Efficacy of a low-dose fipronil bait in controlling blacklegged tick (Ixodes scapularis) larvae parasitizing white-footed mice (Peromyscus leucopus) under field conditions in the northeastern United States.

BACKGROUND: Lyme disease, caused primarily by Borrelia burgdorferi sensu stricto, remains the most common vector-borne disease in the United States despite control efforts. Peromyscus leucopus (white-footed mouse) is a key natural pathogen reservoir and Ixodes scapularis ticks, commonly infesting this rodent, serve as vectors for spirochetes causing human Lyme disease. Laboratory and simulated field studies demonstrated substantial efficacy of a 0.005% fipronil bait in systemically controlling I. scapularis parasitizing treated P. leucopus. To evaluate fipronil bait efficacy under natural conditions, a multi-year field study was conducted in Connecticut.

METHODS: Reusable rodent bait stations containing fipronil bait were deployed on residential properties during the I. scapularis nymphal and larval seasons (May-August) from 2022 to 2024. Bait consumption was monitored and bait replenished as needed. Peromyscus leucopus were captured during larval season each year within treated and control properties to assess parasitizing I. scapularis and determine fipronil concentrations in P. leucopus plasma. Drag sampling/flagging was performed during I. scapularis nymphal seasons to estimate abundance of host-seeking nymphs. Molecular assays were used to determine B. burgdorferi infection in P. leucopus and host-seeking nymphs.

RESULTS: An average of 4.9 kg of fipronil bait, containing approximately 247 mg fipronil, was distributed/property/year. The majority of plasma samples analyzed (100% in 2022 and 2023; 83.3% in 2024) had detectable concentrations of fipronil sulfone. Results indicated that within treated properties, parasitizing larvae and successfully engorging larvae were significantly reduced by up to 84.2% and 92.5%, respectively. Additionally, B. burgdorferi infection in P. leucopus was significantly reduced by up to 34.1% relative to control. While reductions were noted in density and B. burgdorferi infection in host-seeking nymphs within treatment properties, there were no detectable significant differences relative to control.

CONCLUSIONS: Our results demonstrate potential for a low-dose fipronil bait to significantly reduce parasitizing tick burden, pathogen prevalence in wild mice, with potential to reduce host-seeking nymph density and pathogen infection. Future research should consider increasing the bait application rate during the larval season to more effectively reduce density and B. burgdorferi infection rate in host-seeking nymphs. Additional field studies would aid in confirming these findings and further investigation of the potential impact of largescale treatment.