Acaricidal activity in vitro of spilanthol against cell lines derived from acaricide-resistant and acaricide-susceptible Rhipicephalus microplus ticks with or without Ehrlichia minasensis infection

Abstract

Ticks in general represent a great public health concern, since they cause considerable livestock losses and are vectors of many pathogens, such as bacteria, viruses, protozoa and helminths, affecting domestic/wild animals and human beings as well. The widely used method to control ticks is the use of synthetic chemical acaricides. However, the indiscriminate use of such products can induce the selection of resistant ticks and lead to the accumulation of chemical residues in the environment, contaminating the soil and water streams, and consequently affecting other animals, including human beings. This innovative study aims to establish an in vitro protocol for the study of acaricidal products as an alternative to traditional animal tests. The objectives of the present study will be firstly to investigate the effects of spilanthol, an aliphatic alcamid abundantly found in the flowers, leaves and stem of the plant Acmella oleracea and with known acaricidal effect in vivo, on two Rhipicephalus microplus cell lines: BME/CTVM6, derived from ticks resistant to organophosphates, organochlorines, and amitraz, and BME/CTVM2, derived from acaricide-susceptible ticks. The second objective will be to understand the direct effect of sublethal acaricide doses on pathogens carried by ticks; for this, the two tick cell lines will be infected with Ehrlichia minasensis, an obligate intracellular bacterium isolated from the cattle tick R. microplus, whose pathogenicity for cattle is unknown. If ticks survive acaricide application, they may go on to infest new hosts and potentially transmit any pathogens that they harbor. It is not known whether acaricide exposure has any effect directly on intracellular tick-borne pathogens or indirectly on the tick's ability to transmit them; such effects would have important implications for the role of acaricides in integrated tick and tick-borne disease control programmes. For both objectives, morphological techniques will be applied, comparing the results with untreated control cultures. The results will provide information relevant for the development of sustainable strategies to control these ticks.