Efficacy of low-dose fipronil bait against blacklegged tick (Ixodes scapularis) larvae feeding on white-footed mice (Peromyscus leucopus) under simulated field conditions

Abstract

Background: Lyme disease, caused primarily by Borrelia burgdorferi sensu stricto, is the most prevalent vector-borne disease in the United States. Treatment of rodent pathogen reservoirs with an oral acaricide may suppress the production of infected host-seeking ticks posing a risk for human infection. A previous study showed that an oral fipronil bait effectively controlled larval Ixodes scapularis ticks on white-footed mice (Peromyscus leucopus) up to 15 days post-bait exposure. The present study expands upon this finding by exposing group-housed white-footed mice to fipronil bait under simulated field conditions prior to tick infestation.

Methods: Mice (n = 80) were housed in groups of 10 within large enclosures and offered a choice between fipronil bait within a commercial bait station and an alternative diet. The mice were assigned to two treatment groups and two control groups to undergo bait exposure durations of either 24 h (reduced) or 168 h (extended). Groups were further differentiated by the time point post-bait exposure when larval ticks were applied to mice within feeding capsules (reduced day 1, day 15; extended day 21, day 35). For 4 days post-tick introduction, attached larvae were observed by microscopy and replete larvae were recovered. Replete larvae were monitored for molting success. Plasma was collected from all treatment group mice to obtain fipronil plasma concentrations (CP).

Results: The fipronil bait (0.005% fipronil) was palatable and controlled larval ticks on white-footed mice when presented under simulated field conditions. Efficacy in preventing attached larvae from feeding to repletion was 100% (day 1), 89.0% (day 15), 85.8% (day 21), and 75.2% (day 35). When also considering molting success, the fipronil bait prevented 100% (day 1), 91.1% (day 15), 91.7% (day 21), and 82.5% (day 35) of larvae attaching to mice from molting. The mean CP per mouse was 191.5 ng/ml (day 1), 29.4 ng/ml (day 15), 10.6 ng/ml (day 21), and 1.0 ng/ml (day 35).

Conclusions: The results suggest that fipronil bait will be consumed by white-footed mice in the presence of an alternative diet, and effectively control larval ticks on treated mice. A field trial is needed to confirm the results of this study. Low-dose fipronil bait may provide a cost-effective means of controlling blacklegged ticks to be integrated into tick management programs.

Keywords: Acaricides; Blacklegged ticks; Borrelia burgdorferi sensu stricto; Fipronil bait; Ixodes scapularis; Peromyscus leucopus; Simulated field conditions; Systemic insecticides; Vector control; White-footed mice.

Fig. 1

Conceptual diagram for the experimental design of the study. a Mice group-housed in a large enclosure and presented with bait station, alternative diet, water source, and shelter. b Control group mice received no fipronil bait prior to being transferred to the insectary for tick exposure. Larvae were allowed to feed to repletion and molt. c Treatment group mice were presented with bait stations loaded with fipronil bait. d Mice transferred to the insectary for tick exposure. Fipronil bait prevents larvae from feeding to repletion and molting

Fig. 2

Overhead view of an enclosure housing 10 treatment group mice. The bait station and alternative diet are positioned equidistance from the water source and shelter

Fig. 3

Replete blacklegged tick larvae collected from moats being dried on a sheet of filter paper

Fig. 4

Comparison of a engorging larvae feeding on control mouse, and b non-engorged, deceased larvae on treatment mouse. Engorging larvae are bloated and excreting red feces (green arrows). Non-engorged, deceased larvae are brown and desiccated (red arrows)

Fig. 5

a Test tube containing replete larvae and molted nymphs, b closer view of a test tube with several molted nymphs