Three parasitic worms are recognized by the World Health Organization’s International Agency for Research on Cancer (IARC) as definitive causes of cancer in humans, placed in the same risk category as tobacco smoke and asbestos. Beyond those three confirmed carcinogens, a growing body of research links several other parasites to elevated cancer risk through chronic inflammation, immune suppression, and direct DNA damage. The relationship is more varied than a simple “infection equals tumor,” though, and some parasites are even being studied for potential anti-cancer properties.
The Three Parasites Officially Classified as Carcinogens
IARC classifies biological agents into groups based on how strong the evidence is that they cause cancer. Group 1 means “carcinogenic to humans,” the highest level of certainty. Three helminth (worm) parasites hold that designation: two liver flukes, Opisthorchis viverrini and Clonorchis sinensis, and one blood fluke, Schistosoma haematobium.1PubMed Central. Why does infection with some helminths cause cancer? Each is tied to a specific cancer type: the liver flukes to cholangiocarcinoma (bile duct cancer) and the blood fluke to bladder cancer.2PubMed. Parasite-Associated Cancers (Blood Flukes/Liver Flukes) No other parasites currently carry that Group 1 label, though several others are under active investigation.
A recent systematic review and meta-analysis pooling data across multiple studies found that liver fluke infections were associated with a roughly fivefold increase in the odds of cholangiocarcinoma, with a pooled odds ratio of 4.82. Schistosoma haematobium infection and bladder cancer showed a pooled odds ratio of about 3.19.3International Journal of Medical and Pharmaceutical Research. Infection-Related Carcinogenesis: The Impact of Parasitic Infections and Antiparasitic Treatment on Cancer Risk – A Systematic Review and Meta-Analysis Those are substantial increases in risk, especially for populations where these infections are endemic.
Liver Flukes and Bile Duct Cancer
Opisthorchis viverrini and Clonorchis sinensis are flatworms that people pick up by eating raw or undercooked freshwater fish. The parasites migrate to the bile ducts inside the liver and can live there for years, sometimes decades. The cancer connection is not about a single moment of infection. It takes years of chronic inflammation, tissue damage, and repair for the process to tilt toward malignancy.
Research has shown that these flukes secrete growth factors, digestive enzymes, and tiny membrane-bound packages called extracellular vesicles while grazing on the lining of the bile ducts. This constant irritation triggers cycles of wounding and repair. Over time, the bile duct tissue thickens and scars, a process called fibrosis, and the cells lining the ducts start dividing abnormally.4PubMed Central. Mechanistic insights into liver-fluke-induced bile-duct cancer Laboratory studies on cells exposed to extracts from C. sinensis found that the cells shifted into more rapid division, produced molecules associated with cancer, showed changes in the activity of cancer-related genes, and lost normal cell-to-cell communication.5PubMed Central. Clonorchis sinensis and Cholangiocarcinoma Multi-omics research has further confirmed that the parasite drives lasting changes in how genes are expressed in bile duct tissue, including epigenetic abnormalities and the formation of precancerous lesions.6PubMed Central. Clonorchis sinensis and cholangiocarcinoma: molecular mechanisms and biomarker advances
Cholangiocarcinoma is a relatively rare cancer globally, but in parts of Thailand, Laos, Cambodia, Vietnam, South Korea, and China where liver fluke infection is common, it ranks among the leading causes of cancer death. The geographic overlap between infection rates and cancer incidence is one of the strongest pieces of epidemiological evidence for the connection.
Schistosoma and Bladder Cancer
Schistosoma haematobium is a blood fluke found across much of sub-Saharan Africa and parts of the Middle East. People become infected through contact with freshwater where infected snails release the parasite’s larvae. The adult worms settle in the blood vessels surrounding the bladder and lay eggs that burrow into the bladder wall, triggering an intense inflammatory response.
Over years of infection, this inflammation can lead to changes in the bladder lining that increase the risk of cancer. Historically, the cancer linked to schistosomiasis was predominantly squamous cell carcinoma of the bladder, a type less common in non-endemic countries, where transitional cell carcinoma dominates. However, researchers have noted that as treatment programs reduce chronic heavy infections, the ratio of cancer types in affected regions is shifting.7PubMed Central. Controversies and challenges in research on urogenital schistosomiasis-associated bladder cancer The precise biological mechanisms connecting the parasite’s eggs to cancerous transformation remain an active area of research, partly because good animal models for this specific process have been hard to develop.
