In vitro bioassays used in evaluating plant extracts for tick repellent and acaricidal properties: A critical review

Ticks are haematophagous arthropods which rank closely with mosquitoes in their capacity to transmit disease pathogens of importance to animals and humans. Current control of ticks is based on the routine use of synthetic chemicals administered to animals or their environment. However, years of use and overuse of these chemicals have resulted in the development of resistance in these parasites and negative environmental impacts, hence the need for cheaper, safer and more environmentally friendly alternatives with alternate modes of action. There has been a large interest in using plants for these purposes. Peer-reviewed articles on plants evaluated for their tick repellent and/or acaricidal activities against immature and adult stages of ticks were collected from nine scientific databases with the aim of reviewing the bioassays employed. Search words included "acaricidal", "tick repellent", "antitick assays" and "phytomedicine". Many different methods were used to determine repellency and acaricidal activity. These include, among a few others, petri dish, tick climbing, olfactometer, larval packet and immersion bioassays. Tick climbing repellency and adult immersion bioassays were most commonly used. Ethanol was the most widely used solvent and Rhipicephalus (Boophilus) microplus was the most commonly studied tick across all the reviewed papers. It is unclear whether the outcome of these experiments on a one-host tick can be applied to other species of ticks that infest animals and humans. Also, most of the assays on repellency did not discriminate between olfaction and tactile chemoreception-based repellency and though some of the observed methods were similar, results differ significantly. These aspects will need further evaluation. Standardized laboratory methods are required to enable the valid comparisons between results from different laboratories.