Fipronil chewable tablets: development, pharmacokinetic profile, and efficacy in treating beagles naturally infested with Cochliomyia hominivorax.

Abstract

The New World screwworm (NWS), Cochliomyia hominivorax, is an obligate ectoparasite whose larvae cause severe myiasis, leading to extensive tissue damage and potentially fatal outcomes in dogs if not promptly treated. Dogs play an important epidemiological role in maintaining and disseminating infestations, requiring rapid and effective therapeutic options. Fipronil (FIP) is an insecticide widely used for ectoparasite control with systemic efficacy after oral administration to different species, representing a promising alternative for treating canine myiasis. This study describes the development of molded FIP chewable tablets, the evaluation of its pharmacokinetics, and its clinical efficacy in dogs naturally infested by NWS larvae. For pharmacokinetic assessment, eight healthy dogs received a single dose (6 mg/kg) chewable tablet, and blood samples were collected at selected time points to determine plasma concentrations. To assess efficacy, eight dogs naturally infested with NWS larvae received the same dosage. Larvae expelled from hosts were collected over 24 h, after which remaining larvae were mechanically removed. The tablets met established quality control criteria. Following oral administration, FIP was rapidly absorbed, being detectable 30 min after treatment and reaching a maximum plasma concentration (Cmax) of 3126.5 ± 1269.5 ng/mL at 2.2 ± 0.94 h (Tmax). Clinically, larval expulsion increased progressively from 2 h onward, reaching 97.8% effectiveness within 24 h. No live larvae were observed in the lesions at the end of evaluation, resulting in 100% overall effectiveness. These findings demonstrate that the FIP chewable tablet is a viable alternative option for treating canine myiasis from NWS larval infestation.