Malaria and Burkitt Lymphoma
The connection between malaria and cancer is less direct than with the flukes, but no less real. In equatorial Africa, where both the malaria parasite Plasmodium falciparum and Epstein-Barr virus (EBV) are widespread in children, Burkitt lymphoma occurs at dramatically higher rates than elsewhere. The prevailing scientific model involves a two-hit process: malaria provides one hit and EBV provides the other.
Laboratory research has demonstrated that proteins from the malaria parasite can directly trigger EBV to switch from its dormant state to active replication inside infected B cells, a type of white blood cell.8PLOS Pathogens. A Molecular Link between Malaria and Epstein–Barr Virus Reactivation Meanwhile, the massive expansion of B cells driven by repeated malaria infections creates more opportunities for a cancer-promoting genetic error, specifically a rearrangement involving a gene called c-MYC. EBV then delivers a second blow by helping the damaged cells survive when they would normally self-destruct. A recent review synthesized the epidemiological and mechanistic evidence into a combined model: malaria drives the initial genetic damage and cell expansion, while EBV immortalizes those cells, allowing the cancer to take hold.9PubMed Central. The Complex Interplay of Malaria and EBV in Burkitt Lymphoma
This explains why reducing malaria transmission in a region tends to reduce Burkitt lymphoma rates, even though EBV remains nearly universal in those populations. It also illustrates how a parasite can promote cancer without being a direct carcinogen itself: malaria acts as a co-factor that amplifies the cancer-causing potential of a virus the child was already carrying.
Suspected but Unconfirmed Links
Beyond the confirmed carcinogens and the well-established malaria-lymphoma connection, several other parasites have generated concerning signals without yet reaching the level of proof needed for an IARC classification.
Toxoplasma gondii, the common cat-associated parasite that infects a large fraction of the world’s population, has drawn attention because of its tendency to form cysts in the brain. Studies have found that people with brain tumors are more likely to carry antibodies against T. gondii than healthy controls.10PubMed Central. Possible role of Toxoplasma gondii in brain cancer through modulation of host microRNAs In one Korean study, about 18% of brain tumor patients tested positive for T. gondii antibodies, compared with roughly 9% of healthy controls.11PubMed Central. High Toxoplasma gondii Seropositivity among Brain Tumor Patients in Korea The parasite is known to alter the expression of tiny regulatory molecules called microRNAs in host cells, which could theoretically push infected tissue toward cancerous changes. But these are correlation studies, and causation has not been established. It is possible, for instance, that the immune environment created by a brain tumor makes people more susceptible to reactivation of a dormant Toxoplasma infection rather than the other way around.
Trichomonas vaginalis, a sexually transmitted protozoan, has been explored as a possible contributor to aggressive prostate cancer. The parasite secretes a protein that mimics a human inflammatory molecule and, in lab experiments, this protein increased the growth and invasiveness of prostate cancer cells.12PubMed Central. Trichomonas vaginalis homolog of macrophage migration inhibitory factor induces prostate cell growth, invasiveness, and inflammatory responses That is a plausible mechanism, but population studies have not produced consistent results. One study found no significant link between T. vaginalis antibodies and prostate-specific antigen levels, a common marker for prostate issues.13PubMed Central. Trichomonas vaginalis Infection and Prostate-Specific Antigen Concentration: Insights into Prostate Involvement and Prostate Disease Risk Another found no association between T. vaginalis infection and prostate cancer mortality.14PubMed Central. Association between Trichomonas vaginalis and prostate cancer mortality The evidence here is best described as suggestive at the cellular level but unconvincing in human populations so far.
