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During the last decade many improvements have taken place in aquaculture, especially in prawn and shrimp farming. The shift from extensive to intensive and semi-intensive farming has brought about an increase in disease outbreaks, especially by bacteria. To control these diseases, antibiotics have been used indiscriminately. To avoid the use of antibiotics and the development of resistant strains of bacteria, we studied the efficacy of benzalkonium chloride (BKC), a quaternary ammonium compound as an antibacterial and immunostimulant in Macrobrachium rosenbergii (de Man). Efficacy was evaluated in vitro by minimum lethal concentration (MLC) and found to be 1.0 ppm for P. fluorescens and 1.5 ppm for E. tarda, V. alginolyticus, S. aureus and A. salmonicida. The 24 hr LD50 of BKC for two month old M. rosenbergii was 10.0 ppm. Its immunostimulant effect was evaluated by challenging BKC treated M rosenbergii (1.5 ppm BKC bath for 15 days) with P. fluorescens and V. alginolyticus. The NBT (nitroblue tetrazolium) assay showed that BKC treatment stimulated nonspecific immune response by the activation of granular cells. It also protected M. rosenbergii from disease when compared to untreated controls. A single treatment of BKC was effective for P. fluorescens for 49 hours and 46 hours for V. alginolyticus. Bath treatment at 2.0 ppm for one hour showed successful control of bacterial infection in M. rosenbergii previously infected with the bacteria. The results showed that BKC could be used as an effective antibacterial and immunostimulant in M. rosenbergii.

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