Current knowledge and systematic review of viruses associated with Crohn's disease.

The aetiology of Crohn's disease (CD) is currently unknown. A viral trigger was proposed more than 40 years ago and has been the focus of many investigations. We summarised the current literature surrounding the association between viruses and CD and conducted a systematic review of all studies investigating this association quantitatively. Studies were identified by searching for 13 specific virus names or the general term 'virus' and 'Crohn's disease' in search engines PubMed and OVID. A total of 1315 studies were identified, of which 78 studies had a laboratory result. Of the 78, 46 case-control studies met all the inclusion criteria for forest plot analysis. The most common viruses studied were EBV, CMV and measles virus (MV). Forest plot analysis for each virus was carried out (fitted using random effects) and identified evidence of an association between CD and CMV (risk ratio [RR] 1.602, 95% confidence interval [CI] 1.069 to 2.400) with some suggestion that EBV may also be associated with CD (RR 1.366, 95% CI 0.996 to 1.873). However, there was evidence of large heterogeneity in the results from the identified studies for EBV. There was little evidence of an association with CD for MV, human herpes virus 6, human herpes virus 8, human simplex virus, varicella-zoster virus, mumps virus, Rubella virus, rotavirus, norovirus and adenovirus. There is still some question around whether CD is associated with the presence of a currently known virus.