Potentials and Applications of Microalgae and Spirulina (Cyanobacterium) in Pet Nutrition and Health: A Comprehensive Review with a Special Focus on Euglena gracilis

The pet food industry is undergoing a significant transformation, driven by the growing trend of companion animal humanization and increasing concerns over the environmental sustainability of conventional protein and lipid sources. Consumers now seek diets that not only meet basic nutritional requirements but also offer preventive health benefits. Microalgae, including Arthrospira (Spirulina, a kind of cyanobacterium), Chlorella, Schizochytrium, and Euglena gracilis, have emerged as versatile biological platforms capable of addressing both functional and sustainability challenges. These microorganisms produce high-quality proteins, omega-3 long-chain polyunsaturated fatty acids (particularly docosahexaenoic acid, DHA), natural pigments, and immunomodulatory polysaccharides. This review synthesizes findings from peer-reviewed studies on the application of microalgae in pet nutrition, covering dogs, cats, and aquatic companion animals. We examine how algal ingredients influence gut microbiota, for instance, by enriching beneficial genera such as Turicibacter and Peptococcus, enhance vaccine responses and mucosal immunity, support cognitive function in aging pets, and contribute to weight management. Particular attention is given to Euglena gracilis and its paramylon (β-1,3-glucan), a pathogen-associated molecular pattern that engages the Dectin-1 pathway to train innate immunity and has demonstrated antiviral activity through host defense mechanisms. The review also surveys the patent landscape, highlighting trends in palatability enhancement, hypoallergenic formulations, and novel delivery formats. Key challenges remain, including ingredient standardization, safety validation, palatability optimization, and consumer acceptance. We outline a translational roadmap that prioritizes well-designed clinical trials in target species and processing methods that preserve bioactivity. Collectively, the evidence positions microalgae, and Euglena gracilis in particular, as promising candidates for next-generation functional pet foods that deliver health benefits alongside ecological sustainability.