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Dr. Claudia Bechteler: Versuche zur Immunisierung von Garnelen (Penaeus monodon) gegen Vibrioneninfektionen


6 Summary

The hygienic problems in prawn production in Thailand have aggravated during the past few years due to intensified aquaculture systems. Especially the opportunistic disease Vibriosis has caused heavy economic losses. The prophylactically and therapeutically used antibiotics and sulfonamides work only insufficiently and contain severe health and ecological risks. Thus the search for a vaccine against Vibriosis of shrimp has intensified.

The submitted paper was suggested and financially supported by B. B. Holding Co., Ltd., Bangkok/Thailand. The goal was the development of a vaccine against Vibriosis of giant tiger prawns (P. monodon). A literature survey on the present knowledge of the mechanisms of immune response in crustacea suggests that there is a primitive active immunity in prawns. This makes the application of a vaccine against Vibriosis promising. Although first lab trials have been encouraging, many questions are still unsolved, particularly there is no evidence of the practicality of such vaccines.

According to the literature and experiences of the IBT, a successful fish vaccine should consist of homologous, site-specific pathogenes, be polyvalent, and demonstrate a high degree of purity. In order to achieve this twenty vibrio species from the south of Thailand, where the vaccine would eventually be used, were, according to the usual methods, identified and characterizied to species level. An IBT-developed continuous fermentation process („Göttinger Bioreaktor") with additional cascade ultrafiltration through hollow-fibre-filters was used to produce the actual vaccine. This resulted in three formaline-inactivated and highly concentrated vaccine charges from three especially virulent species of vibrio: V. parahaemolyticus, V. vulnificus, and V. alginolyticus. They consisted only of the bacteria cells and/or the corresponding toxoids and differed in formaline content and ultrasonic treatment.

The bio-availability and effectiveness of the three charges of the vaccine, measured as survival rate, development of body weight and health status of the immunized prawns as compared to non-immunized (control) animals, were tested under practical conditions in several regions in the south of Thailand. The vaccine-charges were applied to the prawns (PL 5 and 12 to 14) in various concentrations, incubation times and vaccination-procedures via the water using short-time bathing. The most important results of the lab and field trials can be summarized as follows:

  • The treatment with formaline increased the mortality of the prawns significantly. There was no difference between treatment with inactivated bacteria and toxoids. As a consequence of these results, for the second and third vaccine-charges, the cell- and toxoid fraction were mixed at a ratio 1:1 and the content of formaline was reduced to a minimum.
  • Increased vaccine concentration increased survial rate to a maximum, beyond this survival rate decreased. Longer incubation time had a positive effect on the survival rate. It is suggested that vaccine concentration and incubation time can replace each other to some extent.
  • Over the short term, lower vaccine concentrations led to higher survival rates, whereas for longer periods higher vaccine concentrations were superior. The bio-availability of the vaccine and its influence on the short-term survival rate should therefore not be overestimated.
  • A protection against Vibriosis was evident already after one day and continued over a period of three months. This indicates that a vaccine-protection is initially mediated by chemostimulation and later by an active immunity and is maintained through natural boosterings during the entire production period. This confirms the hypothesis that prawns possess a passive as well as an active immunity.
  • An immune protection could be induced in a very early PL-stage (PL 5). At this age, the prawns are still in the hatchery, so that they could be protected against Vibriosis infection while still there, which is especially important due to technical and organisational limitations for vaccination on farms.
  • The contact with the vaccine in the antigen-bath, especially at higher concentrations, and the handling connected with the vaccination, were found to stress the prawns. The splitting of the vaccine dose could not compensate for the negative effects of higher vaccine concentrations due to the intensified handling.
  • The bio-availability and effectiveness of the vaccine depended on the quality and age of the prawns. They were also influenced by transporting, packing, water-exchanging and the specific conditions of husbandry and hygiene the prawns were exposed to before and after vaccination. Thus among other things, the survival rate of the test animals was higher in the hatchery than on the farm. The vaccination of prawns therefore should be transfered from the farm to the hatchery.

These findings lead to new possibilities of improving the vaccination process. E.g., the inactivation of the vaccine components with formaline could probably be replaced by heatstabilisation. Instead of short-time bathing, oral immunization („bioencapsulation") could be tried. The optimum combination of vaccine concentration and incubation time remains to be evaluated. It needs also to be clarified to what extent a vaccination of PL 5 remains effective.

The field trial did prove that the tested vaccine provides an immune protection under the very heterogenous conditions of farms. After a proven outbreak of Vibriosis, the survival rate and development of body weight were clearly increased with a noteworthy and statistically significant improvement in the final harvest. It should be noted however, that it was not possible to work with the ceteris-paribus condition in the field trial and that the relationship between the actual effectivity of the vaccine and the development of the prawn immunue system remains unknown. Thus, it is still unclear whether these results in further field trials can again be so clearly demonstrated.

Despite these limitations, it seems likely that this vaccine against Vibriosis can improve or even guarantee the economic viability of prawn production in the south of Thailand. With the help of the vaccine, the potentially harmful, both ecological and human, application of chemical agents can also be reduced.

 

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