Cryptosporidium parvum, a waterborne protozoan best known for causing diarrhea, has shown alarming cancer-causing potential in immunocompromised mice. Even a single parasite cyst was enough to trigger invasive digestive-tract tumors in mice lacking a functional immune system, with cancerous lesions appearing within weeks.15PubMed Central. Cryptosporidium parvum infection in SCID mice infected with only one oocyst: qPCR assessment of parasite replication in tissues and development of digestive cancer Longer-term infection in mouse models also created a tumor-friendly local environment complete with immune-suppressing cells and pro-inflammatory signals.16PubMed Central. Persistent Cryptosporidium parvum Infection Leads to the Development of the Tumor Microenvironment in an Experimental Mouse Model: Results of a Microarray Approach Whether this translates to meaningful cancer risk in humans, particularly in immunocompetent people who clear the infection within weeks, remains unknown. The concern is primarily for people with severely weakened immune systems, such as transplant recipients or those with advanced HIV.
Strongyloides and Adult T-Cell Leukemia
A different kind of parasite-cancer interaction plays out with Strongyloides stercoralis, a soil-transmitted roundworm common in tropical regions. This worm does not cause cancer on its own, but in people already carrying a cancer-associated virus called HTLV-1, the worm infection appears to accelerate the development of adult T-cell leukemia/lymphoma (ATL). In a study comparing 19 co-infected patients with 19 who had ATL without Strongyloides, the co-infected group developed leukemia at a much younger age, with a median onset of 39 years compared to 70 years in the uninfected group.17PubMed. Effect of Strongyloides stercoralis infection and eosinophilia on age at onset and prognosis of adult T-cell leukemia Pilot research has suggested that the co-infection may expand the population of HTLV-1-infected cells and push them toward malignant transformation, though the exact mechanism remains under investigation.18PubMed Central. CADM1 Overexpression and CD7 Downregulation in Strongyloides stercoralis and HTLV-1 Coinfection as Possible Markers for Adult T-cell Leukemia/Lymphoma progression Like the malaria-EBV story, this illustrates how a parasite can be a co-factor rather than a standalone carcinogen.
The Shared Mechanisms Behind Parasite-Driven Cancer
Despite the variety of parasites and cancer types, a few common pathways emerge across the research. A systematic review of both lab and animal evidence identified chronic inflammation, oxidative stress, unchecked cell division in damaged tissues, and immune manipulation as the main pro-tumor effects associated with parasitic infections.19PubMed Central. The dual role of parasites and parasite-derived products in cancer biology: a systematic review of in vitro and in vivo evidence
Chronic inflammation is probably the most important thread. Parasites that persist in tissues for years generate a constant immune response, and that sustained inflammation produces reactive oxygen species that can damage DNA. Over time, the accumulation of DNA damage raises the odds of a cell acquiring the mutations needed to become cancerous.20PubMed. DNA damage induced by parasitic infections in humans and animals Alongside the direct genetic damage, long-lived parasites often manipulate the host immune system to avoid being destroyed. Research has shown that some helminths cause immune cells to upregulate the same inhibitory checkpoint molecules, such as PD-L1 and PD-L2, that cancer cells exploit to evade immune attack.21PubMed Central. Similarities and differences between helminth parasites and cancer cell lines in shaping human monocytes: Insights into parallel mechanisms of immune evasion In other words, the immune-suppressing tricks that keep the worm alive may simultaneously make the body less able to detect and destroy abnormal cells.
Parasites, the Gut Microbiome, and Colorectal Cancer Risk
A newer line of investigation looks at how intestinal worm infections reshape the community of bacteria living in the gut. A systematic review found that certain helminth infections were associated with shifts in the microbiome that could promote cancer. Infections with Trichuris trichiura (whipworm) and Strongyloides stercoralis were linked to the expansion of bacterial species associated with inflammation and oncogenic processes, along with reduced overall microbial diversity. Ascaris lumbricoides (roundworm) infections altered the microbiome at a broad level, increasing certain bacterial groups and decreasing others, alongside metabolic shifts in amino acid and lipid pathways that have been associated with tumor development.22PubMed Central. The Hidden Link Between Intestinal Helminthiasis, Gut Microbiome Alterations, and Colorectal Cancer Risk: A Systematic Review
This is still early-stage science. The same review noted that other studies found opposite patterns, with some helminth infections appearing to increase beneficial microbial diversity. The direction of the effect may depend on the specific worm species, the intensity and duration of infection, and the baseline state of the person’s microbiome. But it opens a fascinating indirect pathway: rather than the parasite directly damaging tissue, it may alter the microbial ecosystem in ways that increase cancer risk elsewhere in the gut.
