Emergence of multi-acaricide resistant Rhipicephalus ticks and its implication on chemical tick control in Uganda.

BACKGROUND

Acaricide failure has been on the rise in the western and central cattle corridor of Uganda. In this study, we identified the tick species associated with acaricide failure and determined their susceptibility to various acaricide molecules used for tick control in Uganda.

METHODS

In this cross sectional study, tick samples were collected and identified to species level from 54 purposively selected farms (from 17 districts) that mostly had a history of acaricide failure. Larval packet test was used to screen 31 tick populations from 30 farms for susceptibility at discriminating dose (DD) and 2 × DD of five panels of commercial acaricide molecules belonging to the following classes; amidine, synthetic pyrethroid (SP), organophosphate (OP) and OP-SP co-formulations (COF). Resistance was assessed based on World Health Organization criteria for screening insecticide resistance.

RESULTS

Of the 1357 ticks identified, Rhipicephalus (Rhipicephalus) appendiculatus and Rhipicephalus (Boophilus) decoloratus were the major (95.6%) tick species in farms sampled. Resistance against SP was detected in 90.0% (27/30) of the tick populations tested. Worryingly, 60.0% (18/30) and 63.0% (19/30) of the above ticks were super resistant (0% mortality) against 2 × DD cypermethrin and deltamethrin, respectively. Resistance was also detected against COF (43.3%), OP chlorfenvinphos (13.3%) and amitraz (12.9%). In two years, 74.1% (20/27) of the farms had used two to three acaricide molecules, and 55.6% (15/27) rotated the molecules wrongly. Multi-acaricide resistance (at least 2 molecules) was detected in 55.2% (16/29) of the resistant Rhipicephalus ticks and significantly associated with R. decoloratus (p = 0.0133), use of both SP and COF in the last 2 years (p < 0.001) and Kiruhura district (p = 0.0339). Despite emergence of amitraz resistance in the greater Bushenyi area, it was the most efficacious molecule against SP and COF resistant ticks.

CONCLUSION

This study is the first to report emergence of super SP resistant and multi-acaricide resistant Rhipicephalus ticks in Uganda. Amitraz was the best acaricide against SP and COF resistant ticks. However, in the absence of technical interventions, farmer-led solutions aimed at troubleshooting for efficacy of multitude of acaricides at their disposal are expected to potentially cause negative collateral effects on future chemical tick control options, animal welfare and public health.