Determination of a Discriminant Dose to Identify Resistance to Amitraz in Rhipicephalus sanguineus s.l. (Acari: Ixodidae) from Mexico

Simple Summary The brown dog tick is cosmopolitan with medical and veterinary importance. Control with acaricides is the commonly used alternative; however, its indiscriminate use can generate resistance. A discriminant dose (d.d) can be used to rapidly and inexpensively identify resistant populations in samples collected in the field; however, to date, there is limited information on the subject. This study aimed to determine the d.d of amitraz to identify resistance in larvae natives from Mexico and to evaluate its application in field-collected ticks. Because there was no reference strain, the search for samples susceptible to amitraz was carried out in naturally infested rural dogs using a larval immersion test (LIT); the d.d. was determined as a consensus value by multiplying the LC99 x 2, and then, we proceeded to evaluate it in in-field samples by using the LIT technique. The d.d. calculated was 4 ppm. The in-field evaluation found 64% of the samples resistant to amitraz, with mortality percentages between 98.3% and 0.35%; these samples were widely distributed in all the areas of study. With this, bases are established so that scrutiny can be initiated in order to document amitraz resistance in field populations. The brown dog tick, Rhipicephalus sanguineus s.l., is considered the most widely distributed three-host tick in the world and has medical and veterinary importance; the control of infestation is carried out with acaricides, towards which it can develop resistance. This study aimed to determine the discriminant dose (d.d) of amitraz to identify resistance in R. sanguineus s.l. larvae natives from Mexico and to evaluate its application in field-collected ticks. Engorged ticks were collected from naturally infested dogs residing in rural communities and were incubated for 25 days, and their progeny was used in a larval immersion test (LIT) to be exposed to the d.d. determined in Rhipicephalus microplus, and those that were susceptible were analyzed using the LIT in six concentrations. Mortality was analyzed through probit methodology to calculate the lethal concentration (LC) 50 and 99. The d.d. was determined as a consensus value by multiplying the LC99 x 2, and then, we proceeded to evaluate it in in-field samples by using the LIT technique. The d.d. calculated was 4 ppm. The in-field evaluation found 64% of the resistant samples to amitraz with mortality percentages between 98.3% and 0.35%. This dose can be used to rapidly and inexpensively identify resistant populations in samples collected in the field.