# Anthelmintic Efficacy of Rosemary Oil and Its Nanoemulsion Against the Monogenean Parasite, Zeuxapta seriolae, in Yellowtail Kingfish (Seriola lalandi)

## TL;DR

This study shows that rosemary oil, especially in a nanoemulsion form, reduces parasitic infections in yellowtail kingfish without harming their health.

## Contribution

The study introduces a nanoemulsion delivery method for rosemary oil that enhances its anthelmintic efficacy in fish.

## Key findings

- Nanoemulsified rosemary oil increased cineole uptake and reduced parasite abundance more effectively than regular rosemary oil.
- All rosemary oil treatments significantly reduced juvenile parasite recruitment compared to the control.
- Fish health metrics like growth, feed intake, and plasma biochemistry were unaffected by the treatments.

## Abstract

This study evaluated the anthelmintic efficacy of rosemary oil delivered as a nanoemulsion compared with regular rosemary oil against the monogenean parasite Zeuxapta seriolae in yellowtail kingfish (YTK; Seriola lalandi). A 20‐day feeding trial tested three dietary treatments: two levels of regular rosemary oil (providing 0.85 and 1.7 g cineole·kg−1 feed) and a rosemary‐oil nanoemulsion (providing 0.85 g cineole·kg−1 feed), against a control diet without rosemary oil. Juvenile fish (293 ± 5 g) were pre‐exposed to Z. seriolae oncomiracidia before the trial, resulting in an initial mean parasite abundance of 97 ± 4 per fish. At trial completion, plasma cineole concentrations were highest in fish receiving the high‐dose regular rosemary oil diet, followed by the nanoemulsion diet, and then the low‐dose regular rosemary oil. These plasma levels closely matched treatment efficacy: fish fed the high‐dose diet exhibited the lowest mean abundance of Z. seriolae, followed by those receiving the nanoemulsion and low‐dose diets. A similar pattern was observed for juvenile parasite recruitment. Across parasite developmental stages, all rosemary‐oil treatments significantly reduced the proportion of juvenile Z. seriolae relative to the control. Growth performance and feed intake did not differ among treatments. Plasma biochemistry remained unchanged, and no histopathological alterations were detected in liver or kidney tissues. Overall, these findings demonstrate that dietary rosemary oil effectively transfers cineole into the blood of YTK and reduces Z. seriolae infection and that application of the rosemary oil in a nanoemulsion further increases cineole uptake.