Abstract:
BACKGROUND: One bioassay for detecting acaricide resistance in livestock ticks is the adult immersion test (AIT), wherein
engorged ticks are briefly immersed into a solution of a particular acaricidal compound and examined for mortality, their
egg-laying capacity and offspring hatchability in vitro. Usually, the recommended label dose or an established discriminating dose of an acaricide is used to determine high mortality (≥95%) of susceptible tick strains. Such a test intends to detect
the presence of resistance in a tick population. However, the adult immersion test does not directly translate the bioassay
results to the predicted efficacy in the field. In this paper, we used the AIT as an initial screening bioassay supplemented
with the resistance intensity test (RIT), a novel larval-based bioassay, wherein the resistance level can be determined
and translated to the expected field efficacy. This was done by adopting World Health Organisation (WHO) guidelines
for resistance detection in mosquitoes, which combines a 1×recommended dose with 5×and 10×concentrated doses
to reveal low, moderate and high resistance intensity, respectively.
METHODS: Engorged Rhipicephalus microplus ticks were collected from cattle at six different ranches across Rio Grande
do Sul, Brazil, as part of the state’s acaricide resistance surveillance program. Groups of adult ticks from each field collection were subjected to the AIT from each field sample. Additionally, engorged female ticks from each ranch were allowed
to lay eggs, and their larval progeny aged 14 to 28 days were then used in the RIT. Deltamethrin and a combination
of cypermethrin, chlorpyrifos and piperonyl butoxide were used in both tests, and the results were statistically analysed.
RESULTS: The in vitro efficacy of deltamethrin against adult ticks in the AIT ranged between 8.74% and 25.38%. The
corresponding RIT results on their larval progeny indicated a high resistance level. In the immersion test, the in vitro
efficacy of the combination of cypermethrin, chlorpyrifos, and piperonyl butoxide against adult ticks ranged
between 49.31% and 100%. The corresponding RIT results on their larval progeny indicated a similar response ranging
from fully susceptible to low or moderate resistance. The Pearson correlation coefficient (r=0.883) showed a high correlation between tick mortality at the 1×recommended concentrations of acaricides in both tests.
CONCLUSIONS: The resistance intensity test is a valuable addition to the range of bioassays currently available
for detecting acaricide resistance by determining the level of acaricide resistance. This is relevant to whether or
not to continue using a particular acaricidal class for controlling cattle ticks.
