Parasites Misdiagnosed as Other Diseases

This page looks at the point that parasitic infections are often mistaken for other illnesses, such as irritable bowel syndrome, Crohn's disease and other autoimmune or chronic conditions. It sets that point, made in a 2026 integrative-oncology presentation, beside the published research: case reviews of parasitic colitis first diagnosed as inflammatory bowel disease, the dangerous combination of a hidden Strongyloides infection and corticosteroid medicines, the long debate over Blastocystis and Dientamoeba in irritable bowel syndrome, and the lasting bowel symptoms that can follow Giardia.

What the Presentation Said
Why Parasites Can Look Like Other Diseases
Parasitic Colitis First Diagnosed as Inflammatory Bowel Disease
Strongyloides Hyperinfection After Corticosteroids
Screening Before Immune-Suppressing Treatment
Blastocystis, Dientamoeba and Irritable Bowel Syndrome
Irritable Bowel Syndrome After Giardia
How the Claim Compares With the Research
What Is Not Known
Connections
Key Research Papers
Featured Videos

What the Presentation Said

Among "things you may not know about parasites", the speaker listed misdiagnosis first. The presentation said parasitic infections are often misdiagnosed as autoimmune and chronic disorders, naming irritable bowel syndrome, Crohn's disease and urinary tract infections as examples.

To illustrate the point, the speaker told a story described as true, set in a well-known hospital in Houston, Texas. As told:

A hospitalised patient had stomach and other vague symptoms.
On daily ward rounds, a first-year resident, originally from Mexico, repeatedly said he thought the cause was parasites.
The senior doctors did not accept this at first and prescribed other medicines, treating it as a digestive problem, and the patient did not improve.
Eventually the team looked beyond Crohn's disease or another digestive diagnosis and found a parasitic infection.
The resident suggested antiparasitic medicines, and the patient got better.

The speaker offered the story to show that parasitic infection is "oftentimes" missed. Elsewhere in the same talk, the speaker linked parasites to chronic fatigue, autoimmune diseases including lupus, allergies, skin problems and digestive dysfunction, and said that a negative standard stool test does not rule parasites out.

About the hospital story

The story names no patient, no parasite, no date and no published case report, so it cannot be checked. It is reported here as the speaker told it. The rest of this page looks at what published case reviews, cohort studies and guidelines say about parasitic infections that were first labelled as something else.

Why Parasites Can Look Like Other Diseases

Several features of intestinal parasites make them easy to mistake for other conditions, and the published literature describes each of them.

Symptoms are common and non-specific. Diarrhoea, belly pain, bloating, weight loss, wheezing and itchy rashes have many causes. In a hospital record review of 151 people diagnosed with Strongyloides in the United States, presenting complaints were described as non-specific, and diagnosis came on average 56 months after the person had immigrated (Boulware and colleagues, 2007).
The bowel can look the same under the camera. Amoebic colitis (inflammation of the large bowel caused by the amoeba Entamoeba histolytica) produces ulcers that can resemble Crohn's disease, ulcerative colitis or intestinal Behçet's disease (Cheng and colleagues, 2020; Fukui and colleagues, 2022).
Biopsies can mislead. Many worm infections draw in eosinophils (white blood cells that respond to parasites and allergies). A biopsy full of eosinophils can be read as "eosinophilic colitis" of unknown cause unless the parasite itself is found (Corsetti and colleagues, 1999).
Stool tests can miss the organism. Strongyloides releases few larvae, and not every day. In the 151-case review, a stool "ova and parasites" examination (microscope search for eggs, larvae and cysts) detected the infection about half the time (51%) (Boulware and colleagues, 2007). Reviews describe low larval output as a basic limit of stool testing for this worm (Buonfrate and colleagues, 2023).
The infection may be decades old. Strongyloides can re-infect its own host from inside the gut, so an infection picked up in childhood can persist for decades with few symptoms (Czeresnia and Weiss, 2022).

These are the same features the presentation pointed to. The research describes them for particular parasites, not for parasites as a whole.