Can Treating the Infection Prevent the Cancer?
The logic seems straightforward: if the parasite drives cancer, killing the parasite should reduce the risk. For schistosomiasis, mass drug administration programs using praziquantel have been credited with declining rates of bladder cancer in some formerly high-burden regions. For liver flukes, animal experiments support the idea. In a hamster model of liver fluke infection, three rounds of O. viverrini infection followed each time by praziquantel treatment did not lead to bile duct cancer, whereas untreated infections did.23PubMed Central. Decreased risk of cholangiocarcinogenesis following repeated cycles of Opisthorchis viverrini infection-praziquantel treatment: Magnetic Resonance Imaging (MRI) and histopathological study in a hamster model
In humans, though, the picture gets murkier. A hospital-based case-control study found that people who had received repeated praziquantel treatments actually had a higher risk of cholangiocarcinoma. Before you conclude the drug is the problem, the researchers noted a strong association between repeated treatments and habitual consumption of raw freshwater fish, the very behavior that causes reinfection. In practice, people who keep eating raw fish keep getting reinfected, keep needing treatment, and keep accumulating tissue damage during the intervals between treatments. Repeated praziquantel use appeared to be a marker for ongoing risky eating habits rather than a cause of cancer itself.24PubMed Central. Association between praziquantel treatment and cholangiocarcinoma: a hospital-based matched case-control study The real lesson is that curing the current infection matters, but preventing reinfection is just as important.
The Flip Side: Parasites as Potential Cancer Therapy
In a twist that catches most people off guard, some researchers are investigating whether parasites or parasite-derived molecules could be useful in treating cancer. The same immune-modulating abilities that make parasites dangerous in chronic infection might be harnessed therapeutically. A review of preclinical studies found that parasites offer a variety of strategies being explored for cancer treatment, including acting as cancer vaccines, adjuvants that boost the immune response against tumors, and sources of molecules that can inhibit tumor blood vessel growth and trigger cancer cell death.25PubMed Central. Bridging the gap for diverse applications of parasites as advanced cancer therapeutics: current progress and future directions
The rationale is that parasites are exceptionally good at provoking strong immune responses. If that immune activation can be redirected toward tumor cells, it could complement existing cancer immunotherapy. Researchers have catalogued a range of biological activities from parasite-derived molecules: remodeling the tumor microenvironment, triggering programmed cell death in cancer cells, blocking the formation of new blood vessels that feed tumors, and even reprogramming tumor metabolism.26PubMed Central. Parasite in cancer therapy: molecular mechanisms and translational potential All of this remains in preclinical stages, with no parasite-based cancer treatments approved for use in people. But it underscores how the relationship between parasites and cancer is not one-dimensional.
Why Detection Still Lags Behind in Affected Regions
The populations most affected by parasite-associated cancers tend to be in low-resource settings where cancer screening and treatment infrastructure are weakest. Liver fluke-driven cholangiocarcinoma in Southeast Asia and schistosomiasis-driven bladder cancer in sub-Saharan Africa both peak in rural communities where diagnosis often comes late. Even the deworming programs designed to prevent these outcomes face access problems. A study of community health workers implementing a soil-transmitted helminth deworming program in the Philippines documented that reaching geographically isolated communities remains a persistent barrier, with some areas simply too remote for health teams to deliver medicine reliably.27PubMed Central. Lived Experiences of Health Workers in the Implementation of Soil-transmitted Helminthiasis Deworming Program in South Cotabato: A Phenomenological Inquiry
One emerging tool that could help is the detection of circulating microRNAs shed by helminth parasites into the host’s bloodstream. These tiny RNA molecules can be measured in blood, urine, or saliva, potentially allowing early detection of infection from non-invasive samples before the chronic damage accumulates.28PubMed Central. Circulatory microRNAs in helminthiases: Potent as diagnostics biomarker, its potential role and limitations If these biomarkers can be translated into affordable point-of-care tests, they could allow earlier treatment and potentially reduce the years of chronic infection that lead to cancer. That translation, though, depends on the kind of investment that has historically been slow to reach neglected tropical diseases.