The Spread of Pyrethroid Resistance in Brown Dog Ticks: A Growing Concern for Veterinarians

The brown dog tick, Rhipicephalus sanguineus sensu lato (s.l.), poses a significant threat as a vector for Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever. Recent research has identified a mutation associated with resistance to synthetic pyrethroids (SPs), highlighting an emerging challenge in tick management.

Study Overview

The study involved a geographically diverse collection of Rhipicephalus sanguineus s.l. ticks from domestic pets across the US and northern Mexico. Researchers employed a highly multiplexed amplicon sequencing approach to investigate the genetic diversity at specific loci within the voltage-gated sodium channel (VGSC) gene, the gamma-aminobutyric acid-gated chloride channel (GABA-Cl) gene, and three mitochondrial genes (COI, 12S, and 16S).

Key Findings

Pyrethroid Resistance: A single nucleotide polymorphism (T2134C) in the VGSC gene, linked to SP resistance, was widespread in the tropical lineage of Rhipicephalus sanguineus s.l., present in over 50% of these ticks. This mutation was absent in the temperate lineage, suggesting a significant resistance concern primarily in the tropical lineage.

Geographic Distribution: The tropical lineage of Rhipicephalus sanguineus s.l. accounted for 79% of the ticks analyzed and was found at 13 out of 14 collection sites, indicating its widespread presence. Both tropical and temperate lineages co-occurred in four US states, reaching as far north as New York.

GABA-Cl Gene Findings: Ticks from both lineages exhibited multiple copies of the GABA-Cl gene, with some containing mutations associated with fipronil resistance in other species. However, the impact of these mutations on fipronil resistance in Rhipicephalus sanguineus s.l. remains unclear.

Rickettsial Infection: None of the ticks examined tested positive for Rickettsia rickettsii or Rickettsia massiliae, providing some reassurance regarding the immediate risk of these pathogens.

Clinical Implications

Challenges in Tick Management: The widespread presence of pyrethroid-resistant ticks in the tropical lineage complicates current tick control strategies, which heavily rely on SP-based acaricides. Veterinarians need to be aware of this resistance and consider alternative tick control methods.

Need for Regular Monitoring: Routine monitoring for acaricide resistance is crucial to ensure effective tick management. Incorporating genetic testing for resistance markers into regular veterinary practice can help identify and mitigate resistance issues early.

Alternative Control Measures: Given the resistance to SPs, veterinarians should explore and recommend alternative acaricides and integrated pest management strategies. Educating pet owners on comprehensive tick control measures, including environmental management and regular tick checks, is essential.

Conclusion

The study by Nathan E. Stone et al. underscores a significant challenge in controlling Rhipicephalus sanguineus s.l., particularly the tropical lineage, due to widespread resistance to synthetic pyrethroids. For veterinarians, staying informed about resistance patterns and adopting diverse tick control strategies is critical in managing this evolving threat effectively.Read full study here: A mutation associated with resistance to synthetic pyrethroids is widespread in US populations of the tropical lineage of Rhipicephalus sanguineus s.l