Parasitic Colitis First Diagnosed as Inflammatory Bowel Disease

The strongest published evidence on this point is a 2025 systematic review of case reports from high-income countries (Lees and colleagues, 2025). The authors searched PubMed and Scopus for reports published between 2012 and January 2025 of colitis (bowel inflammation) whose cause turned out to be a parasite, leaving out people who had travelled to an endemic area within the four weeks before they became ill.

What the review found

52 articles described 54 patients: 33 with amoebiasis, 15 with strongyloidiasis and 6 with schistosomiasis.
37 of the 54 patients (69%) were misdiagnosed at first, and 28 of those 37 (76%) were misdiagnosed as inflammatory bowel disease.
Substantial harm was reported in 31 of 54 patients (57%), including seven deaths.
Among people with strongyloidiasis, major illness or death (hyperinfection, sepsis, emergency surgery) occurred in 8 of 9 who received corticosteroids (89%), compared with 1 of 6 who did not (17%).
Among people with amoebiasis, the figures were 10 of 13 who received corticosteroids (77%) compared with 2 of 20 who did not (10%).

The authors concluded that misdiagnosis as inflammatory bowel disease is commonly reported and that poor outcomes were linked to corticosteroid treatment. A review of case reports cannot say how often this happens in the population; it shows that it does happen and what tends to follow.

Individual case reports

Amoebiasis. A 54-year-old man with more than a year of right-sided belly pain had been diagnosed with inflammatory bowel disease; colonoscopy biopsies showed amoebic infection of the caecum, and the ulcers healed after anti-amoebic treatment (Cheng and colleagues, 2020). In Japan, a woman with Behçet's disease had four colonoscopies before amoebiasis was found; she had received high-dose glucocorticoids for suspected intestinal Behçet's disease, and her condition worsened (Fukui and colleagues, 2022).
Schistosomiasis. A 23-year-old farmer in Sudan with six months of bloody diarrhoea had been treated for dysentery and inflammatory bowel disease; biopsies showed schistosome eggs, and symptoms resolved within two weeks of praziquantel (Alajab and colleagues, 2024).
Giardia. An adolescent with marked whole-body features suggesting inflammatory bowel disease had Giardia in biopsies from the small intestine, and the illness resolved after metronidazole (Gunasekaran and Hassall, 1992).
Cryptosporidium. During the 1993 Milwaukee waterborne outbreak, 12 people with stable ulcerative colitis or Crohn's disease had sudden flares caused by Cryptosporidium, and two people with no bowel history developed ileitis or colitis that at first suggested Crohn's disease. All recovered (Manthey and colleagues, 1997).

Case reports are the weakest form of clinical evidence on their own, because they describe single people and unusual cases are more likely to be written up. Their value here is that the same pattern repeats across countries and parasites.

Strongyloides Hyperinfection After Corticosteroids

The most serious consequence of a missed parasite in the published record involves the threadworm Strongyloides stercoralis. Unlike most worms, it can complete its life cycle inside one person: larvae hatched in the gut can re-enter the body through the bowel wall or the skin around the anus. This "autoinfection" keeps a small, quiet infection going for decades (Czeresnia and Weiss, 2022; Buonfrate and colleagues, 2023).

What hyperinfection is

When the immune system is suppressed, the number of larvae can rise very fast. In hyperinfection, large numbers of larvae pierce the bowel wall and spread through the lungs and other organs, and they can carry gut bacteria with them into the blood, causing sepsis or meningitis (Czeresnia and Weiss, 2022; Buonfrate and colleagues, 2023). A 2008 review reported a mortality rate for hyperinfection syndrome of 15% to 87% across published series (Marcos and colleagues, 2008).

Corticosteroids are the most frequent trigger

Corticosteroids (anti-inflammatory steroid medicines such as prednisone and dexamethasone) are the most frequent risk factor for hyperinfection, according to the 2008 review. Other listed risk factors include infection with the HTLV-1 virus, organ transplantation, blood cancers (especially lymphoma), tuberculosis and malnutrition (Marcos and colleagues, 2008). Cases have also been described after corticosteroids given for COVID-19 pneumonia (Czeresnia and Weiss, 2022).

