode capable of lifelong autoinfection, often resulting in delayed diagnosis because of nonspecific symptoms and the low sensitivity of stool testing. Isolated colonic involvement is uncommon and may be overlooked in the absence of hyperinfection.

Case presentation

A 67-year-old man with longstanding iron deficiency anemia presented with fatigue and intermittent eosinophilia. Upper endoscopy with duodenal biopsies was unrevealing. Colonoscopy demonstrated diffuse polypoid lesions with surrounding erythema and edema throughout the colon. Histopathologic examination of colonic biopsies revealed invasive Strongyloides stercoralis larvae with marked eosinophilic inflammation. Stool ova and parasite examinations were negative on two occasions, whereas Strongyloides IgG serology was positive. No evidence of hyperinfection or disseminated disease was identified. The patient was treated with ivermectin.

Conclusion

This case highlights a rare presentation of isolated colonic strongyloidiasis causing chronic iron deficiency anemia. Negative stool studies and unrevealing duodenal biopsies did not exclude infection, with diagnosis established only through colonic histopathology. Strongyloidiasis should be considered in patients with unexplained eosinophilia, iron deficiency anemia, or eosinophilic colitis despite negative stool studies and unrevealing upper gastrointestinal biopsies.

Background

Iron deficiency anemia (IDA) in older adults is most attributed to occult gastrointestinal (GI) blood loss and mandates structured endoscopic evaluation to identify bleeding sources or malabsorptive pathology [1]. Chronic IDA is frequently multifactorial, and premature diagnostic closure after identification of a plausible lesion can result in missed or delayed diagnoses [1], [2], [3], [4].

Although anemia is classically associated with hookworm infection due to direct intestinal blood loss, Strongyloides stercoralis can also contribute to chronic anemia via mucosal inflammation, impaired nutrient absorption, and occult GI bleeding [2], [3], [4]. Chronic strongyloidiasis may persist for decades through autoinfection and is increasingly recognized in the United States, including among socioeconomically vulnerable populations [5]. Adult female Strongyloides stercoralis typically inhabit the duodenum and proximal jejunum, making duodenal biopsy a high-yield diagnostic site. Colonic involvement is uncommon and is usually reported in patients with hyperinfection, disseminated disease, or immunosuppression [5]. Diagnosis is challenging due to low-sensitivity stool O&P testing from intermittent larval shedding and variable or absent eosinophilia, leading to delays, especially in nonendemic settings.

Case presentation

A 67-year-old man with a history of congestive heart failure, hypertension, chronic obstructive pulmonary disease, and longstanding anemia was transferred from a detention facility for evaluation of severe anemia. A detailed travel and exposure history was not available from the medical record, limiting identification of a specific source of infection. He reported chronic dizziness and fatigue lasting for more than one year. He denied melena, hematochezia, hematemesis, abdominal pain, diarrhea, rash, or unintentional weight loss. A colonoscopy performed seven years earlier was reportedly normal.

He had been hospitalized four months earlier for symptomatic anemia requiring transfusion but did not complete recommended outpatient gastroenterology follow-up. There was also no known history of systemic corticosteroid use, organ transplantation, chemotherapy, or other immunosuppressive therapy. HIV and HTLV-1/2 antibody testing were negative.

On presentation, the patient was hemodynamically stable but hypertensive. Physical examination revealed pale mucous membranes and mild abdominal distention without tenderness. No rash was noted. Laboratory evaluation demonstrated severe microcytic anemia with hemoglobin of 6.6 g/dL (Table 1). Absolute eosinophil count was mildly elevated at 0.7 × 10⁹ L. He received one unit of packed red blood cells resulting in an improvement of hemoglobin of 7.4 g/dL.

Table 1.

Evaluation of iron deficiency anemia.

Investigation	Result
Panel A. Laboratory Evaluation
Hemoglobin	6.6 g/dL
Hematocrit	22.70%
Mean corpuscular volume (MCV)	72.3 fL
Red cell distribution width (RDW)	19.90%
Ferritin	8 ng/mL
Iron Saturation	9.00%
Absolute eosinophil count	0.6 × 10⁹/L
Peak eosinophil %	11.20%
Panel B. Diagnostic Evaluation
CT abdomen/pelvis	No acute pathology
Fecal occult blood	Positive
Esophagogastroduodenoscopy	Diffuse nodular gastropathy
Gastric biopsy	Positive for Helicobacter pylori
Duodenal biopsy	Preserved villous architecture; no parasitic infection identified
Colonoscopy	Multiple diffuse flat and sessile polypoid lesions throughout the colon
Colonic biopsy	Strongyloides stercoralis larvae with marked eosinophilic inflammation

Eosinophil counts fluctuated during hospitalization and peaked at 11.2% on hospital day four. Computed tomography (CT) of the abdomen showed no acute pathology and no evidence of small bowel inflammation, with only moderate stool burden noted. Given concern for occult GI bleeding, esophagogastroduodenoscopy (EGD) and colonoscopy were performed.

Pathology and diagnostic evaluation

Endoscopic evaluation revealed diffuse nodularity of the gastric mucosa; however, gastric biopsies did not demonstrate trophozoites or other specific infectious pathology. Duodenal biopsies were obtained to evaluate for celiac disease in the setting of iron deficiency anemia and demonstrated preserved villous architecture without increased intraepithelial lymphocytosis or other significant histopathologic abnormality.

