Formulation and In Vitro Evaluation of Red Palm Oil with Rhinacanthus nasutus, Curcuma longa, Zingiber montanum, and Zingiber officinale Extracts as a Possible Pet Shampoo Formula with Antibacterial, Antifungal, and Anti-Inflammatory Activities

Bacterial and fungal skin infections are common in companion animals and often require antimicrobial therapy, contributing to antimicrobial resistance. This study developed shampoos containing red palm oil (RPO) and extracts of Rhinacanthus nasutus (RN), Curcuma longa (CL), Zingiber montanum (ZM), and Zingiber officinale (ZO), which provide antibacterial, antifungal, antioxidant, and anti-inflammatory activities. Extracts were obtained using MAE and were evaluated for phytochemical composition, total phenolic content, antioxidant activity (DPPH assay), anti-inflammatory activity (COX-2 inhibition), and antimicrobial activity. RPO contained high levels of vitamin E and β-carotene. Rhinacanthin, curcumin, terpinen-4-ol, and 6-gingerol were the major phytochemical constituents of RN, CL, ZM, and ZO extracts, respectively. All extracts significantly reduced COX-2 expression (p < 0.05), while RN exhibited the strongest antioxidant activity (DPPH IC₅₀ = 15.23 ± 0.53 µg/mL). RN and ZM demonstrated the strongest antibacterial activity against Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MIC = 8 µg/mL; MBC = 8–16 µg/mL), while ZO exhibited the strongest antifungal activity against Malassezia pachydermatis and Microsporum gypseum (MIC = 2, 2 µg/mL; MFC = 2, 16 µg/mL). The developed shampoos containing RPO and the four herb extracts showed suitable pH values, stable viscosity, antibacterial inhibition, and antifungal inhibition (p < 0.05).