[Asymptomatic infections in man: a Trojan horse for the introduction and spread of mosquito-borne arboviruses in non-endemic areas?].

In mosquito-borne arbovirus infections in man the asymptomatic cases are much more frequent than the symptomatic ones, but their true role in the introduction and subsequent spread of such diseases in non-endemic areas remains to be clarified. We have collected pertinent data from English and French literature from 1952 to 2010 through Pubmed and other bibliographic sources. Data were analysed to assess if viremia in asymptomatic human arbovirus infections might be sufficient to represent a true risk for introduction in non-endemic areas. During dengue and chikungunya fever outbreaks, humans are believed to be the only vertebrate hosts. Since a very large number of individuals are infected and since viremic levels are known to vary by many orders of magnitude in symptomatic patients, it is reasonable to augur that a proportion of asymptomatic cases might reach levels of viremia sufficient to infect competent mosquitoes. Moreover, in both dengue and chikungunya fever, nosocomial infections have been identified representing an alternative opportunity for virus introduction in non-endemic areas. In zoonotic mosquito-borne arbovirus infections such as Japanese encephalitis or West Nile infection, the situation is quite different since humans are considered as "dead-end" hosts. However, the very large number of asymptomatic cases arising during outbreaks and the existence of newly recognised ways of contamination (blood transfusion, organ transplantation, transplacental way etc.) may also ensure their introduction and subsequent spread in new areas.