The human polyomaviruses KI and WU: virological background and clinical implications.

In 2007, two novel polyomaviruses KI and WU were uncovered in the respiratory secretions of children with acute respiratory symptoms. Seroepidemiological studies showed that infection by these viruses is widespread in the human population. Following these findings, different biological specimens and body compartments have been screened by real-time PCR in the attempt to establish a pathogenetic role for KI polyomavirus (KIPyV) and WU polyomavirus (WUPyV) in human diseases. Although both viruses have been found mainly in respiratory tract samples of immunocompromised patients, a clear causative link with the respiratory disease has not been established. Indeed, the lack of specific clinical or radiological findings, the frequent co-detection with other respiratory pathogens, the detection in subjects without signs or symptoms of respiratory disease, and the variability of the viral loads measured did not allow drawing a definitive conclusion. Prospective studies carried out on a large sample size including both immunocompromised and immunocompetent patients with and without respiratory symptoms are needed. Standardized quantitative real-time PCR methods, definition of a clear clinical cutoff value, timing in the collection of respiratory samples, are also crucial to understand the pathogenic role, if any, of KIPyV and WUPyV in human pathology.