Acaricidal and repellent activity of the essential oils of Backhousia citriodora, Callistemon viminalis and Cinnamodendron dinisii against Rhipicephalus spp.

The ticks Rhipicephalus microplus and Rhipicephalus sanguineus sensu lato (s.l) are of great importance to agriculture, veterinary medicine and public health. Due to a number of problems related to the use of synthetic acaricides, natural products emerge as promising substances for alternative tick control. In the present study, essential oils of Backhousia citriodora, Callistemon viminalis and Cinnamodendron dinisii were extracted by hydrodistillation, characterized by GC-MS and GC-FID and biologically evaluated for acaricidal activity against R. microplus and repellent activity against R. sanguineus s.l. Inhibition of acetylcholinesterase (AChe) by the essential oils was also evaluated. The major constituent of B. citriodora is citral (98.9 %), whereas the essential oil from C. viminalis is rich in 1.8-cineole (78.1 %), α-pinene (12.5 %) and limonene (3.36 %), and that from C. dinisii contains α-pinene (30.8 %), ß-pinene (12.5 %) and sabinene (11.3 %) as the principal constituents. The median lethal concentrations (LC 50) estimated for the essential oils on engorged R. microplus females were 3.276 µL.mL-1 for B. citriodora, 8.195 µL.mL-1 for C. dinisii and 8.936 µL.mL-1 for C. viminalis. The essential oil of B. citriodora showed the best repellent effect against unfed R. sanguineus s.l. adults, demonstrating repellent action up to 3 h after application. The essential oil of C. viminalis was able to reduce AChe activity, with an average inhibitory concentration (IC 50) of 0.33 µg mL-1. Thus, these oils can be considered as sources of bioactive compounds for tick control.

Effects of andiroba (Carapa guianensis) oil in ticks: Ultrastructural analysis of the synganglion of Rhipicephalus sanguineus (Latreille, 1806) (Acari: Ixodidae).

The present study performed an analysis of the ultrastructural changes induced by andiroba seed oil in the synganglion of Rhipicephalus sanguineus female ticks, aiming to provide scientific grounds to help in the creation of more specific and efficient methods of control. The synganglion consists of a mass of fused nerves externally covered by the neural lamella, a uniform and acellular layer. Just below, the perineurium is found, formed by glial cells. Internally, the synganglion is subdivided into an outer cortical region (cortex), which contains the cellular bodies of the neural cells and an inner region, the neuropile, formed by a set of nerve fibers (extensions of the neural cells). The results showed that the synganglion of females exposed to andiroba oil showed structural changes, such as: irregular and apparently thinner neural lamella, perineurium glial cells presenting large cytoplasmic vacuoles, decrease in the extensions of glial cells, separation of cortex cells, which were formerly attached through their membranes, neural cells presenting irregular plasma membranes and cytoplasm with autophagic vacuoles and mitochondria with disorganized cristae and in process of degeneration. This study confirmed the neurotoxic action of the andiroba oil, which would probably be able to impair the neural functions. Thus, it is suggested that this product has the potential to be used as an alternative method to control ticks.

Histopathological study of ovaries of Rhipicephalus sanguineus (Acari: Ixodidae) exposed to different thymol concentrations.

Thymol is a monoterpene with proven acaricide action for several tick species. In addition to killing these ectoparasites, thymol can also reduce oviposition and egg hatch rate. However, the effects of thymol on the morphophysiology of tick ovaries are still unknown. Thus, the aim of this study was to evaluate the morphophysiological changes caused by this active principle in ovaries of Rhipicephalus sanguineus after a 6-day feeding period, through the application of morphohistochemical techniques. After the feeding period, a total of 50 females were divided into five groups and immersed in the following solutions: (I) distilled water (control), (II) 30% ethanol (control), (III) 1.25 mg/mL thymol, (IV) 2.5 mg/mL thymol, and (V) 5.0 mg/mL thymol. The experimental groups were kept in a climatic chamber (27 ± 1 °C; RH 80 ± 10%) for 5 days. After this period, morphological (hematoxylin/eosin) and histochemical (von Kossa) techniques were applied after remotion of the ovaries. The morphological results revealed large vacuoles in germ cells at different developmental stages and invaginations that represent deformations in the chorionic membrane. From the results obtained in this study, it was concluded that thymol interfered with the development of oocytes, which showed degeneration signs. The treatment containing 5.0 mg/mL thymol affected more accentuately the morphological development. Moreover, thymol also altered the calcium content of yolk granules, which generally showed an intense staining for this element.