Other Suspected Links Between Parasites and Cancer

Table of Contents

1. What the Presentation Said
2. Where These Parasites Stand Officially
3. Trichomonas and Prostate Cancer
4. Trichomonas and Cervical Cancer
5. Cryptosporidium, Blastocystis and Colorectal Cancer
6. Strongyloides, HTLV-1 and Adult T-Cell Leukaemia
7. Pork Tapeworm Cysts and Brain Tumours
8. Echinococcus: Cancer Risk and Cysts Mistaken for Tumours
9. Studies Where Parasites Appeared to Slow Tumours
10. How the Claim Compares With the Research
11. What Is Not Known
12. Connections
13. Key Research Papers
14. Featured Videos

What the Presentation Said

The presentation, a 2026 webinar given by the medical director of an integrative cancer clinic, made a general case that parasites help cause cancer. The speaker said that in a person with cancer, parasites are part of the tumour microenvironment (the mix of cells, blood vessels and signalling molecules surrounding a tumour), and that parasites carry toxins and other microbes, creating what was described as a toxic sink.

The speaker then described a chain of events called parasite-driven oncogenesis (oncogenesis means the formation of cancer). In that account, parasitic infection causes inflammation, the immune system fails to detect the parasites and other microbes, oxidative stress causes mutations, chronic inflammation causes DNA damage, and epigenetic changes (chemical switches that turn genes on or off without changing the DNA code) activate cancer genes. The speaker added that parasites distort cell signals, block oxygen and change the “terrain” in which cancer forms. In answer to a question about stomach cancer, the speaker said parasites can be a factor in that cancer too.

The presentation did not name Trichomonas, Cryptosporidium, Blastocystis, Strongyloides, the pork tapeworm or Echinococcus as causes of any particular cancer, and it cited no study on them. This page therefore tests the speaker’s general claim against these specific cases — the parasites most often proposed in the research literature as possible cancer causes beyond the officially listed ones.

Where These Parasites Stand Officially

The International Agency for Research on Cancer (IARC), part of the World Health Organization, publishes a list of the cancer sites for which each agent has “sufficient” or “limited” evidence of causing cancer in humans. In that list (IARC Monographs volumes 1–141), the parasites named are three flukes and one malaria parasite:

• Sufficient evidence in humans: Schistosoma haematobium (bladder), and the liver flukes Clonorchis sinensis and Opisthorchis viverrini (bile duct).
• Limited evidence in humans: Schistosoma japonicum (liver and colorectum), and malaria caused by Plasmodium falciparum in areas where it is common (Burkitt lymphoma).

None of the parasites on this page — Trichomonas vaginalis, Cryptosporidium, Blastocystis, Strongyloides stercoralis, Taenia solium (the pork tapeworm) or Echinococcus — appears in that list for any cancer site. Giardia and Fasciola do not appear either. The cause of stomach cancer that IARC lists among infections is Helicobacter pylori, a bacterium, not a parasite.

Two things follow. First, absence from the list does not prove that a parasite is harmless; it means the evidence has not reached the level IARC requires. Second, one of the cases below, adult T-cell leukaemia, does have an IARC-listed cause — but that cause is a virus (human T-cell lymphotropic virus type 1, HTLV-1), and the parasite Strongyloides is studied as a possible accelerator, not as the cause.

Trichomonas and Prostate Cancer

Trichomonas vaginalis is a single-celled parasite passed on through sex. It is one of the most common sexually transmitted infections in the world, and in men it often causes no symptoms. Because it can inflame the prostate, researchers have asked whether it raises the risk of prostate cancer. The results are mixed.

