Advances in research on plant (essential oil) extracts in aquaculture

      Plant extracts are made up of a range of plant bioactive ingredients and their derivatives that have long been used as fragrances, preservatives and herbs. Essential oil extract is the core component of plant-derived extracts and is a mixture of aromatic and volatile compounds. Plant materials are often distinguished by their aromatic characteristics (Oyen and Dung, 1999). Essential oils are hydrophobic compounds, including chemical constituents such as alcohols, aldehydes, esters, ethers, ketones, phenols, and hydrazines. The six-carbon ring in the molecular structure of essential oils produces different biochemical components during plant growth and animal feeding, resulting in different biochemical functions. The plant extract feed additive has been identified as a “safe, efficient, stable and controllable” feed additive, and has gradually become a new economic growth point for the green and safe animal food production and feed industry.
      The feeding behavior of aquatic animals makes aquatic animals inevitably face the pathogenic microorganisms. When the disease occurs, the invasion of pathogenic microorganisms often leads to the neglect of the true pathogenic factors. In addition to being induced by viruses, bacteria, fungi and parasites, the disease can be induced by a number of non-pathogenic factors such as malnutrition, accumulation of toxic substances, and deterioration of water quality. In this paper, based on the published research data on plant feed additives, the antibacterial and antioxidant activities of plant extracts and their mechanisms are reviewed. The effects of plant extracts on intestinal microbial, nutrient digestion and growth performance of aquatic animals are emphasized.
      1 Antibiotic problem needs to be resolved
      In view of the legislation prohibiting the use of drug-based feed additives in animal production, and the EU's total ban on the use of probiotics since 2006, plant extracts have received widespread attention both inside and outside the industry. In the 1990s, animal production relied heavily on synthetic drugs such as antibiotics, antibacterials, anticoccidial drugs, synthetic growth promoters, and antiparasitic drugs. Humans' resistance to antibiotics and the residual events of some drugs in food have increased the awareness of plant extracts by consumers and government agencies (Dahiya et al., 2006). Studies have shown that the continued excessive and disorderly use of antibiotic products in animal feed will lead to the rapid emergence of drug-resistant strains, resulting in a situation where there is no drug available (Wegener et al., 1998).
      The EU's strict legislation on the safety of aquatic products may be implemented on a larger scale, which will have a profound impact on aquatic products exporting countries. In 2007, representatives of aquaculture in Southeast Asia signed a draft legislation on the safety of aquaculture, which recommended that the use of human antibiotics in farmed animals be strictly prohibited, while the use of veterinary drugs as a growth promoter was prohibited.
      The main active ingredient of plant extracts is plant essential oil, which acts as a feed additive to improve the immune function of animals and has been widely used in animal production (Bauerman, 2006). The individual components of essential oils vary, for example, the two main active ingredients of valerian essential oils, thymol and carvacrol, can range from 3% to 60% of essential oils (Lawrence and Reynolds, 1984). The essential oil extracted by steaming of Origanum vulgare ssp. hirtum comprises more than 30 components, most of which are phenolic compounds with different activities (Economou et al., 1991; Sivropoulou et al., 1996; Adam et al., 1998). Multiple components have been shown to have efficacy in maintaining gut health and balancing microbial flora (Besra et al., 2002; Rao and Nigam, 1970).