Merkel cell polyomavirus (MCV or MCPyV) was first described in January 2008 in Pittsburgh, Pennsylvania. It was the first example of a human viral pathogen discovered using unbiased metagenomic next-generation sequencing with a technique called digital transcriptome subtraction. MCV is one of seven currently known human oncoviruses. It is suspected to cause the majority of cases of Merkel cell carcinoma, a rare but aggressive form of skin cancer. Approximately 80% of Merkel cell carcinoma (MCC) tumors have been found to be infected with MCV. MCV appears to be a common—if not universal—infection of older children and adults. It is found in respiratory secretions, suggesting that it might be transmitted via a respiratory route. However, it has also been found elsewhere, such as in shedded healthy skin and gastrointestinal tract tissues, thus its precise mode of transmission remains unknown. In addition, recent studies suggest that this virus may latently infect the human sera and peripheral blood mononuclear cells.
Most MCV viruses found in MCC tumors, however, have at least two mutations that render the virus nontransmissible: 1) The virus is integrated into the host genome and 2) The viral T antigen has truncation mutations that leave the T antigen unable to initiate DNA replication needed to propagate the virus.