How a wrong diagnosis leads to steroids

The cases in the literature follow a recognisable pattern: the parasite's symptoms are taken for a disease that is itself treated with steroids.

Asthma. An elderly man from Colombia with an apparent asthma attack grew worse on systemic corticosteroids; larvae were then found in fluid washed from his lungs, and he improved after ivermectin (Salam and colleagues, 2020).
Eosinophilic colitis. A 69-year-old man in Italy was referred for corticosteroid treatment of "idiopathic eosinophilic colitis" after two microscope methods on five stool samples were negative. Ten more samples collected on separate days found Strongyloides (Corsetti and colleagues, 1999).
Inflammatory bowel disease. A 52-year-old woman in Iran with inflammatory diarrhoea and repeatedly negative stool tests was at first suspected of having inflammatory bowel disease; a duodenal biopsy showed Strongyloides larvae in the crypts of the gut lining. The authors wrote that the case was not confirmed by molecular testing or serology (Asdaghi Jahromi and colleagues, 2026).
In a hospital series. In the 151-case United States review, hyperinfection occurred in five patients who had been prescribed corticosteroids, and two of them died (Boulware and colleagues, 2007).

In the 2025 systematic review described above, 8 of 9 people with Strongyloides colitis who received corticosteroids had major illness or died (Lees and colleagues, 2025). Reviews report that ivermectin is the treatment of choice for strongyloidiasis, with a single dose for uncomplicated infection and repeated doses for people with suppressed immunity (Buonfrate and colleagues, 2023).

Screening Before Immune-Suppressing Treatment

Because the danger comes mostly from steroids and other immune-suppressing medicines, much of the published discussion concerns testing people before such treatment begins.

A 2022 review states that patients with exposure to endemic areas and those expected to become immune-suppressed are to be screened and treated before immune-suppressing agents are started, and that empirical treatment (treating without a test result) is considered when timely testing is not available (Czeresnia and Weiss, 2022).
Serology (a blood test for antibodies) is effective for detecting Strongyloides but is less sensitive in people whose immunity is already suppressed; stool tests are limited by low larval output (Buonfrate and colleagues, 2023).
The 2007 United States study also gave a written case to 363 resident doctors in 15 training programmes worldwide: an immigrant with wheezing and eosinophilia. Only 9% of US trainees recognised the need for parasite screening and 23% suggested empirical corticosteroids, compared with 56% recognition among international trainees. Treatment errors in the case series were more frequent among clinicians unfamiliar with immigrant health (relative risk 8.4) (Boulware and colleagues, 2007).
The same study described eosinophilia (more than 5% of white cells, or more than 400 eosinophils per microlitre) in 84% of the 151 patients, a finding on an ordinary complete blood count (Boulware and colleagues, 2007).

This part of the research is close to the presentation's message: it was a lack of familiarity with parasites, not their rarity, that put patients at risk. The difference is that the published concern is specific (Strongyloides, amoebiasis and schistosomiasis in people with exposure risk who are about to receive steroids), while the presentation's claim was general. Decisions about testing and treatment belong with a clinician.

Blastocystis, Dientamoeba and Irritable Bowel Syndrome

Irritable bowel syndrome is a long-term pattern of belly pain with changed bowel habits, without visible damage to the bowel. Two single-celled organisms, Blastocystis and Dientamoeba fragilis, are often found in stool samples, and whether they cause irritable bowel symptoms has been argued for decades.

Blastocystis

A 2022 meta-analysis of observational studies (29 papers, 31 datasets for Blastocystis) found that people carrying Blastocystis had higher odds of irritable bowel syndrome (odds ratio 1.78, 95% confidence interval 1.29 to 2.44). The authors wrote that further studies are needed to determine the effect (Abedi and colleagues, 2022).
A 2021 cross-sectional study in Barcelona of 36 people with irritable bowel syndrome (Rome intravenous (IV) criteria) and 36 people without symptoms, using microscopy and polymerase chain reaction (PCR) (a DNA test), found Blastocystis in 19.4% of the patients and 36.1% of the symptom-free group, a difference that was not statistically significant (Salvador and colleagues, 2021).

