In Vitro Larvicidal Efficacy of a Fipronil-Based Nanoixodicide Against Rhipicephalus microplus

Controlling Rhipicephalus microplus is currently one of the main challenges in livestock farming due to the significant economic losses it causes. Traditionally, managing this parasite has been based on the use of synthetic ixodicides, among which fipronil has proven to be highly effective. However, its low water solubility and the limitations of commercially available formulations can affect the bioavailability of this compound, favoring the emergence of resistance in tick populations. In this context, fipronil-loaded nanoparticles were developed using the Eudragit^® E PO polymer (NP_F) (Helm, Naucalpan, Mexico, Mexico), which were physicochemically characterized and evaluated against fipronil-susceptible R. microplus larvae. NP_F had an average size of 143.43 ± 1.88 nm, a polydispersity index (PDI) of 0.162 ± 0.01, a ζ (P ζ) of 21.16 ± 0.54, an encapsulation percentage (%E) of 7.36 ± 0.30, and an encapsulation efficiency percentage (%EE) of 66.28 ± 3.5%. Free fipronil showed an LC₅₀ of 0.582 µg/mL and an LC₉₀ of 2.503 µg/mL against R. microplus. The NP_F formulation showed an LC₅₀ of 0.427 µg/mL and an LC₉₀ of 2.092 µg/mL. These results suggest that incorporating fipronil into nanoparticles improves its ixodicide efficacy, positioning it as an innovative and promising alternative for the development of effective tick control formulations.