Does Aedes albopictus (Diptera: Culicidae) play any role in Usutu virus transmission in Northern Italy? Experimental oral infection and field evidences

- Our experiment showed that Aedes albopictus has a low vector competence for USUV as no evidence was found of PCR positivity in the saliva.
- Significant rate of Aedes albopictus field collected pools were PCR positive to USUV in the period 2009-2012, while in the period 2013-2016 all Ae. albopictus pools were found negative.
- USUV circulation was observed to be continuous in Culex pipiens in the period 2009-2016.
- The controversial results are puzzling and requires that attention is paid to the trend of development of this new relationship as a possible source of public health risk.

This study evaluated the vector competence of Aedes albopictus in transmitting USUV after oral infection under laboratory conditions. Ae. albopictus showed a low vector competence for USUV, although the positive body sample found with a very high number of viral copies at one week post infection indicates that a replication in the mosquito body can occur, and that USUV can escape the midgut barrier. Field data from an extensive entomological arboviruses surveillance program showed a relevant incidence of Ae. albopictus USUV positive pools in the period 2009-2012 while all pools were negative from 2013 on. No conceivable explanation regarding this field evidence was addressed, suggesting that attention must be paid to the trend of development of this vector-pathogen association, being aware of the potential rapid arbovirus' adaptation to new vectors, to prevent possible new disease's emergence.

USUV was detected in Ae. albopictus wild collected pools at consistent infection rate for four consecutive years (2009-2012) while disappear from 2013 on. In contrast the USUV infection rate resulted very similar in Cx. pipiens collected in the same area during the whole surveillance period (2009-2016). In lab vector competence study the transmission rate of USUV in Ae. albopictus resulted very low.244