INTRODUCTION

Aquaculture has experienced substantial global growth over the past several decades and, in 2022, accounted for just over half of total aquatic animal production worldwide, marking the first time farmed production exceeded that of capture fisheries [1]. Projections indicate that global aquaculture production will increase to approximately 111 million tons by 2032, underscoring its growing importance in meeting the rising global demand for high-quality protein [1, 2]. Within freshwater aquaculture systems, _Oreochromis niloticus _(Nile tilapia) remains one of the most extensively farmed species, ranking second globally with an estimated production of 5.3 million tons in 2022 [3]. However, the intensification of aquaculture systems, although enhancing productivity, has simultaneously increased vulnerability to infectious disease outbreaks, resulting in considerable economic losses and threatening production sustainability [4, 5].

Streptococcosis is recognized as one of the most economically devastating bacterial diseases affecting tilapia farming worldwide [6, 7]. Streptococcus agalactiae (Group B Streptococcus) is responsible for recurrent disease outbreaks characterized by septicemia and meningoencephalitis, leading to high mortality rates and reduced profitability of aquaculture operations [8, 9]. Streptococcosis caused by S. agalactiae represents a major economic burden in tilapia aquaculture, with global losses estimated to exceed USD 1 billion annually [10]. Under intensive production conditions, outbreaks frequently result in mortality rates ranging from 30% to 70%, substantially compromising production efficiency, farm profitability, and trade sustainability in major tilapia-producing regions [11, 12]. Although disease management strategies, including vaccination, selective breeding for resistance, and probiotic supplementation, have shown promise, their implementation remains inconsistent, and the level of protection achieved is often incomplete under commercial farming conditions [11, 13, 14]. Consequently, the development of complementary and sustainable approaches to improve disease resilience remains a major research priority.

Traditionally, bacterial diseases in tilapia farming have been controlled primarily through antimicrobial drugs and chemical treatments. However, the prolonged and widespread use of these agents has resulted in the emergence and dissemination of antimicrobial resistance, reduced therapeutic efficacy, and increasing concerns regarding environmental contamination and food safety [15–17]. These interconnected challenges have created an urgent need to shift toward more sustainable and biologically compatible disease management strategies. Consequently, increasing attention has been paid to alternative approaches that enhance fish health and disease resistance without exacerbating ecological risks [18, 19]. In this context, phytobiotics, probiotics, prebiotics, and synbiotics have emerged as promising options because they provide antimicrobial and immunomodulatory benefits while avoiding many of the limitations associated with conventional antibiotic therapies [16, 20–23].

Among phytobiotics, Curcumin (CUR), a naturally occurring polyphenol derived from Curcuma longa, has attracted considerable scientific interest because of its potent antioxidant and anti-inflammatory properties [24, 25]. Beyond these activities, CUR exhibits hepatoprotective effects and modulates immune responses, thereby contributing to improved physiological condition and enhanced stress tolerance in aquatic organisms [26–30]. In addition, its antibacterial activity against several fish pathogens further underscores its potential for disease management [26, 27, 31]. Despite these functional advantages, the practical use of CUR remains limited by poor aqueous solubility, chemical instability, and low bioavailability, which collectively reduce its efficacy when incorporated into conventional feed systems [32–34].

Nanostructured lipid carriers (NLCs) have emerged as efficient delivery systems for hydrophobic and chemically unstable bioactive compounds, such as CUR, because they enhance molecular stability, facilitating intestinal absorption and enabling controlled release [35, 36]. Compared with conventional solid lipid nanoparticles, NLCs possess a less ordered lipid matrix formed by combining solid and liquid lipids, thereby enhancing drug-loading capacity and minimizing compound expulsion during storage [37]. In contrast to liposomal systems, NLCs offer superior physical stability, greater compatibility with lipophilic compounds, and enhanced protection of encapsulated phytochemicals under gastrointestinal conditions [38]. By incorporating CUR into a lipid-based matrix, NLCs may facilitate sustained physiological exposure and prolonged biological activity, thereby enabling compound-specific effects extending beyond growth promotion [39]. Previous studies by our group and others have demonstrated the utility of lipid-based nanocarriers for delivering various bioactive compounds in aquatic species, including hormonal, anesthetic, and phytogenic substances [30, 40–43]. Nevertheless, these studies involved compounds with physicochemical characteristics and biological activities fundamentally different from those of CUR, and the biological implications of CUR delivery through NLCs in Nile tilapia remain largely unexplored. Compared with other conventional nano-CUR delivery systems, NLCs may provide superior physicochemical stability and sustained-release characteristics suitable for long-term dietary application of hydrophobic phytochemicals.

Although the beneficial effects of CUR on growth performance, antioxidant status, and immune responses have been reported in several aquatic species, its practical application in aquafeeds remains limited by poor solubility, instability, and low bioavailability. Nanoencapsulation approaches have been investigated to improve the delivery of various bioactive compounds; however, studies specifically evaluating CUR-loaded NLC systems in Nile tilapia remain scarce. Furthermore, previous investigations have mainly focused on physicochemical characterization or isolated biological responses, with limited attention given to the integrated effects of nanoencapsulated CUR on growth performance, feed utilization efficiency, hepatosomatic condition, growth allometry, antibacterial activity, and post-challenge survival against S. agalactiae. In addition, information regarding the ability of sustained-release CUR delivery systems to enhance disease resistance and production efficiency in Nile tilapia under experimental challenge conditions remains insufficient. Therefore, a comprehensive evaluation of CUR encapsulated in NLCs (CUR-NLCs) as a functional dietary strategy for improving both productive performance and host resilience is warranted.

Accordingly, the present study was undertaken to develop and comprehensively evaluate CUR-NLCs, with particular emphasis on their physicochemical characteristics and their effects on growth performance, feed utilization efficiency, hepatosomatic condition, length–weight relationships, and resistance to streptococcosis in Nile tilapia. In addition to in vitro antibacterial assessment, the study investigated post-challenge survival responses following experimental exposure to S. agalactiae ENC06, a serotype Ia strain frequently associated with streptococcosis outbreaks in tilapia farming systems in Thailand. By integrating physicochemical characterization with biological and disease resistance evaluations, this study aimed to clarify the potential of CUR delivered via NLCs as a sustainable, phytochemical-based nutritional strategy to enhance fish health, disease resilience, and production efficiency in Nile tilapia aquaculture.