Yeasts have also furnished nucleotides which have shown promise with control

Yeasts have also furnished nucleotides which have

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Yeasts have also furnished nucleotides, which have shown promise with control- ling infections caused by Str. iniae. Using a commercial product, Ascogen, which comprises oligonucleotides from brewer's yeast, feeding trials were carried out for 7-8 weeks with hybrid striped bass followed by bath challenge with Str. iniae, with the result that experimental groups showed a higher level of protection when compared with the controls (Li et al., 2004). Aloe has been found to increase resistance to V. alginolyticus infections in rock- fish when fed at 5 g aloe/kg of diet for 6 weeks (Kim et al., 1999). Yeast RNA incorporated in diets at 0.4% (w/v) and fed for 60 days reduced mortalities caused by Aer. hydrophila in rohu (Labeo rohita) and enhanced the phagocyte respiratory burst activity (Choudhury et al., 2005). There is a growing interest in the use of whole plant extracts for disease control. As an example, rosemary (Rosmarinus officinalis) was used as dried and powdered leaves and as ethyl acetate extracts in feeds in a ratio of 1:17 and 1:24, respectively, to tilapia for 5 days followed by infection with Str. iniae and feeding with the rosemary
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344 Bacterial Fish Pathogens and its extract for a further 10 days with a resulting marked reduction in mortahties (Abutbul et ai, 2004). The Indian medicinal herb, Azardirachta indica, demonstrated marked in vitro inhibitory activity against Aer. hydrophila. Furthermore, aqueous and ethanol extracts of equi-mixtures o^ Azardirachta indica, Curcuma longa and Ocimum sanctum had demonstrable in vitro inhibitory activity (Harikrishnan and Balasun- daram, 2005). Synergism between low levels of iron and high amounts of long-chain poly- unsaturated fatty acids led to a RPS of 70 and 96% after challenge with Aer. salmonicida and V. salmonicida, respectively (Rorvik et aL, 2003). VACCINES The rationale for the development of fish vaccines parallels that of other aspects of veterinary and human medicine, i.e. a Utopian desire to rid fish stocks of disease coupled with a healthy regard for profit. In practical terms, the aquaculturist needs to control specific diseases which may be financially crippling in terms of high mor- talities. From the opposite viewpoint, the vaccine manufacturer needs substantial (perhaps even multinational) markets in order to ensure profitability of the products. A complicating factor concerns cost of the vaccines to the user. Generally, fish farmers who produce fish for human consumption demand inexpensive, easy-to- use, reliable products; whereas the vaccine supplier needs to charge high fees, which are sufficient to recoup developing and licensing costs, pay current expenses and invest for the future. This difference in opinion between user and supplier may lead to difficulty. Moreover, with a comparatively small aquaculture industry, private vaccine manufacturers are likely to invest resources only in developing vaccines against diseases which are prevalent in many countries, rather than those restricted to small geographical areas or representing novel and emerging conditions. This attitude undermines the whole basis of prophylaxis. No easy solution is envisaged
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  • Bacteria, representative, gram-negative bacteria

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