Tick resistance to acaricides is an increasing problem in South Africa and poses a real economic threat to the livestock and veterinary pharmaceutical industries. New acaricides are extremely expensive to develop so the present acaricides should be seen as an ever-diminishing resource, which should be protected by all means possible. The main objective of the study was to detect the levels of tick resistance to acaricides at selected commercial and communal farms in South Africa. Also to compare the in vitro adult and larval test methods and to investigate acaricide management strategies which may increase the lifespan of the presently used acaricides. To meet these objectives a field survey (February 2000 to August 2001) was carried out at selected communal and commercial farms in the Eastern Cape and Northwest Provinces of South Africa to monitor levels of field tick resistance to acaricides. The larvae were originally obtained from engorged female A. hebraeum, B. decoloratus, R. appendiculatus and R. evertsi evertsi. The larvae were tested against different concentrations of amitraz, chlorfenvinphos and cypermethrin using the Shaw Larval Immersion Test (SLIT). Mortality dose data were subjected to probit analysis using a BMDP statistical package. Factors of resistance (FOR) were calculated by comparing the larval response of ticks from the field, which had been exposed to acaricides, with baseline data from very susceptible laboratory strains of ticks, on the basis of the LC50 values. On the communal farms high levels of tick resistance were detected to cypermethrin as well as partial resistance to chlorfenvinphos whilst no resistance was detected against amitraz. On the commercial farms, however, ticks were equally resistant to amitraz, cypermethrin and chlorfenvinphos. The populations of B. decoloratus on these farms had developed higher levels of resistance to the test acaricides than the equivalent R. evertsi evertsi, R. appendiculatus and A. hebraeum populations. Higher levels of tick resistance to amitraz was observed on commercial farms than on communal farms, however, there was no significant differences in tick resistance to chlorfenvinphos and cypermethrin at both the commercial and communal farms. It was surmised that inappropriate use of acaricides might have resulted in higher tick resistance to the currently available acaricides on the commercial as well as the communal farms. Correct acaricide usage may solve this problem to a limited extent. Comparative in vitro tests were also carried out on the larvae and adults of B. decoloratus to determine the susceptibility of this tick to different concentrations of the currently used acaricides, (amitraz, chlorfenvinphos and cypermethrin) at three commercial dairy farms, (“Brycedale”, “Sunny Grove” and “Welgevind”) near East London in the Eastern Cape Province of South Africa. Resistance of field strains of B. decoloratus were determined using the SLIT and the Adult Immersion Test (AIT) as the latter test took into account factors such as oviposition assessment and reproductive ability. At “Brycedale”, resistance to amitraz and chlorfenvinphos was detected with the AIT method. Emerging resistance to amitraz and resistance to chlorfenvinphos were also detected at “Brycedale” with the SLIT method. At “Sunny Grove” resistance was detected to cypermethrin and at “Welgevind” resistance was detected to chlorfenvinphos with the SLIT whilst no resistance was detected using AIT. It would appear that the B. decoloratus populations tested on these dairy farms were more resistant to chlorfenvinphos than to amitraz or cypermethrin. Variable results were obtained using the SLIT, the Reproductive Estimate Test (RET) and the Egg laying Test (ELT). Nearly 50% of the dairy farms sampled showed resistance to chlorfenvinphos and the majority had susceptible B. decoloratus populations to both amitraz and cypermethrin. “Brycedale” had a serious resistance problem whilst “Sunny Grove” and “Welgevind” dairies had much less resistance problems. At “Brycedale”, the SLIT, RET and ELT methods all recorded resistance to amitraz and chlorfenvinphos whilst cypermethrin resistance was also detected with the ELT. At “Sunny Grove”, the SLIT detected emerging resistance to chlorfenvinphos and resistance to cypermethrin while the other two test methods were negative. At “Welgevind” the SLIT detected resistance to chlorfenvinphos and the ELT resistance to cypermethrin whilst the RET did not detect any resistance at “Welgevind”. In general there was a good correlation between the RET and the ELT whilst in many cases there was poor correlation between the SLIT and the two AIT methods (RET and ELT). From this study it would appear that the ELT was a good method to detect resistance within seven days, as opposed to the 42 days required for the RET and the 60 days for the SLIT. The ELT and the RET could possibly be used as screening methods to detect acaricide resistance on farms whilst the SLIT would remain the test of choice for National surveys. In addition the ELT is less costly and does not require sophisticated equipment for field testing for resistance, compared with other in vitro test methods. This method, however, still needs to be validated and standardized for use in South Africa and the rest of Africa where tick control is important.
Dissertation (MSc (Veterinary Science))--University of Pretoria, 2006.