Diagnostic performance of a Rapid Tick exposure Test (RaTexT) to detect acaricide resistance in cattle ticks in East Africa.

Background: The Rapid Tick Exposure Test (RaTexT) is a new method for detecting acaricide resistance in cattle ticks. This test provides rapid pen-side results based on the exposure of partially engorged adult ticks to a specially designed acaricide-impregnated matrix. RaTexT has been utilized in Brazil, where it identified resistance to deltamethrin in both laboratory colonies and field strains of Rhipicephalus microplus. The resistance levels in adult ticks tested in Brazil corresponded with those in larvae when using the resistance intensity test (RIT), a modification of the FAO-recommended larval packet test. In this paper, RaTexT was validated in East Africa using laboratory colonies of cattle ticks from Tanzania and field-collected ticks from Uganda. The resistance levels in adult ticks measured by RaTexT were compared with those in larvae using the RIT against synthetic pyrethroids, organophosphates, and formamidines.

Methods: The diagnostic validation involved 15,400 adult cattle ticks distributed across 110 RaTexT boxes and approximately 99,000 larval cattle ticks in 110 RIT tests conducted in Tanzania (n = 45) and Uganda (n = 65). In Tanzania, semi-engorged adult ticks and larvae from two laboratory colonies of R. decoloratus and one strain of R. appendiculatus were tested using RaTexT and the RIT. In Uganda, semi-engorged adult R. decoloratus ticks were collected from cattle and immediately tested with RaTexT in the field. The larval progeny of fully engorged ticks, collected simultaneously from the same cattle, were tested six weeks later under laboratory conditions. Statistical analysis consisted of a combination of categorical (Z-test, Kappa) and continuous (Bland-Altman, CCC, regression) agreement analyses between RaTexT and RIT.

Results: The results of deltamethrin tests with laboratory ticks in Tanzania and field ticks in Uganda were highly consistent, exhibiting the same high resistance level in adults and larvae after 24 h of exposure. The cypermethrin/chlorpyrifos/PBO tests demonstrated that laboratory tick strains were fully susceptible when the exposure time in RaTexT was extended to 72 h. In Uganda, field strains demonstrated high resistance to cypermethrin/chlorpyrifos/PBO in RaTexT while showing low resistance in RIT. The chlorfenvinphos tests revealed that laboratory strains of R. decoloratus were susceptible after 48 h of exposure in RaTexT. Both tests identified a low resistance level in adults and larvae of the laboratory strain of R. appendiculatus ticks. Resistance to chlorfenvinphos was confirmed in R. decoloratus collected in the field in Uganda, where the resistance level in RaTexT consistently exceeded that in RIT. Comparisons of both tests with amitraz showed that laboratory R.decoloratus ticks were susceptible after an extended exposure of 96 h. In field ticks, RaTexT detected resistance against amitraz, with the resistance level in RaTexT consistently exceeding that in RIT. The two-proportion Z-Test (P > 0.01) indicated that no significant differences existed between the percentage mortality in 72 out of 168 comparisons between RaTexT and RIT (42.9%). Cohen's Kappa statistical analysis of the entire dataset demonstrated moderate to substantial agreement between RaTexT and RIT for detecting resistance in cattle ticks between 48 and 72 h of tick exposure. RaTexT demonstrated adequate repeatability, since variance between test boxes was negligible. Overall statistical analysis revealed that RaTexT can serve as a reliable proxy for RIT, provided that exposure time and acaricidal mode of action are adequately considered in the test design.

Conclusions: RaTexT detected resistance to three different acaricidal classes in one-host and multi-host cattle ticks in East Africa. The test can differentiate between resistant and susceptible ticks and potentially become a useful decision-support tool for tick control management.