Use of Rosmarinus officinalis as a treatment against Streptococcus iniae in tilapia (Oreochromis sp.) S. Abutbul, A. Golan-Goldhirsh, O. Barazani, D. Zilberg * Albert Katz Department of Dryland Biotechnologies, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990, Israel Received 19 January 2004; received in revised form 4 May 2004; accepted 11 May 2004 Abstract The use of Rosmarinus officinalis as a treatment against Streptococcus iniae in Oreochromis sp. (tilapia) is described. Sixteen accessions of R. officinalis were extracted in several solvents and tested in vitro against S. iniae by disk diffusion assay. All extracts were effective in inhibiting bacterial growth. The most effective was accession no. 1 when extracted in ethyl acetate, which inhibited bacterial growth by 37.5 mm/mg extract. R. officinalis was found to have a bacteriostatic effect in a broth dilution assay, as the inhibited bacterial growth was recovered after 48 h. For oral administration, R. officinalis was mixed in the feed either as whole dry leaves or as dried ethyl acetate extract in a ratio of 1:17 or 1:24 (w/w), respectively. There was a significant ( p < 0.012 and p < 0.003, respectively) reduction in mortality rate in fish fed on both diets following infection with S. iniae. D 2004 Elsevier B.V. All rights reserved. Keywords: Fish; Disease; Bacteria; Rosemary 1. Introduction The disease caused by Streptococcus iniae accounts for significant economical losses in the aquaculture industry worldwide (Shoemaker and Klesius, 1997), leading to mortality, reduced growth, and unmarketable appearance. The bacterium affects a wide range of cultured fish species, including: hybrid striped bass (Morone chrysops  Morone saxatilis; Shoemaker and Klesius, 1997), rabbitfish (Siganus canaliculatus; Yuasa et al., 1999), 0044-8486/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2004.05.016 * Corresponding author. Tel.: +972-8-659-6818; fax: +972-8-659-6742. E-mail address: dzilberg@bgumail.bgu.ac.il (D. Zilberg). www.elsevier.com/locate/aqua-online Aquaculture 238 (2004) 97 – 105