Studies in people

• A 2019 meta-analysis (a study that pools the results of earlier studies) combined six case-control studies with 2,677 men with prostate cancer and 2,913 men without it, from the United States, Kuwait and Taiwan. The pooled odds ratio was 1.17 (95% confidence interval 1.01–1.36). An odds ratio of 1 means no difference; 1.17 means past infection was about 17% more common among the men with cancer. The authors described the result as not statistically significant and said firmer conclusions would need further studies (Najafi and colleagues, 2019).
• A US study of 146 men with advanced (spread or fatal) prostate cancer and 181 matched men without it found that antibodies to Trichomonas were not linked to a higher risk of advanced or fatal disease; the odds ratios were below 1 (Shui and colleagues, 2016).
• Among 1,485 men with prostate cancer followed for a median of about 13 years in two large US cohorts of health professionals, Trichomonas antibody status was not linked to death from prostate cancer or from any cause (Tsang and colleagues, 2019).
• A case-control study in Egypt found antibodies to Trichomonas in 19% of 126 men with prostate cancer and in 8.3% of 120 healthy men. The antibody levels rose with PSA and tumour stage, but seropositivity was not linked to overall survival (Saleh and colleagues, 2021).

A 2022 review that weighed many possible infectious causes of prostate cancer judged Trichomonas vaginalis as “possible but unlikely” to have a role, ranking it below human papillomavirus and several bacteria (Lawson and Glenn, 2022).

Laboratory studies point in both directions

• In mice and in cell culture, mouse prostate cancer cells exposed to Trichomonas released inflammatory signals and grew, moved and invaded more; prostates injected with the parasite plus cancer cells grew larger tumours over four weeks than prostates given cancer cells alone (Kim and colleagues, 2021).
• In cell culture, a different team found the opposite: fluid from Trichomonas cultures slowed the growth of two human prostate cancer cell lines and pushed the cells toward programmed cell death (Zhu and colleagues, 2016).

Trichomonas and Cervical Cancer

In women the association is more consistent, but it is tangled up with the known cause of cervical cancer. IARC lists infection with several types of human papillomavirus (HPV) as having sufficient evidence of causing cervical cancer. Trichomonas and HPV are both sexually transmitted, so women who have one are more likely to have had the other.

• A 2023 meta-analysis of 35 observational studies, covering 67,856 women with cervical neoplasia (abnormal or cancerous cervical cells) and 933,697 controls in 14 countries, found an adjusted odds ratio of 2.17 (95% confidence interval 1.82–2.60) for Trichomonas infection (Fazlollahpour-Naghibi and colleagues, 2023).
• Another 2023 meta-analysis of 29 studies and about 474,000 women found that women with Trichomonas had higher odds of HPV co-infection (odds ratio 1.79), of high-grade precancerous lesions (2.34) and of cervical cancer (5.23) (Hamar and colleagues, 2023).

Both analyses are built from observational studies. They show that the two conditions occur together; they cannot show whether Trichomonas adds risk of its own, whether it helps HPV persist, or whether it simply marks women with more exposure to HPV. The authors of the larger meta-analysis called for longitudinal and experimental studies to settle that question.

Cryptosporidium, Blastocystis and Colorectal Cancer

Cryptosporidium

Cryptosporidium is a single-celled parasite spread through contaminated water and food. In healthy people it usually causes diarrhoea that clears on its own; in people with weak immune systems it can be long-lasting and dangerous.

• In mice with no working immune system (SCID mice), some also given the steroid dexamethasone, Cryptosporidium parvum infection led to adenocarcinoma (a gland-forming cancer) in the last part of the small intestine and the start of the colon, even with very small doses of the parasite. The researchers found abnormal placement of proteins in the Wnt signalling pathway and of p53, but no mutations in the genes they sequenced (Benamrouz and colleagues, 2014).
• In people, a 2022 meta-analysis found Cryptosporidium in about 12.7% of patients with colorectal cancer, and in the two case-control studies available the odds of infection were about five times higher in patients than in controls (odds ratio 5.06) (Taghipour and colleagues, 2022). These studies tested people who already had cancer, so they cannot show whether the infection came first.