An association in observational studies does not show which comes first: Blastocystis may cause symptoms, or a disturbed gut may simply suit it, and many healthy people carry it.

Dientamoeba fragilis

In a Dutch primary-care study of 107 children with digestive complaints, 55.1% carried Dientamoeba. In a comparison with their symptom-free siblings, it was not found more often in children with symptoms (odds ratio 1.02, 95% confidence interval 0.28 to 3.65) (Holtman and colleagues, 2017).
In a case-control study in Saudi Arabia, Dientamoeba was found in 9 people in the comparison group without digestive symptoms and in 3 with digestive symptoms (Hawash and colleagues, 2020).
A 2019 clinical guideline review notes that the organism is frequently found in people with belly pain and diarrhoea but also commonly in people without symptoms, so its clinical relevance is disputed; it adds that wider use of polymerase chain reaction testing has raised the number of positive results and that treatment guidelines differ between countries (van Gestel and colleagues, 2019).

For these two organisms, the research does not settle whether a positive test explains a person's irritable bowel symptoms. That is different from the presentation's framing, in which a parasite is the hidden real diagnosis.

Irritable Bowel Syndrome After Giardia

A different and better-supported link runs the other way: an infection that has been treated and cleared can leave irritable bowel syndrome behind. This is called post-infectious irritable bowel syndrome.

The Bergen outbreak. In 2004 a waterborne Giardia outbreak hit the city of Bergen, Norway. Three years later, 46.1% of 817 people with laboratory-confirmed giardiasis reported irritable bowel syndrome, compared with 14.0% of 1,128 matched controls (adjusted relative risk 3.4). Chronic fatigue was reported by 46.1% of the exposed group and 12.0% of the controls (Wensaas and colleagues, 2012). The data came from mailed questionnaires.
Travellers. At a Barcelona travel-medicine referral centre, 68 of 669 travellers diagnosed with travellers' diarrhoea (10.2%) went on to develop post-infectious irritable bowel syndrome, defined as symptoms lasting at least six months with negative stool tests after treatment. Parasitic infection, mostly Giardia, was an independent risk factor (odds ratio 3.0) (España-Cueto and colleagues, 2023).
Across pathogens. A 2019 meta-analysis found pooled rates of post-infectious irritable bowel syndrome of about 11% to 14% after bacterial gut infections, and noted that few studies exist for parasitic causes (Svendsen and colleagues, 2019). The 2022 meta-analysis found a strong association between Giardia and later irritable bowel syndrome in cohort studies (odds ratio 5.47), but not a statistically significant one in case-control studies (Abedi and colleagues, 2022).

In these studies the parasite was found and treated, and the symptoms that remained were irritable bowel syndrome. That is the opposite of the presentation's picture of irritable bowel syndrome as a label hiding an active parasite: here the irritable bowel syndrome is real and outlasts the parasite.

How the Claim Compares With the Research

Supported

Parasitic infections are sometimes first diagnosed as other diseases, including inflammatory bowel disease, asthma and eosinophilic colitis. A systematic review of 54 published cases in high-income countries found an initial misdiagnosis in 69%, most often as inflammatory bowel disease (Lees and colleagues, 2025).
A negative stool test does not always rule out a parasite. For Strongyloides, single stool examinations missed about half of infections in one large series (Boulware and colleagues, 2007), and repeated samples found what earlier samples missed (Corsetti and colleagues, 1999).
Unfamiliarity among clinicians is documented. In a survey of trainee doctors, only 9% of US residents recognised the need for parasite screening in a written Strongyloides case (Boulware and colleagues, 2007).

