Bioactive-based spray and spot-on formulations: development, characterization and in vitro efficacy against fleas and ticks

The flea Ctenocephalides felis felis and the tick Rhipicephalus sanguineus are ectoparasites of great importance in veterinary medicine that can affect pets, also impacting the lives of owners due to the possible transmission of zoonoses. Due to the negative impact on animals, owners and the environment, synthetic chemicals are being replaced by natural alternatives. Eugenol and carvacrol stand out as bioactive substances with potential antiparasitic activity. The aim of the present study was to develop and characterize physicochemically spray and spot-on formulations containing bioactives alone and in combination and to evaluate their ectoparasiticidal activity through in vitro bioassays of the knockdown effect and residual efficacy against adult fleas and ticks. Regarding flea knockdown, spot-on formulations achieved maximum mortality in a shorter period than spray formulations, 2 hours for carvacrol plus eugenol (SCE) and 4 hours for carvacrol (SC) and eugenol (SE) alone, compared to 15, 30 and 45 minutes for the spot-on formulations of carvacrol (PC), both substances together (PCE) and eugenol alone (PE), respectively. For tick control, the formulations required longer exposure times than for fleas, so that 100% control took 6 hours for PCE, followed by 12 hours for SCE and PE, and 24 hours for PC, SC and PE. Regarding residual efficacy against fleas, the spray and spot-on formulations remained active for approximately 90 and 45 days, respectively. In general, formulations containing associated bioactives had a faster knockdown effect than formulations with only one in their composition, indicating a synergistic effect of the two bioactive substances against fleas and ticks.