The extent of acaricide resistance in the eastern region of the Eastern Cape Province

Abstract

The control of ticks, and to some extent tick-borne diseases, over much of South Africa is currently dependent on acaricides and will probably remain so for the foreseeable future. Resistance to these chemicals by ticks thus poses a major threat to the livestock industry especially as these chemicals constitute an ever-diminishing resource with fewer being discovered and the cost of their development becoming prohibitive. In order to determine the extent of acaricide resistance in the eastern region of the Eastern Cape Province one-, two- and three-host ticks were collected from cattle at 58 dip-tanks over a period of 2 years. The one-host tick selected was Rhipicephalus (Boophilus) microplus, the two-host tick Rhipicephalus evertsi evertsi and the three-host tick Rhipicephalus appendiculatus. The ticks were tested for resistance to three compounds, namely amitraz, cypermethrin and chlorfenvinphos. The Shaw Larval Immersion Test detected emerging resistance to amitraz in the one-host tick R. (Boophilus) microplus at two dip-tanks and resistance at a third. It also revealed resistance in this tick to cypermethrin at one dip-tank, and emerging resistance to chlorfenvinphos at eight dip-tanks and resistance at two. The two-host tick R. evertsi evertsi was susceptible to amitraz and cypermethrin at all dip-tanks, but showed emerging resistance to chlorfenvinphos at seven dip-tanks and resistance at four. The three-host tick R. appendiculatus was susceptible to amitraz and chlorfenvinphos at all dip-tanks and demonstrated emerging resistance to cypermethrin at one. With the exception of R. (Boophilus) microplus, in which emerging resistance to amitraz was detected at one dip-tank by the Reproductive Estimate Test, all three tick species at all dip-tanks at which sufficient numbers of ticks had been collected, were susceptible to the three acaracides in both the Egg Laying Test and the Reproductive Estimate Test. Thus despite its fairly long and widespread use in the eastern region of the Eastern Cape Province very little or no resistance to amitraz was detected in three tick species regularly encountered on cattle in this region, namely R. (Boophilus) microplus, R. evertsi evertsi and R. appendiculatus. On the other hand resistance to chlorfenvinphos was detected in both R. (Boophilus) microplus and R. evertsi evertsi at a number of dip-tanks even though it, or other organophosphorous-based compounds, had probably not been used for tick control in the region for a number of years. The localities at which ticks were collected had already been mapped and the localities at which acaricide resistant ticks were encountered were mapped during this study. The three tick species that were targeted for acaricide resistance testing were widespread throughout the study region, but no pattern of geographic distribution for the acaricide resistant strains of these species that were detected, emerged. The rapidity of selection for acaricide resistance appeared to be closely related to the life cycles of the three ticks and the number of days that they theoretically would spend annually on their preferred host animals. Thus a greater number of acaricide resistant strains were encountered amongst the one-host tickR. (Boophilus) microplus and the two-host tick R. evertsi evertsi than the three-host tick R. appendiculatus. The fist two ticks both complete more than one life cycle a year and hence spend a longer time on their cattle hosts than the three-host tick R. appendiculatus, which completes only one life cycle a year and in addition is a rapid feeder in all its stages of development. To counter selection for acaricide resistance it is proposed that regular testing for resistance should be carried out, and that as soon as emerging resistance is detected in ticks on cattle at a particular dip-tank, that the acaricide in use at that dip-tank should be changed to a compound belonging to a completely different group of chemicals. Copyright