Supported only in part

The research concerns a few specific parasites (Entamoeba histolytica, Strongyloides, Schistosoma, Giardia, Cryptosporidium) in people with relevant exposure. It does not show that parasites are a common hidden cause of irritable bowel syndrome or Crohn's disease in general.
Blastocystis is associated with irritable bowel syndrome in a meta-analysis of observational studies, but individual studies disagree and cause and effect are not established. For Dientamoeba, two studies found it no more often, or less often, in people with symptoms.
Giardia is linked to irritable bowel syndrome, but mainly as a lasting effect after the infection is cleared, not as an active infection mislabelled as irritable bowel syndrome.

Not found in the research

No published study was found showing that parasites are commonly misdiagnosed as lupus or other autoimmune diseases, or as urinary tract infections, as a general pattern.
The Houston hospital story could not be checked against any published case.

What the research adds

The presentation did not mention the finding that has the most direct consequences for patients: when a parasitic infection is mistaken for an inflammatory disease, the treatment for that disease (corticosteroids) can turn a quiet Strongyloides infection into a frequently fatal hyperinfection, and can worsen amoebic colitis (Marcos and colleagues, 2008; Lees and colleagues, 2025).

What Is Not Known

How often misdiagnosis happens. The best evidence is a review of published case reports, which shows the pattern but cannot give a rate. No population study was found that measures how many people diagnosed with inflammatory bowel disease or irritable bowel syndrome actually have an undetected parasite.
How common Strongyloides is. Estimates of worldwide infection range from 30 to 100 million (Yang and colleagues, 2025) to 300 to 600 million (Buonfrate and colleagues, 2023), and a 2024 review describes global prevalence as largely unknown because of limited surveillance (Gordon and colleagues, 2024).
The best test. Reviews describe a lack of standardised diagnostic methods for Strongyloides, and serology is less sensitive in people who are already immune-suppressed (Buonfrate and colleagues, 2023; Gordon and colleagues, 2024).
Whether Blastocystis or Dientamoeba cause symptoms. Both are common in healthy people, and studies disagree.
Why post-infectious irritable bowel syndrome develops. A review describes how Giardia increases the leakiness of the gut lining and disturbs the gut's bacteria (Allain and Buret, 2020), but in a Norwegian study of people with lasting symptoms after Giardia, three proposed blood markers did not differ from healthy controls (Hanevik and colleagues, 2022).
Any link to cancer. The research on misdiagnosis concerns digestive and lung illness; none of the studies above examined cancer.

