Melissa officinalis L. extract and its main phenolic compound rosmarinic acid as phytoprophylactic feed additives against koi herpesvirus infection in a pilot study

Koi herpesvirus (KHV) causes an important emerging disease that causes considerable economic losses in common carp (Cyprinus carpio) and koi aquaculture, so the development of effective treatment and prophylactic measures is important. Melissa officinalis L. is one of the best known medicinal plants with documented antiviral effects in vitro and in vivo. The effects on supplementing the feed of koi carp with lemon balm dry extract (1.62 %) and purified rosmarinic acid (0.16 %), a major compound in lemon balm, were investigated by feeding to koi after challenge with KHV. Viral infection of fish was confirmed by PCR for the DNA of KHV in organ pools of dead koi and cumulative survival was assessed. LC/MS revealed a plasma level of 27.5 ng/ml of rosmarinic acid in koi fed the rosmarinic acid diet. Koi fed with lemon balm extract showed the lowest cumulative mortality, in particular at an early stage of infection, but the differences were not statistically significant. Further studies with a greater number of fish would be required to assess the potential benefit of supplying lemon balm in the diet against emerging viral diseases that threaten fish populations.