Research Article: The current status of resistance to alpha-cypermethrin, ivermectin, and amitraz of the cattle tick (Rhipicephalus microplus) in Ecuador

Date Published: April 7, 2017

Publisher: Public Library of Science

Author(s): Richar Rodríguez-Hidalgo, Ximena Pérez-Otáñez, Sandra Garcés-Carrera, Sophie O. Vanwambeke, Maxime Madder, Washington Benítez-Ortiz, Luciano Andrade Moreira.


Rhipicephalus microplus is widely distributed in tropical and subtropical areas of the world where livestock is a principal activity with great veterinary and economic importance. In Ecuador, this hematophagous ectoparasite has been observed between 0 and 2600 masl. One of the main tick control measures is the use of acaricides, which have been indiscriminately used worldwide and in Ecuador. In this country, no studies on acaricide resistance in Rhipicephalus microplus have been published. The current study aims to characterise the level of resistance of R. microplus against three main acaricides commonly used in Ecuador i.e. amitraz, alpha-cypermethrin and ivermectin to determine the level and pattern of dose-responses for R. microplus in 12 field populations (farms). The level of acaricide resistance was evaluated using three different bioassays: adult immersion test (AIT), larval package test (LPT) and larval immersion test (LIT), as recommended by the FAO. The predictive dose-responses were analysed by binomial logistics regression of the larval survival rate (resistance). In general, we found resistance of 67% for amitraz; 50% for alpha-cypermethrin and from 25 to 42% for ivermectin in the twelve field populations analysed. Resistance levels were studied in larval and adult bioassays, respectively, which were slightly modified for this study. For larval bioassays based on corrected mortality i.e. high (above 51%), medium (21–50%) and low (11–20%) resistance, percentages less than 10% were considered as susceptible. For the adult test, two resistance levels were used i.e. high (more than 76%) and medium (51 to 75%) resistance. Percentages lower than 50% were considered as susceptible. In this context, for larval bioassays, amitraz showed 21%, 38% and 8% for high, medium and low resistance, respectively. Alpha-cypermethrin presented 8%, 4 and 38% for high, medium and low resistance, respectively. Ivermectin presented 8%, 25% and 8% for high, medium and low resistance, respectively. For adult tests with amitraz 50% and 17% of the field populations showed average and high resistance, with evidences of average resistance to alpha-cypermethrin in 50% of the samples and average resistance against ivermectin in 25% of the farms. No statistical difference amongst the three bioassays was found and acaricide resistance was confirmed by logistic regression analysis; hence resistance (dose-responses) in each field populations differed, depending on the choice of the acaricide, frequent usage, frequency of treatment and farm management. The effective estimated dose needed to eliminate 99% of ticks is higher than dose recommended by the manufacturer. In conclusion, amitraz showed the highest resistance followed by ivermectin and alpha-cypermethrin and reveals differences on resistance in each individual field population. This information is important in order to establish the monitoring of resistance on each farm individually, contributing to the rational use of acaricides included in an integrated control program for R. microplus.

Partial Text

In Ecuador, livestock farming is one of the main economic activities with 75% of Ecuadorian livestock farms located in tropical and subtropical coastal areas and in the Amazon region. These farms are infested or at risk to be infested by ticks [1], and acaricidal treatment are part of their control programs [2]. Rhipicephalus microplus is the main cattle tick in Ecuador, and is distributed between 0 and 2600 meters above sea level (masl) [3,4]. Amblyomma spp. and Ixodes spp. are present but are less important than Rhipicephalus microplus.

All bioassays performed in this study demonstrated elevated resistance in R. microplus to amitraz, alpha-cypermethrin and ivermectin. According to these protocols as defined by the FAO (2004), the resistance percentage was based on the calculation of the percentage of eggs produced per female and the percentage of hatching eggs or surviving larvae (Table 2). Resistance was found for each of these criteria. As expected, resistance values are higher for amitraz than for other compounds. This relates to the continuous and massive use of amitraz in the 1990’s [2]. Amitraz and alpha-cypermethrin are the recommended acaricides to control ticks. In Ecuador, ivermectin is used to control external and internal parasites but its use in production animals is forbidden, because of its residual effect. This is probably the reason why resistance to this acaricide is lower than amitraz but higher than alpha-cypermethrin. In this survey, one population did not show resistance to any of the products tested. This likely relates to the farm management system relying on chemical products different from those tested in this study.




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