Action of the chemical agent fipronil on the reproductive process of semi-engorged females of the tick Rhipicephalus sanguineus (Latreille, 1806) (Acari: Ixodidae). Ultrastructural evaluation of ovary cells

The ovary of the tick Rhipicephalus sanguineus consists of a wall of epithelial cells and a large number of oocytes in five different developmental stages (I-V), which are attached to the wall by a pedicel. The present study provides ultrastructural information on the effects (dose-response) of the acaricide fipronil (Frontline®) on ovaries of semi-engorged females of R. sanguineus, as well as it demonstrates some possible defense mechanisms used by oocytes to protect themselves against this chemical agent. Individuals were divided into four groups. Group I was used as control while groups II, III and IV were treated with fipronil at the concentrations of 1, 5 and 10 ppm, respectively. Fipronil at the concentration of 10 ppm had the strongest effect on the development of oocytes. At this concentration, even oocytes that reached the final developmental stage exhibited damaged cell structures. Moreover, the observation in fipronil-treated R. sanguineus ticks of damaged cellular components such as plasmic membrane, mitochondria and protein granules (due to alteration in the protein synthesis), and cellular defense mechanisms such as increase in the amount of cytoplasmic microtubules and large amounts of digestive vacuoles and myelin figures, were only possible by means of ultrastructure.