Viral infections and colorectal cancer: a systematic review of epidemiological studies.

Numerous studies have found the presence of viral DNA in colorectal tumor tissues. However, whether viral infections contribute to the risk of colorectal cancer (CRC) is still under debate. We aimed to provide an overview of published epidemiological studies on the association between viral infections and CRC. A systematic literature search was performed in PubMed to find relevant studies published until 8 May 2014. Information collected included study population, sample type, laboratory method and prevalence of viral infection in cancer or precancer patients and controls. We found 41 studies that fulfilled the selection criteria, all of which had cross-sectional or case-control designs, and most of which were of small to moderate size. Viral infections included human papillomaviruses (HPV), human polyomaviruses, human herpesviruses, human bocavirus and Inoue-Melnick virus. Inconsistent results were observed across studies. Many studies reported higher viral DNA prevalence in tumor tissues than in normal noncancerous tissues either in the same patients or in CRC-free controls. However, potential contamination or temporal sequence of the infection and cancer development were often unclear. Seroprevalence studies assessing antibody titers indicative of viral infections did not find statistically significant differences between CRC cases and healthy controls. Overall published evidence on the role of viral infections in CRC etiology remains limited. Given the potential importance of viral infections and their implication for prevention, there is a strong need for large, methodologically rigorous epidemiological studies.