Effects of carvacrol on oocyte development in semi-engorged Rhipicephalus sanguineus sensu lato females ticks (Acari: Ixodidae)

Currently, the most commonly used method to control ticks is the use of synthetic acaricides. However, these compounds are potentially harmful to hosts and the environment, in addition to causing the selection of resistant individuals. Therefore, several studies have been conducted to find sustainable methods to control ticks, such as Rhipicephalus sanguineus sensu lato, the most important vectors of pathogens for dogs. There has been increasing research on natural products with acaricidal action, especially with regard to plant-derived compounds as carvacrol, an aromatic monoterpene with several biological properties, including repellent and acaricidal activities, besides exerting cytotoxic effects on the exposed ticks. The objective of the present study was to evaluate the action of different carvacrol concentrations on the ovaries morphophysiology in semi-engorged R. sanguineus s.l. females to determine its effect on oocyte development. The results showed the occurrence of significant morphological alterations in the shape of oocytes (from round-shaped to irregular) and in the germinal vesicles, in addition to extensive cytoplasmic vacuolation.These effects were observed after the application of carvacrol at a concentration of 20 μL/mL. The most significant alterations were observed at the highest concentration (100 μL/mL), at which the oocytes could not develop further than stage II (total absence of oocytes III, IV and V). These data showed that even though carvacrol was unable to kill all ticks at these concentrations, surviving females could have had an altered reproduction, which would hinder the generation of new individuals, resulting in a long-term control. Data regarding the inhibition of oocyte development are unprecedented and indicate the use of carvacrol as a natural product with the potential to control R. sanguineus s.l. ticks.