Human protozoal infections and their potential for causing neoplasms

Protists causing human infections generally produce local or systemic disease through direct cellular injury, and inflammatory response. Some protists have been found associated with human neoplasms, and their carcinogenic potential has received much attention in recent years. Here we outline the epidemiologic and experimental evidence linking Cryptosporidium sp., malaria and Trichomonas vaginalis to neoplastic changes in humans. Experimental studies in mammalian cells have unraveled the disruptive alterations in many of the normal signaling pathways that are critcal in innate and adaptive immunity. The immune deficient states, induced by protists or other concurrent infections, most likely increase susceptibility to infection and contribute to tumorigenesis. The neoplasms that occur in malaria and trichomoniasis often contain Epstein-Barr virus (EBV) and human papilloma virus (HPV), respectively. The relative contribution of protists and different viruses, including HIV, requires delineation. The role of local microbiota in determining susceptibility to Cryptosporidium sp. or T. vaginalis infection and in carcinogenesis also requires additional investigation. The elucidation of precise mechanisms of tumorigenesis in mono- and polymicrobial infections is expected to identify targets for intervention and treatment.