Emergence of multidrug-resistant Haemaphysalis longicornis populations in China

Many chemical acaricides have been used to mitigate the detrimental effects of Haemaphysalis longicornis. However, the excessive use of chemical acaricides may facilitate the development of resistance in H. longicornis. Larval packet tests (LPTs) were conducted on larvae aged 14-21 days collected from 10 counties in China to evaluate the effectiveness of different acaricides, including cyhalothrin, cypermethrin, deltamethrin, amitraz, and fipronil. Additionally, genomic DNA was extracted from tick samples collected from 14 counties, and partial fragments of the voltage-gated sodium channel (VGSC), beta-amino-oxidoreductase (β AOR), and gamma-aminobutyric acid (GABA)-gated chloride channel genes were amplified via PCR. According to the LPT findings, it can be inferred that ticks from all localities exhibited resistance to at least three different acaricides. Multidrug resistance to five acaricides was detected in 40% of the samples, including those from Liaoning (LYG), Hebei (HZY), Henan (HSY), and Hubei (HSG) Provinces. The resistance factors for cyhalothrin, cypermethrin, deltamethrin, amitraz, and fipronil ranged from 2.71-9.57, 1.14-10.4, 0.80-24.2, 5.2-56.97, and 4.12-41.17, respectively. Hence, the resistance levels to the different acaricides ranged from susceptible (S) to level IV. Level IV resistance was observed only for fipronil and amitraz. Despite substantial resistance, no nonsynonymous mutations in the VGSC or β AOR genes were detected. However, one novel nonsynonymous mutation (M295L) in transmembrane 2 (TM2) of the GABA-gated chloride channel gene, which may have led to conformational changes in this channel, was detected in ticks from Shandong Province (SQL). In this study, LPTs were used to evaluate the effects of various acaricides on H. longicornis, and the findings revealed that this tick species is resistant to many acaricides. This study provides a foundation for the development of preventative and control measures against H. longicornis.