In vitro virucidal effect of nebulised silver on equine herpesvirus 1 and murine norovirus

Viruses can be involved in subclinical respiratory disorders affecting performance in horses, for which limited therapeutic options are available. Silver nanoparticles (AgNP) have shown some bactericidal properties after nebulisation. The aim of the present study was to assess the in vitro virucidal activity of AgNP after instillation or nebulisation on equine herpesvirus 1 (EHV1) and murine norovirus (MNV), the latter used as surrogate of equine rhinitis viruses. Firstly, either EHV1 or MNV were instilled with AgNP of increasing concentrations (100, 500, 1000 and 2,000 mg/kg). Secondly, the viruses were exposed to nebulised AgNP in a nebulisation chamber. Treated viruses were distributed onto cell monolayers using ten-fold serial dilutions, which were incubated for 72 hours before titration by the Reed and Muench method. Treatments were performed in triplicate and matched pairs t-tests were applied to assess the effect of treatment. The reduction of the EHV1 titer was at least 99.99% after instillation and 99.91% after nebulisation of all AgNP concentrations (higher titer reductions could not be assessed owing to assay detection limits). Only the instillation of AgNP at 2000 mg/kg significantly reduced the MNV titer (by 98.22%). Nebulised AgNP did not reduce the MNV titer. Nebulisation of AgNP could represent a new therapeutic approach against some viral respiratory disorders in horses, while AgNP concentration seems to be relevant for treatment. Further investigations are needed to assess the virucidal activity on other equine respiratory viruses, the toxicity of AgNP on the equine respiratory mucosa and the effective concentration for in vivo use.