Histologia e composição elementar da cutícula de diferentes populações do carrapato Rhipicephalus microplus submetidas ou não ao tratamento com fluazuron

Abstract

Ticks are associated with numerous economic and health impacts. The bovine tick, Rhipicephalus microplus, with its hematophagous habits, transmits pathogens causing

various diseases, reducing milk and meat production, and affecting the health and well- being of animals. The indiscriminate and repeated use of acaricides has led to the

emergence of resistant populations, a phenomenon that has increased in the last decade. Resistance can be attributed to factors such as reduced penetration of the product into the cuticle, which can modify the chemical composition of the cuticle. This study aims to evaluate the elemental chemical composition and histology of the cuticle of larvae from different populations of R. microplus obtained from females treated or not with fluazuron. This dissertation is presented in two chapters. Chapter I is titled “Elemental Composition of the Cuticle of R. microplus Larvae from Different Populations in Brazil” and aims to characterize the elemental composition of the cuticle of R. microplus larvae from different geographical populations, correlating this information with susceptibility or resistance levels to the main synthetic acaricides used to control this species. The reference population POA showed susceptibility to all the tested acaricides, while the other populations exhibited variable resistance (low, medium, and high) to cypermethrin, chlorpyrifos, and fipronil, maintaining universal susceptibility to doramectin and moxidectin. In the electron microscopy analysis (Experiment II), a predominantly carbon composition was observed in the larval cuticle, with statistically significant differences between populations and between dorsal and ventral regions, with the dorsal region showing a higher carbon percentage in 9 of the 11 populations analyzed. In addition, a strong and significant positive correlation between dorsal carbon percentage and resistance to cypermethrin was identified (r = 0.7728, p = 0.0081). Although the correlation for chlorpyrifos and fipronil was positive, it was not statistically significant. The carbon composition in the ventral region showed a very weak and non-significant negative correlation with the evaluated acaricides. It was not possible to correlate the cuticular composition with resistance to doramectin and moxidectin due to universal susceptibility to these compounds. Chapter II is titled “Histology and Elemental Composition of the Cuticle of Rhipicephalus microplus Larvae from Females Treated with Fluazuron” and aims to evaluate the elemental chemical composition and histology of the cuticle of larvae from different populations of R. microplus obtained from females treated or not with fluazuron. R. microplus females treated with fluazuron showed a significant reduction in egg weight (1.59 mg vs. 9.77 mg in the untreated group), hatchability rate (20.18% vs. 56.61% in the untreated group), and egg production index (3.00 vs. 42.99 in the untreated group), with an effective control of 97.52%. Larvae from treated females showed a lower carbon percentage in the cuticle at various developmental stages, with significant reductions at 7 and 21 days (dorsal region) and at 14 and 21 days (ventral region) compared to the control group. In the control group, the three cuticle layers were preserved in the evaluated larvae. In the treated group, thinning of the procuticle, absence of the epicuticle, and severe structural damage were observed, indicating degradation resulting from the treatment of females with fluazuron. This dissertation represents the first report attempting to elucidate the chemical elemental composition of the cuticle of larvae from different populations of R. microplus, and it was possible to detect a potential correlation between the cuticular composition and resistance to acaricides. It also demonstrates for the first time the elemental-level alterations in the cuticle of R. microplus larvae from engorged females treated with fluazuron, a growth inhibitor belonging to the benzoylphenylurea class.