At least 11 colonic biopsies were obtained from the cecum, cecal polyp, ascending colon polyps, and transverse colon polyp. Histopathologic examination of the ascending colon polyp specimen, which contained eight biopsy fragments, demonstrated helminth larvae within crypt lumina and focally invading the lamina propria, accompanied by a dense eosinophilic inflammatory response. No parasitic organisms were identified in the remaining colonic specimens. Stool ova and parasite examinations were negative on two occasions, whereas Strongyloides IgG serology was positive, supporting the diagnosis of strongyloidiasis.

Blood cultures were negative and sputum culture revealed normal respiratory flora.

There was no clinical, laboratory, or radiographic evidence of hyperinfection or disseminated disease (Fig. 1, Fig. 2, Fig. 3).

Fig. 1.

Duodenum findings. The examined duodenum appeared endoscopically normal, with preserved mucosal folds and no evidence of scalloping, nodularity, erosions, or ulceration.

Fig. 2.

Colon findings. Colonoscopy demonstrated multiple diffuse flat and sessile polypoid lesions throughout the colon with surrounding mucosal erythema and edema. The appearance initially raised concern for inflammatory or polyposis-related pathology prior to histopathologic identification of Strongyloides stercoralis.

Fig. 3.

Histopathologic findings of colonic biopsies. Arrowheads indicateStrongyloides stercoralisorganisms/larvae. A. Low-power view (200 ×) demonstrating multiple organisms within the crypt lumen with surrounding inflammatory infiltrate. B. Low-power view (200 ×) showing organism invasion into the lamina propria with dense eosinophil-rich inflammation. C-E. Intermediate magnification (400 ×) demonstrating intramucosal organisms with prominent eosinophilic infiltrate. F. High-power view (1000 ×) demonstrating a helminth larva consistent with Strongyloides stercoralis, with surrounding eosinophils.

Management and outcome

The patient was referred for treatment with ivermectin, with dosing and duration coordinated through the detention facility infirmary. Given the presence of residual colonic lesions and a sessile serrated adenoma, repeat colonoscopy within 6 months was recommended for completion of polypectomy and surveillance. Capsule endoscopy was not performed or planned based on the available records. The patient was also scheduled for gastroenterology follow-up to assess response to antiparasitic therapy and confirm eradication of concurrent Helicobacter pylori infection.

No complications occurred, and full recovery was anticipated.

Discussion

This case highlights a rare and diagnostically challenging presentation of strongyloidiasis characterized by isolated colonic involvement with negative duodenal biopsies and stool testing. Strongyloides stercoralis classically localizes to the duodenum and proximal jejunum, where adult female worms reside and eggs hatch in situ; consequently, duodenal biopsy is generally considered a high-yield diagnostic site [5]. In contrast, colonic involvement is uncommon and has most frequently been described in patients with hyperinfection syndrome, disseminated disease, or significant immunosuppression [5]. Our patient demonstrated histologically confirmed colonic invasion without evidence of hyperinfection or an identifiable immunocompromising condition. The mechanism underlying isolated colonic involvement remains unclear; however, this case demonstrates that clinically significant colonic infection can occur in immunocompetent hosts.

The diagnosis was particularly challenging because both stool ova and parasite examinations and duodenal biopsies were negative. Chronic strongyloidiasis is known to be difficult to diagnose due to intermittent larval shedding and patchy gastrointestinal involvement, which can reduce the sensitivity of stool microscopy and endoscopic sampling [5]. Negative stool studies and unrevealing duodenal biopsies do not exclude strongyloidiasis when clinical suspicion remains high.

Endoscopically, the colon demonstrated numerous flat lesions that initially raised concern for polyposis. Histopathologic examination ultimately revealed larvae within crypt lumina and invading the lamina propria with a dense eosinophilic inflammatory response. Because strongyloidiasis may mimic inflammatory, infectious, or neoplastic colonic processes, recognition requires careful histopathologic evaluation and a high index of suspicion [5].

Notably, the patient had only mild peripheral eosinophilia. Prior studies have demonstrated that eosinophilia may be absent, intermittent, or mild in chronic strongyloidiasis and should not be used to exclude the diagnosis [6], [7], [8]. In this setting, tissue-based diagnosis remains particularly important.

Ultimately, this case adds to the limited literature describing isolated colonic strongyloidiasis in an apparently immunocompetent host. It emphasizes that clinically significant infection may be confined to the colon despite negative stool studies and unrevealing upper gastrointestinal biopsies, highlighting the importance of considering strongyloidiasis in patients with unexplained eosinophilia, chronic iron deficiency anemia, or eosinophilic colitis.

CRediT authorship contribution statement

Shaghayegh Abdollahi: Conceptualization, Data curation, Formal analysis, Writing – original draft, Writing – review & editing. Maria Margarita Epino: Conceptualization, Writing – original draft, Writing – review & editing. Sarah Salama: Writing – review & editing, Writing – original draft, Investigation, Data curation, Conceptualization. Ericka Charley: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. Amin Valencia Leal: Data curation, Formal analysis, Writing – original draft, Writing – review & editing.

Consent

Written informed consent was obtained from the patient for publication of this case report and accompanying images.

Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the authors used ChatGPT (OpenAI), solely for grammatical review and language refinement of the manuscript. The authors reviewed and edited the content as needed and take full responsibility for the content of the article.

Funding

No funding was received for this work.

Ethics

Written informed consent for publication of this case report and the accompanying images was obtained from the patient. Institutional Review Board approval was not required for this single-patient case report in accordance with institutional policy.

Conflicts of Interest

The authors declare no conflicts of interest.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or person relationships that could have appeared to influence the work reported in this paper.