Connections

Key Research Papers

Lees RD, Fyfe J, Woods LM, et al. Parasitic colitis misdiagnosis as inflammatory bowel disease in high-income settings and association with poor clinical outcomes when exposed to corticosteroids: a systematic review of case reports. BMJ Open Gastroenterol. 2025;12(1):e002080. PubMed PMID: 41360440
Boulware DR, Stauffer WM, Hendel-Paterson BR, et al. Maltreatment of Strongyloides infection: case series and worldwide physicians-in-training survey. Am J Med. 2007;120(6):545.e1-8. PubMed PMID: 17524758
Marcos LA, Terashima A, Dupont HL, Gotuzzo E. Strongyloides hyperinfection syndrome: an emerging global infectious disease. Trans R Soc Trop Med Hyg. 2008;102(4):314-8. PubMed PMID: 18321548
Czeresnia JM, Weiss LM. Strongyloides stercoralis. Lung. 2022;200(2):141-148. PubMed PMID: 35396957
Buonfrate D, Bradbury RS, Watts MR, Bisoffi Z. Human strongyloidiasis: complexities and pathways forward. Clin Microbiol Rev. 2023;36(4):e0003323. PubMed PMID: 37937980
Gordon CA, Utzinger J, Muhi S, et al. Strongyloidiasis. Nat Rev Dis Primers. 2024;10(1):6. PubMed PMID: 38272922
Yang R, Xu M, Zhang L, et al. Human Strongyloides stercoralis infection. J Microbiol Immunol Infect. 2025;58(2):164-179. PubMed PMID: 39142910
Salam R, Sharaan A, Jackson SM, et al. Strongyloides hyperinfection syndrome: a curious case of asthma worsened by systemic corticosteroids. Am J Case Rep. 2020;21:e925221. PubMed PMID: 33347427
Corsetti M, Basilisco G, Pometta R, et al. Mistaken diagnosis of eosinophilic colitis. Ital J Gastroenterol Hepatol. 1999;31(7):607-9. PubMed PMID: 10604103
Asdaghi Jahromi H, Davoodi A, Hosseini A. Duodenal strongyloidiasis mimicking inflammatory bowel disease: a case report of a 52-year-old immunocompetent woman. Clin Med Insights Case Rep. 2026;19:11795476261474993. PubMed PMID: 42553508
Cheng CW, Feng CM, Chua CS. Cecal amebiasis mimicking inflammatory bowel disease. J Int Med Res. 2020;48(5):300060520922379. PubMed PMID: 32475192
Fukui A, Nakayama Y, Yoshida T, et al. A case of intestinal amoebiasis mimicking intestinal Behçet's disease. Mod Rheumatol Case Rep. 2022;6(2):270-272. PubMed PMID: 35388431
Alajab MB, Rafei A, Abdo AE. Schistosomal colitis mimicking inflammatory bowel disease: a case report from Sudan. Ann Med Surg (Lond). 2024;86(8):4895-4897. PubMed PMID: 39118742
Gunasekaran TS, Hassall E. Giardiasis mimicking inflammatory bowel disease. J Pediatr. 1992;120(3):424-6. PubMed PMID: 1538290
Manthey MW, Ross AB, Soergel KH. Cryptosporidiosis and inflammatory bowel disease. Experience from the Milwaukee outbreak. Dig Dis Sci. 1997;42(8):1580-6. PubMed PMID: 9286220
Abedi SH, Fazlzadeh A, Mollalo A, et al. The neglected role of Blastocystis sp. and Giardia lamblia in development of irritable bowel syndrome: a systematic review and meta-analysis. Microb Pathog. 2022;162:105215. PubMed PMID: 34592369
Salvador F, Lobo B, Goterris L, et al. Blastocystis sp. carriage and irritable bowel syndrome: is the association already established? Biology (Basel). 2021;10(4):340. PubMed PMID: 33921803
Holtman GA, Kranenberg JJ, Blanker MH, et al. Dientamoeba fragilis colonization is not associated with gastrointestinal symptoms in children at primary care level. Fam Pract. 2017;34(1):25-29. PubMed PMID: 27784723
Hawash YA, Ismail KA, Saber T, et al. Dientamoeba fragilis infection in patients with digestive and non-digestive symptoms: a case-control study. Korean J Parasitol. 2020;58(2):129-134. PubMed PMID: 32418381
van Gestel RS, Kusters JG, Monkelbaan JF. A clinical guideline on Dientamoeba fragilis infections. Parasitology. 2019;146(9):1131-1139. PubMed PMID: 30165915
Wensaas KA, Langeland N, Hanevik K, et al. Irritable bowel syndrome and chronic fatigue 3 years after acute giardiasis: historic cohort study. Gut. 2012;61(2):214-9. PubMed PMID: 21911849
Allain T, Buret AG. Pathogenesis and post-infectious complications in giardiasis. Adv Parasitol. 2020;107:173-199. PubMed PMID: 32122529
Hanevik K, Saghaug C, Aaland M, et al. No difference in serum levels of B-cell activating receptor and antibodies against cytolethal distending toxin B and flagellin in post-infectious irritable bowel syndrome and chronic fatigue syndrome after Giardia infection. JGH Open. 2022;6(3):185-188. PubMed PMID: 35355666
España-Cueto S, Oliveira-Souto I, Salvador F, et al. Post-infectious irritable bowel syndrome following a diagnosis of traveller's diarrhoea: a comprehensive characterization of clinical and laboratory parameters. J Travel Med. 2023;30(6):taad030. PubMed PMID: 36881659
Svendsen AT, Bytzer P, Engsbro AL. Systematic review with meta-analyses: does the pathogen matter in post-infectious irritable bowel syndrome? Scand J Gastroenterol. 2019;54(5):546-562. PubMed PMID: 31112663