Susceptibility of Amblyomma sculptum, Vector of Rickettsia rickettsii, Ticks from a National Park and an Experimental Farm to Different Synthetic Acaricides

Abstract

Amblyomma sculptum is a relevant tick species from a One Health perspective, playing an important role as a vector of Rickettsia rickettsii, the main agent of spotted fever rickettsiosis in Brazil. In this study, we evaluated the susceptibility of two A. sculptum populations from Goiás state (midwestern Brazil) to different acaricides. The first tick population (GYN strain) originated from an experimental farm, where the ticks are annually exposed to acaricides. The second (PNE strain) was collected in a national park (Emas National Park), where the ticks had not been exposed to acaricides. Immersion tests were conducted with 21-day-old laboratory-reared larvae and nymphs originating from adult ticks collected in the areas mentioned above. The chosen acaricides were two synthetic pyrethroids (cypermethrin and deltamethrin), one organophosphate (chlorfenvinphos), one formamidine (amitraz), and two combinations of pyrethroids and organophosphates (cypermethrin, chlorpyrifos and citronellal; cypermethrin, fenthion and chlorpyrifos). Mortality data were used to determine the lethal concentration (LC) values at which 50%, 90%, and 99% of the ticks died (LC₅₀, LC₉₀, and LC₉₉, respectively), and resistance ratios (RR) were calculated based on the LC values. The RR revealed differences between the acaricide-exposed (GYN) and unexposed (PNE) tick strains. The PNE strain larvae and nymphs were susceptible to all the tested acaricides. The GYN strain larvae were tolerant to cypermethrin, whereas the nymphs were tolerant to deltamethrin, chlorfenvinphos, and the combination of cypermethrin, chlorpyrifos, and citronellal (2 < RR ≤ 10). The GYN strain nymphs were resistant to amitraz (RR > 10). This is the first report of A. sculptum nymphs with resistance to amitraz and tolerance to deltamethrin, chlorfenvinphos, and the combination of cypermethrin, chlorpyrifos, and citronellal.

Keywords: Amblyomma cajennense complex; amidines; amitraz; organophosphates; pyrethroids; resistance.

Figure 1

Location of tick collection sites in Goiás state (Cerrado biome), midwestern Brazil. (A) Experimental farm of the School of Veterinary and Animal Science of the Federal University of Goiás (EVZ/UFG) (red). (B) Emas National Park (blue).

Figure 2

Lethal concentrations (LC₅₀, LC₉₀, and LC₉₉) either in ppm (A–D) or percentage (E,F) for Amblyomma sculptum larvae from Goiás state, which were exposed to different acaricides using larval immersion tests. (A) Deltamethrin. (B) Cypermethrin. (C) Chlorfenvinphos. (D) Amitraz. (E) Chlorpyrifos + cypermethrin + citronellal. (F) Chlorpyrifos + cypermethrin + fenthion. Error bars represent 95% confidence intervals. An asterisk denotes statistically significant differences (p < 0.05).

Figure 3

Lethal concentrations (LC₅₀, LC₉₀, and LC₉₉) either in ppm (A–D) or percentage (E,F) for Amblyomma sculptum nymphs from Goiás state, which were exposed to different acaricides using nymphal immersion tests. (A) Deltamethrin. (B) Cypermethrin. (C) Chlorfenvinphos. (D) Amitraz (LC₅₀ and LC₉₀ of PNE are very low, thus not visible; for details, see Table 4). (E) Chlorpyrifos + cypermethrin + citronellal. (F) Chlorpyrifos + cypermethrin + fenthion. Error bars represent 95% confidence intervals. An asterisk denotes statistically significant differences (p < 0.05).