Influence of selected Indian immunostimulant herbs against white spot syndrome virus (WSSV) infection in black tiger shrimp, Penaeus monodon with reference to haematological, biochemical and immunological changes
Introduction
White spot syndrome virus (WSSV) is an economically significant shrimp disease, which causes high mortalities and severe damages to shrimp cultures. The principle clinical sign of the affected shrimp will be white spots in the exoskeleton and epidermis [1]. This virus can cause 100% mortalities within 3–10 days of the onset of the above symptoms .The WSSV has a wide host range and it has been observed not only in shrimps but also in crabs and other arthropods such as copepods, insects and pest prawns [2].
Strategies for prophylaxis and control WSSV theoretically include improvement of environmental conditions, stocking of specific pathogen free shrimp post-larvae and enhancement of disease resistance by using immunostimulants. Immunostimulants are the substances, which enhance the non-specific defence mechanism and provide resistance against pathogenic organism. Perusal of the literatures indicated that immunostimulants are proven very successfully in treating/preventing microbial diseases in cultured shellfishes [3]. Protective efficacies of various immunostimulants have been reported against WSSV infection, for example oral administration of immunostimulants like Peptidoglucan, Lipopolysaccharides, etc. [4]. Therefore, the prevention and control of shrimp diseases appear as an integrated approach in which knowledge about immunity has to be improved, considering other research areas related not only to pathology, shrimp physiology and genetics, but also to nutrition, environment and ecotoxicology.
Thousands of research works are known to exert both specific and non-specific effects on the immune system of many animals, and potent non-specific effects in crustaceans [5]. Almost all of the published observations with LPS have been geared towards showing an anti-bacterial effect. Many plant derived compounds have been found to have non-specific immune stimulating properties in animals, of which more than a dozen have been evaluated in fish and shrimp [6], [7]. However, only a few have been tested to determine the impact against viruses affecting shrimp, specifically WSSV.
Many substances from different sources (bacterial components, chemical agents, animal or plant extracts, etc.) have been studied as prospective immunostimulants for fish (reviewed by Sakai [8]), and some have been reported to confer a degree of protection against several diseases commonly found in farmed fishes, caused by Vibrio anguillarum, Aeromonas salmonicida or Yersinia ruckeri, etc. [9].
Natural adjuvants, synthetic agents, antibody reagents are used as immunosuppressive and immunostimulative agents. But there are major limitation to the general use of these agents such as increased risk of infection and generalised effect throughout the immune system [10]. Even though glucan and other chemical immunostimulants have positive effects on fishes and prawns [11], [12], [13], but some demerits have been found by the application of these natural immunostimulants like heat intolerable and indigestibility effects while using the products. Hence, it is advisable to select alternative immunostimulant products from herbal plants.
Many herbals have been used for millennia as home remedies and some of these have potent anti-viral properties. Among them, a few have been found to have anti-viral activity against fish viruses in tissue culture [14] and some have been investigated for their ability against shrimp viruses [15].
Plants are the storehouses and rich sources of safer and cheaper chemical compounds. These natural plant products have been reported to have various activities like antistress, growth promoters, appetiser, tonic, immunostimulants and antimicrobials [16]. Moreover, the substances are obtained from natural sources, besides possessing other interesting properties like non-toxic, biodegradable and biocompatible [17].
Further studies are essential to screen various identified herbal plants as immunostimulant. In the present study, it is proposed to screen the immunostimulant property of few herbal plants against WSSV in shrimp Penaeus monodon.
Section snippets
Screening and extraction of the herbal immunostimulant active principles
Five immunostimulant characteristics herbs such as Cyanodon dactylon, Aegle marmelos, Tinospora cordifolia, Picrorhiza kurooa and Eclipta alba were selected based on the previous screening. The collection of the above plants was based on the availability from various geographical locations of south India.The plant parts were cleaned well, shadow dried, ground to fine texture and sieved. The individual sieved powder was then soaked in methanol (1:1 ratio) for 48 h. The slurry was then filtered,
Challenge test – survival (%)
The survival of the control (E) and different concentrations (A–D) of the herbal immunostimulant incorporated diets fed shrimps is given in Fig. 1. Within 20 days of challenging experiment with WSSV, the control group of shrimp fed on diet devoid of immunostimulant succumbed to death (100%) within 7 days. The survival of shrimps was increased significantly (P < 0.05) when they fed on increasing concentrations of immunostimulants. The maximum survival (74%) was recorded in group D. One way ANOVA
Discussion
Immunological approaches to prevent fish and shrimp diseases have normally involved antibiotics, chemicals or vaccination against specific pathogens, while the use of immunostimulants is relatively a new and developing area [40]. In general decapod crustaceans have three major categories of blood cells (haemocytes) such as hyaline cells, semi-granular cells and granular cells. Each has distinctive morphological features and physiological functions [28]. Haemocytes are responsible for clotting,
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