Ticks are among the most important vectors of diseases affecting dogs [1, 2]. Brown dog ticks,, particularly Rhipicephalus sanguineus and Rhipicephalus linnaei, which are the most widely distributed globally [3–6]. Brown dog ticks are vectors of a range of bacterial and protozoan pathogens, causing canine ehrlichiosis and babesiosis, adversely affecting canine health worldwide [7]. With all stages feeding on dogs, their three-host life cycle can lead to an uncontrolled build-up of large ticks [8] [4].
Tick control depends on applying acaricidal compounds to animals [9]. In cattle, where ticks are repeatedly exposed to the same chemicals, selected highly resistant tick populations pose a significant global challenge to the livestock industry [10]. Plausible resistance mechanisms include target-site mutations and increased metabolic detoxification. Resistance to ticks in dogs is often anecdotal [11]. Although treatment failure may result from incorrect application of the acaricide, the persistence of ticks after correctly applied treatments indicates that resistance has developed [12]. Resistance can be confused with misuse of acaricides because resistance tests are not readily available, and this is the rationale behind creating a rapid test that can be used in a clinic or kennel next to the dogs [11].
Resistance in R.sanguineus ticks has been reported against different acaricides, particularly organophosphates, macrocyclic lactones, synthetic pyrethroids and phenylpyrazoles. The first report of resistance to the cyclodiene compound dieldrin was in 1954 in R. sanguineus ticks, not cattle ticks [13]. The American dog tick, Dermacentor variabilis, has also been found resisting dieldrin [14]. Furthermore, resistance to ivermectin, introduced in the mid-1980s, has also been reported in R. sanguineus ticks [15–18]. Reports of resistance against synthetic pyrethroids, sodium channel modulators that cause nerve excitation, have been documented in R. sanguineus ticks [16, 19, 20]. Finally, fipronil, a phenylpyrazole extensively used on dogs, has resulted in cases of resistance in dog ticks [16].
The Food and Agriculture Organization (FAO) recommends using the Larval Packet Test (LPT) for resistance detection and monitoring in ticks [21]. LPT employs a range of drug concentrations to cover a 0-100% mortality of larvae [22]. Serial dilutions of acaricides are impregnated into filter papers in triplicate with a mixture of olive oil and trichloroethylene as the solvent. For running the tests, engorged female ticks must be collected, and female ticks' weight must be above a certain threshold for producing eggs. Proper guidance is required in collecting, handling and submitting engorged ticks to diagnostic laboratories. For most tick species, incubation conditions for engorged female ticks for eggs and larval production should be 27°C and 85% relative humidity. The main drawbacks of the LPT are that the test is laborious, needs laboratory facilities and trained personnel, and takes at least six weeks before testing with larvae derived from engorged female ticks collected from animals in the field can be completed. Moreover, it demands a significant number of fully engorged female ticks collected simultaneously from the host, which may be difficult for companion animals.
Recently, we introduced RaTexT®, a Rapid Tick exposure Test, to detect acaricide resistance in livestock ticks (Jongejan et al., submitted for publication). The novel rapid test is based on the principle that free-living unfed ticks collected from the environment or partly engorged ticks recently removed from their hosts can be utilised to detect resistance by exposing them to lethal concentrations of an acaricidal compound. During their movement on the impregnated matrix, ticks get exposed to the acaricides, which mimics the behaviour of ticks that embark on an acaricide-treated host. Moreover, the test is also based on the resistance intensity protocol from the latest WHO guidelines for resistance detection in malaria mosquito vectors, which proposed evaluation under 1x, 5x and 10x drug concentrations, revealing a low, moderate, and high resistance intensity, respectively [23] [24].