The Threat and Reality of Drug Resistance in the Cattle TickRhipicephalus (Boophilus) microplus

Use of acaricides is one of the key strategies to protect cattle herds on pasture against tick infestation with Rhipicephalus (Boophilus) microplus. Control of cattle ticks with chemical compounds started at the end of the nineteenth century with the use of arsenic. This acaricide class was followed by organochlorines, organophosphates, amidines, synthetic pyrethroids, phenylpyrazols, macrocyclic lactones, growth regulators, and natural products. Unfortunately, the introduction of new acaricide classes has been followed by reports of treatment failure and finally the identification of resistance. Responsible for resistance are genetic changes in a cattle tick population that cause modifications to the target site, increased metabolism or sequestration of the acaricide, or reduced ability of the acaricide to penetrate through the outer protective layers of the tick's body. The mechanisms of resistance have been widely examined for some classes like the synthetic pyrethroids and organophosphates, while for other acaricides many aspects still remain unclear. Nowadays, resistance can be found to almost all acaricide classes in the endemic areas. As a consequence, alternative strategies have been proposed to reduce the dependency on chemical products. While the use of indicine cattle breeds – which are less susceptible to tick-infestation – needs to be balanced with reduced productivity, other strategies like tick vaccines, may simply not be available in some countries or regions. Finally, the use of acaricides will remain an important pillar for control of cattle ticks, which increases the pressure on their sustainable use.