Blastocystis

Blastocystis is one of the most common organisms found in human stool. Whether it causes disease at all is still debated; it was long regarded as a harmless or even helpful member of the gut community (Tocci and colleagues, 2024).

• In Poland, Blastocystis was found by microscope in 12.15% of 107 patients with colorectal cancer, tested before any cancer treatment, compared with 2.42% of 124 people without cancer. The authors interpreted this as cancer raising the risk of an opportunistic Blastocystis infection, not the reverse (Sulżyc-Bielicka and colleagues, 2021).
• The same 2022 meta-analysis found a pooled odds ratio of 2.10 for Blastocystis in colorectal cancer patients compared with controls (Taghipour and colleagues, 2022).
• A 2024 review describes proposed mechanisms — inflammation, oxidative stress, changes to gut bacteria and damage to the gut lining — and states that a causal link needs further investigation (Tocci and colleagues, 2024).

For both organisms, the human evidence is an association found in people who already have cancer. Cancer, surgery, chemotherapy and hospital stays can all make gut infections more likely, which is one reason the direction of the link is unknown.

Strongyloides, HTLV-1 and Adult T-Cell Leukaemia

Adult T-cell leukaemia/lymphoma is a blood cancer caused by human T-cell lymphotropic virus type 1 (HTLV-1), a virus listed by IARC with sufficient evidence for this cancer. Most people with the virus never develop the cancer; a 2023 review puts the lifetime share at 5–10% of carriers (Nakahata and colleagues, 2023).

Strongyloides stercoralis, a threadworm that can live in the gut for decades, is common in the same tropical regions as HTLV-1. Research on the pair reports:

• In co-infected people, Strongyloides is linked to a higher viral load of HTLV-1 than infection with the virus alone, and HTLV-1 shifts the immune response in a way that makes the worm harder to control (Dykie and colleagues, 2020).
• A 2023 review states that co-infection with Strongyloides promotes the development of adult T-cell leukaemia, and presents this as evidence that the immune state of the carrier helps decide whether the virus causes disease (Nakahata and colleagues, 2023).
• The traffic runs both ways: a 2022 review reports higher rates of Strongyloides treatment failure in people living with HTLV-1 (Rosadas and Taylor, 2022).

This is the strongest of the suspected links on this page, but it is a link to a virus-caused cancer. The worm is studied as a possible accelerator in people who already carry HTLV-1; no study found here describes Strongyloides causing cancer on its own. A separate and well-documented danger is Strongyloides hyperinfection, in which the worm multiplies out of control, often after steroid treatment — a risk that matters for cancer patients given steroids; the published case series and screening research are described on Parasites Misdiagnosed as Other Diseases.

Pork Tapeworm Cysts and Brain Tumours

Neurocysticercosis is infection of the brain with the larval cysts of Taenia solium, the pork tapeworm. It comes from swallowing the tapeworm’s eggs. Because cysts can sit beside tumours, researchers in Ecuador asked whether one leads to the other.

• In a 1997 case-control study, 8 of 43 patients with a glioma (a tumour arising from the brain’s support cells) had neurocysticercosis, compared with 5 of 172 matched controls, an odds ratio of 7.63 (95% confidence interval 2.03–31.09). In six of the eight patients, calcified cysts lay within or around the tumour (Del Brutto and colleagues, 1997).

The study was small, as the wide confidence interval shows, and came from a single centre in a region where neurocysticercosis is common. The authors proposed that the scarring around dead cysts and the parasite’s suppression of the immune response might contribute to tumour growth; this was a hypothesis, and no large confirming study was found in the evidence gathered for this page.

Echinococcus: Cancer Risk and Cysts Mistaken for Tumours

Echinococcus tapeworms, picked up from the eggs passed by dogs and foxes, form cysts in the liver, lungs and other organs (hydatid disease). They connect to cancer in two separate ways.

