What is the connection between parasitic infections and cancer?

Parasitic Infections and Cancer: Evidence-Based Connections

Certain parasitic infections are definitively carcinogenic to humans, with the strongest evidence linking liver flukes (Opisthorchis viverrini and Clonorchis sinensis) to cholangiocarcinoma and blood flukes (Schistosoma haematobium) to bladder cancer. 1, 2

Parasites Classified as Carcinogenic to Humans

• Three helminth parasites are classified as Group 1 carcinogens by the World Health Organization's International Agency for Research on Cancer (IARC): Clonorchis sinensis (liver fluke), Opisthorchis viverrini (liver fluke), and Schistosoma haematobium (blood fluke) 1
• These parasitic infections are often asymptomatic initially and may go undiagnosed until cancer complications develop 1
• A long time interval between infection and cancer development is typically required 2

Specific Parasite-Cancer Associations with Strong Evidence

• Opisthorchis viverrini and Clonorchis sinensis infections are strongly associated with cholangiocarcinoma (bile duct cancer) 2
• Schistosoma haematobium has a well-established link to bladder cancer 2
• Salmonella Typhi (though bacterial, not parasitic) shows high-level evidence for association with gallbladder and hepatobiliary carcinoma 2

Mechanisms of Parasite-Induced Carcinogenesis

• Chronic inflammation is a primary mechanism by which parasitic infections contribute to cancer development 3
• Modulation of the host immune system by parasites can facilitate malignant transformation 3
• Other mechanisms include:
  • Inhibition of intracellular communication 3
  • Disruption of proliferation-antiproliferation pathways 3
  • Induction of genomic instability 3
  • Stimulation of malignant stem cell progeny 3

Additional Parasites with Potential Cancer Associations

• Strongyloides stercoralis may be a relevant co-factor in the development of lymphomas 4
• Leishmaniasis has been associated with hepatocarcinoma, skin cancer, and lymphomas 4
• Malaria appears to be causative in the carcinogenesis of some cancers, particularly Burkitt lymphoma 4
• Trypanosomiasis has a bi-modal role in cancer development, with both pro- and anti-cancer effects reported 4

Parasites with Potential Anti-Cancer Properties

• Some parasites have demonstrated anticancer activity in experimental models, including:
  • Trypanosoma cruzi
  • Toxoplasma gondii
  • Toxocara canis
  • Acantamoeba castellani
  • Plasmodium yoelii 5
• Shared antigens between parasites and cancers may explain some of these effects, as cancer-associated mucin-type O-glycan compositions are also produced by certain parasites 5

Clinical Significance and Prevention

• Approximately 16% of all cancer cases worldwide are attributable to pathogenic agents, including schistosomes and liver flukes 3
• This represents approximately 1,375,000 preventable cancer deaths per year 3
• Early diagnosis and treatment of parasitic infections is crucial for preventing associated malignancies 1, 3

Cautions and Considerations

• The association between parasitic infections and cancer development requires long latency periods, making causality difficult to establish definitively 2
• Multiple exogenous and endogenous factors interact during these long latent periods, further complicating the establishment of direct causality 3
• While some parasites are definitively carcinogenic, others may only facilitate or contribute to malignant transformation rather than directly causing it 3