WORLD AQUACULTURE 15 luminous bacteria from black tiger shrimp (Penaeus monodon) more effectively than oxytetracycline (Direkbusarakom 2004). Ethanol, methanol and hexane extracts of basil Ocimum basilicum were investigated for their in vitro antimicrobial properties against 146 microbial organisms, including aquaculture pathogens. The hexane extract had a stronger and broader spectrum of antibacterial activity than other extracts (Adigüzel et al. 2005). Indian almond Terminalia catappa extract is an alternative antibacterial remedy against tilapia ectoparasites and the bacterial pathogen A. hydrophila. Growth of two strains of A. hydrophila was inhibited at 0.5 mg/mL (Chitmanat et al. 2005). The antibacterial active components of the herbal extracts may lyse cell walls, block protein and DNA synthesis, inhibit enzyme secretions and interfere with the signaling mechanism of the quorum sensing pathway. Ethanol extracts of roots had excellent inhibitory effects on the growth of Gram positive and negative microorganisms. In practice, most individual herb or spice extracts must be included at a high concentration to observe effects comparable to those of antibiotics. Many extracts contain a multitude of active substances. Oregano (Origanum vulgare) contains more than 30 antibacterial chemicals. Black tiger shrimp post-larvae (PL 1–25) reared in bacteria-inoculated water had an elevated Vibrio load (3.76 x 105 CFU/g) in shrimp tissues (Citarasu 2000, Citarasu et al. 2003a, b). Enriching Artemia nauplii with methanol extracts of Solanum trilobatum, Andrographis paniculata and Psorolea corylifolia reduced Vibrio load in black tiger shrimp post-larvae. Similar results were obtained from inoculation challenge with other pathogens, including Pseudomonas aeruginosa, Staphylococcus aureus and Salmonella typhi. Immanuel et al. (2004) studied the bacterial load of Indian white shrimp (P. indicus) juveniles fed with seaweed and herbal extracts and reared in water inoculated with Vibrio parahaemolyticus. Antiviral Activity Several plant products have potent antiviral activity against fish and shrimp viruses. For example, black tiger shrimp fed ethanol extract of Clinacanthus nutans leaves and exposed to yellowhead virus (YHV) had 95 percent survival rates compared to 25 percent survival in the no-extract (control) group (Direkbusarakom et al. 1996). Methanol extracts of five different herbal medicinal plants, such as Cynodon dactylon, Aegle marmelos, Tinsospora cordifolia, Picrorhiza kurroa and Eclipta alba, were incorporated into a diet for WSSV-infected shrimp. Better values of haematological, biochemical and immunological parameters were found in shrimp fed a diet that incorporated immunostimulants (Citarasu et al. 2006). Antifungal Activity Herbal plant extracts affect lysis of fungal cell walls, alter membrane permeability, affect metabolism and protein synthesis, ultimately leading to death. Many plants have antifungal properties. Infection by molds Aspergillus flavus and Fusarium oxysporum can be controlled with basil (O. basilicum) extract (Adigüzel et al. 2005). A DMSO extract of Tamarix dioica has significant activity against A. flavus and moderate activity against F. solani (Khan et al. (2004). Rhazya stricta is a small glabrous shrub that has antifungal effects against Trichophyton longifusis, A. flavus, Candida albicans and F. solani. Antistress Activity Herbal formulations claimed to enhance physical endurance, mental functions and non-specific resistance of the body have been termed adaptogenic. Safer and cheaper herbal medicines are useful as antistress agents because animals can withstand stress without altering physiological functions. Various herbs, Withinia somnifera, Emblica officanalis, Asparagus racemosus, Ocimum sanctum, Tribulus terresttris and Piper longum, are claimed to have adaptogenic, anabolic effects and the ability to improve vital energy. The herbal antioxidant effect is similar to that of superoxide dismutase, metal-ion chelators and xanthine oxidase inhibitors. The best example is the herb Picrorhiza kurroa, used as an antistress compound for shrimp (Citarasu et al. 1998). Rutin is a bioflavonoid extracted from Toona sinensis with strong antioxidant and antistress activity in crustaceans. Rutin has improved the biochemical, immunological and haematological response to stress in Litopenaeus vannamei caused by Vibrio alginolyticus (Hsieh et al. 2008). Immunostimulant Activity An immunostimulant is a chemical, drug, stressor or action that enhances defense mechanisms or immune response. Immunostimulants as dietary supplements can improve the innate defense of animals and provide resistance to pathogen infection during periods of high stress, such as grading, reproduction, seed transfer and vaccination. Herbal products have been used as immunostimulants for thousands of years. These herbs contain many types of active components, including polysaccharides, alkaloids, and flavonoids. The immunostimulating activities of herbal components have been widely studied in mice, chickens or human cell lines. There is a growing interest in using medicinal herbs as immunostimulants in aquaculture, including extracts of glycyrrhizin, Emblica officinalis, Cynodon dactylon, Adathoda vasica, O. sanctum, W. somnifera and Myristica fragrans, Aegle marmelos, T. cordifolia, P. kurooa and E. alba against pathogenic bacteria and viruses such as Vibrio harveyi and A. hydrophila infected fish, and WSSV-infected shrimp. Aphrodisiac Activity Medicinal plants are also known to have hormonal boosting affects and some herbs have been used as natural ‘viagra’ and in hormone replacement therapy for menopausal women. Black tiger shrimp fed a maturation diet containing W. somnifera, Mucuna pruita, Ferula asafoetida and Piper longum extracts had increased fecundity and gonadal weight and reduced intermolt periods compared to controls (Babu 1999). A commercial, semi-moist maturation diet with herbal extracts (NutraFeed, Nutrakol) fed to P. vannamei broodstock (CONTINUED ON PAGE 14)
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