A pilot study of cancer risk

In Cyprus, 249 people with a past Echinococcus granulosus infection were compared with 753 uninfected relatives and neighbours. Cancer had been reported by 11.65% of the infected group and 8.37% of the comparison group. The hazard ratio (a measure of risk over time) was 1.60, but after adjusting for sex, year of birth, smoking, alcohol and family history it fell to 1.54 with a confidence interval of 0.97–2.45, which includes no difference. Medical histories came from interviews. The authors called it a pilot study whose result would need independent confirmation (Oikonomopoulou and colleagues, 2016).

Cysts that look like cancer

The firmer link runs the other way: Echinococcus lesions are mistaken for cancer. Alveolar echinococcosis, caused by Echinococcus multilocularis, grows into the liver like a tumour. A 2026 radiology review describes it as a tumour-mimicking disease whose solid appearance and contrast patterns often lead to its being classified as primary or secondary liver cancer, and lists the imaging clues — calcifications, and the lack of true arterial-phase enhancement — that help tell them apart (Seyfettin and colleagues, 2026).

Cancer and its treatment also change how the infection behaves. A 2023 review of 870 patients with alveolar echinococcosis found that 8.3% had a cancer; in patients with weakened immune systems the infection was found earlier, looked atypical on scans more often, and antibody tests detected it far less often (25% compared with 81%) (Autier and colleagues, 2023).

Studies Where Parasites Appeared to Slow Tumours

A separate line of research reports the opposite of the presentation’s claim: parasites, or molecules taken from them, slowing tumours in laboratory animals.

• A 2022 systematic review gathered 51 laboratory and animal studies in which material from 13 parasites showed possible anti-tumour activity, among them Toxoplasma gondii, Trypanosoma cruzi, Trichomonas vaginalis, Echinococcus granulosus and Trichinella spiralis. Most of the work used preparations from Toxoplasma (16 studies) and Echinococcus (10 studies). The authors state that the molecular basis remains to be worked out (Asghari and colleagues, 2022).
• A 2024 review describes how some parasites and their products, including Plasmodium, Toxoplasma, Trypanosoma and Echinococcus, can turn “cold” tumours (ones the immune system ignores) into “hot” ones that immune cells attack. It notes that these findings were confirmed in animal studies and discusses the hurdles before any clinical use (Xie and colleagues, 2024).
• The laboratory finding that Trichomonas culture fluid slowed human prostate cancer cells (Zhu and colleagues, 2016), described above, belongs to the same line of work.

All of this is work in cell culture and animals. No human trial of a parasite as a cancer treatment was found in the evidence gathered for this page. A search of PubMed for human studies showing that worm infections protect against cancer returned nothing usable for this page, so no such protective effect in people is asserted here.

How the Claim Compares With the Research

Supported in part

• Parasites can take part in cancer development. For the parasites on this page, the best-supported example is Strongyloides as a possible accelerator of a virus-caused leukaemia in people who carry HTLV-1. Inflammation, immune suppression and oxidative stress — mechanisms the presentation listed — are the same mechanisms researchers propose for Trichomonas, Blastocystis and neurocysticercosis.
• Associations with specific cancers exist. Trichomonas is consistently linked with cervical cancer in observational studies, Cryptosporidium causes intestinal cancer in immune-deficient mice, and Cryptosporidium and Blastocystis are found more often in people with colorectal cancer.

Weak or unconfirmed

• Trichomonas and prostate cancer: a borderline pooled result, two null US studies on advanced and fatal disease, and laboratory studies pointing in opposite directions.
• Neurocysticercosis and glioma: one small case-control study and case reports.
• Echinococcus and cancer risk: one pilot study whose adjusted result included no difference.
• Cryptosporidium, Blastocystis and colorectal cancer in people: associations measured after cancer was diagnosed, which cannot show which came first; one research team interpreted its finding as cancer making infection more likely.

