Avaliação da atividade carrapaticida do timol incorporado a duas formulações de uso tópico sobre estágios imaturos de Rhipicephalus sanguineus (Latreille, 1806) (Acari:Ixodidae)

Abstract

Rhipicephalus sanguineus sensu lato (s.l.) is a tick with known medical and veterinary importance, due to its potential for transmission of pathogens as well as the direct damage caused to hosts, such as blood spoliation, stress, allergies and dermatitis. Control of these parasites is mainly based on the use of synthetic chemical substances, but these often have negative consequences, including environmental contamination, intoxication of animals and handlers/owners, and selection of resistant strains. Thymol, an aromatic monoterpene initially isolated from plants of the family Lamiaceae, has presented good results in controlling R. sanguineus s.l. in in vitro tests. For use as an acaricide, thymol needs to be carried in a suitable pharmaceutical formulation, allowing easy application, reasonable cost and safety for topical use. The objective of this work was to assess, for the first time, the in vitro acaricidal activity of two topical formulations, each with different concentrations of thymol, on larvae and nymphs, both engorged and non-engorged, of R. sanguineus sensu lato. For this purpose, two base formulations were prepared: an oil-in-water (O/W) emulsion and a hydroalcoholic solution, containing different thymol concentrations (0.5 mg/mL to 20 mg/mL). The formulations were analyzed by the larval packet test (modified) in the case of larvae and non-engorged nymphs, with evaluation of mortality after 24 hours; and by the immersion test for larvae and engorged nymphs, with evaluation of mortality after 15 days. The control group was exposed to the base formulations without thymol. There were ten repetitions of each treatment. In the tests with the emulsion, the best average mortality rate was 94.2%, with the concentration of 0.75 mg/mL for non-engorged larvae. In the case of the engorged larvae, the best average mortality was 95.0% at the concentration of 5.0 mg/mL. The average mortality of the non-engorged nymphs treated with the emulsion containing 2.5 mg/mL was 83.3% and in the test with engorged nymphs, the mean mortality was 86.0% for the concentration of 5.0 mg/mL. In the tests with the hydroalcoholic solution, the highest average mortality among the non-engorged larvae was 88.1% for the concentration of 2.5 mg/mL. In turn, for the engorged larvae, the highest mortality was 25.0%, at the concentration of 20 mg/mL, while the test with non-engorged nymphs produced a mortality rate of 91.0% at the concentration of 1.0 mg/mL and in the test with engorged nymphs the maximum mortality was 18.3% with a concentration of 20 mg/mL. Besides the tests with the ticks, preliminary stability tests were carried out to verify possible problems with the formulations. The hydroalcoholic solution remained stable under all the conditions analyzed, at concentrations of 2.5 and 5.0 mg/ml, while the O/W emulsion showed signs of early instability at the concentration of 5.0 mg/ml but not at the concentration of 2.5 mg/ml. The results obtained indicate that the acaricidal activity of thymol, when included in the proposed formulations, was enhanced against non-engorged larvae with topical treatment in comparison with data in the literature. Although there were variations in toxicity between the different stages and forms (engorged and non-engorged), these formulations are promising for future therapeutic use.