Not supported by the research found

• A general rule that parasites cause cancer. None of the parasites on this page is listed by IARC as a cause of any cancer. The research describes a handful of specific associations of varying strength, not a general mechanism that applies to parasites as a group.
• Parasites and stomach cancer. The infectious cause of stomach cancer that IARC lists is the bacterium Helicobacter pylori. No study connecting a parasite to stomach cancer was found in the evidence gathered for this page.
• Parasites only ever favour tumours. Animal and laboratory studies report parasites and their products slowing tumours as well.

Decisions about cancer treatment and antiparasitic medicines belong with a clinician.

What Is Not Known

• Whether Trichomonas adds to cervical cancer risk on its own, or mainly travels with HPV, the established cause.
• Whether Cryptosporidium or Blastocystis infection comes before colorectal cancer in people, or follows it. No long-term study that tested people before cancer appeared was found.
• Whether treating Strongyloides in people with HTLV-1 lowers their risk of adult T-cell leukaemia. No trial of this was found.
• Whether the neurocysticercosis–glioma association holds in larger studies in other countries.
• Whether any of the anti-tumour effects seen in mice can be reproduced safely in people.
• How large a share of cancers, if any, these parasites account for. No estimate of that share was found.

Connections

• Cancer and Parasites
• Parasites Classified as Cancer-Causing Agents
• How Parasites Can Drive Cancer: The Mechanisms
• Toxoplasma and Brain Tumors
• Parasites in the Brain
• Parasites
• Trichomonas Vaginalis
• Cryptosporidium
• Blastocystis
• Strongyloides
• Echinococcus
• Tapeworm
• Prostate Cancer
• Cervical Cancer
• Colorectal Cancer
• Leukemia

Key Research Papers

1. Najafi A, Chaechi Nosrati MR, Ghasemi E, et al. Is there association between Trichomonas vaginalis infection and prostate cancer risk?: A systematic review and meta-analysis. Microbial pathogenesis. 2019;137:103752. PubMed PMID: 31539586
2. Shui IM, Kolb S, Hanson C, et al. Trichomonas vaginalis infection and risk of advanced prostate cancer. The Prostate. 2016;76(7):620-3. PubMed PMID: 26818005
3. Tsang SH, Peisch SF, Rowan B, et al. Association between Trichomonas vaginalis and prostate cancer mortality. International journal of cancer. 2019;144(10):2377-2380. PubMed PMID: 30242839
4. Saleh NE, Alhusseiny SM, El-Zayady WM, et al. Trichomonas vaginalis serostatus and prostate cancer risk in Egypt: a case-control study. Parasitology research. 2021;120(4):1379-1388. PubMed PMID: 33159459
5. Lawson JS, Glenn WK. Multiple pathogens and prostate cancer. Infectious agents and cancer. 2022;17(1):23. PubMed PMID: 35637508
6. Kim SS, Kim KS, Han IH, et al. Proliferation of Mouse Prostate Cancer Cells Inflamed by Trichomonas vaginalis. The Korean journal of parasitology. 2021;59(6):547-556. PubMed PMID: 34974661
7. Zhu Z, Davidson KT, Brittingham A, et al. Trichomonas vaginalis: a possible foe to prostate cancer. Medical oncology (Northwood, London, England). 2016;33(10):115. PubMed PMID: 27613161
8. Fazlollahpour-Naghibi A, Bagheri K, Almukhtar M, et al. Trichomonas vaginalis infection and risk of cervical neoplasia: A systematic review and meta-analysis. PloS one. 2023;18(7):e0288443. PubMed PMID: 37437068
9. Hamar B, Teutsch B, Hoffmann E, et al. Trichomonas vaginalis infection is associated with increased risk of cervical carcinogenesis: A systematic review and meta-analysis of 470 000 patients. International journal of gynaecology and obstetrics. 2023;163(1):31-43. PubMed PMID: 37010897
10. Benamrouz S, Conseil V, Chabé M, et al. Cryptosporidium parvum-induced ileo-caecal adenocarcinoma and Wnt signaling in a mouse model. Disease models & mechanisms. 2014;7(6):693-700. PubMed PMID: 24652769
11. Taghipour A, Rayatdoost E, Bairami A, et al. Are Blastocystis hominis and Cryptosporidium spp. playing a positive role in colorectal cancer risk? A systematic review and meta-analysis. Infectious agents and cancer. 2022;17(1):32. PubMed PMID: 35715853
12. Sulżyc-Bielicka V, Kołodziejczyk L, Adamska M, et al. Colorectal cancer and Blastocystis sp. infection. Parasites & vectors. 2021;14(1):200. PubMed PMID: 33853659
13. Tocci S, Das S, Sayed IM. An Update on Blastocystis: Possible Mechanisms of Blastocystis-Mediated Colorectal Cancer. Microorganisms. 2024;12(9):1924. PubMed PMID: 39338600
14. Nakahata S, Enriquez-Vera D, Jahan MI, et al. Understanding the Immunopathology of HTLV-1-Associated Adult T-Cell Leukemia/Lymphoma: A Comprehensive Review. Biomolecules. 2023;13(10):1543. PubMed PMID: 37892225
15. Dykie A, Wijesinghe T, Rabson AB, et al. Human T-cell Leukemia Virus Type 1 and Strongyloides stercoralis: Partners in Pathogenesis. Pathogens (Basel, Switzerland). 2020;9(11):904. PubMed PMID: 33137906
16. Rosadas C, Taylor GP. HTLV-1 and Co-infections. Frontiers in medicine. 2022;9:812016. PubMed PMID: 35187000
17. Del Brutto OH, Castillo PR, Mena IX, et al. Neurocysticercosis among patients with cerebral gliomas. Archives of neurology. 1997;54(9):1125-8. PubMed PMID: 9311356
18. Oikonomopoulou K, Yu H, Wang Z, et al. Association between Echinococcus granulosus infection and cancer risk - a pilot study in Cyprus. Clinical chemistry and laboratory medicine. 2016;54(12):1955-1961. PubMed PMID: 27327131
19. Seyfettin A, Kantarcı M, Durmaz Ö, et al. Diagnostic pearls and pitfalls in the radiologic evaluation of hepatic alveolar echinococcosis. Diagnostic and interventional radiology. 2026 (online ahead of print). PubMed PMID: 41972949
20. Autier B, Gottstein B, Millon L, et al. Alveolar echinococcosis in immunocompromised hosts. Clinical microbiology and infection. 2023;29(5):593-599. PubMed PMID: 36528295
21. Asghari A, Nourmohammadi H, Majidiani H, et al. Promising effects of parasite-derived compounds on tumor regression: a systematic review of in vitro and in vivo studies. Environmental science and pollution research international. 2022;29(22):32383-32396. PubMed PMID: 35146610
22. Xie Y, Wang J, Wang Y, et al. Parasite-enhanced immunotherapy: transforming the “cold” tumors to “hot” battlefields. Cell communication and signaling. 2024;22(1):448. PubMed PMID: 39327550

PubMed Topic Searches

1. https://pubmed.ncbi.nlm.nih.gov/?term=Trichomonas+vaginalis+prostate+cancer
2. https://pubmed.ncbi.nlm.nih.gov/?term=Cryptosporidium+colorectal+cancer
3. https://pubmed.ncbi.nlm.nih.gov/?term=Strongyloides+HTLV-1+adult+T-cell+leukemia
4. https://pubmed.ncbi.nlm.nih.gov/?term=alveolar+echinococcosis+mimicking+malignancy

Other Sources

1. International Agency for Research on Cancer. List of classifications by cancer sites with sufficient or limited evidence in humans, IARC Monographs Volumes 1–141. monographs.iarc.who.int

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