Comparison of three larval bioassays to evaluate susceptibility of Rhipicephalus (Boophilus) microplus to amitraz

Tânia Regina Bettin Santos,Guilherme Marcondes Klafke,Felipe Geraldo Pappen,Leandro Quintana Nizoli,Patrícia Biegelmeyer,Nara Amélia Rosa Farias

doi:10.1590/s1984-29612013000400008

Tânia Regina Bettin Santos, Guilherme Marcondes Klafke + Show 4 more

Open Access

https://doi.org/10.1590/s1984-29612013000400008

Copy DOI

Abstract

Field samples of Rhipicephalus (Boophilus) microplus from the state of Rio Grande do Sul, Brazil, were assessed using the following methods: larval packet test (LPT), larval immersion test (LIT) and syringe immersion test (SIT). The following parameters were determined for each population and for the Mozo susceptible reference strain: lethal concentration for 50% (LC50) with its 95% confidence interval (95% CI), regression line slope and resistance ratio (RR). Using the LPT, only one population was susceptible to amitraz, presenting a RR of 1.9. Using the same technique, the other populations presented RRs of between 92.9 and 3445.8 and were considered resistant. The LC50 of the Mozo strain calculated using the LPT, LIT and SIT was 2.9, 27.3, and 52.7 µg/mL, respectively. In general, a good fit to the probit statistical model was only achieved using the LPT. The results obtained in this study impair recommendations for using the LIT and SIT to diagnose amitraz resistance in R. (B.) microplus populations. Additional studies are required to improve the sensitivity of these tests in relation to the LPT.

Highlights

Parasitism by Rhipicephalus (Boophilus) microplus has a high impact on cattle production in tropical and subtropical areas around the world
The results from the larval packet test (LPT), larval immersion test (LIT) and syringe immersion test (SIT) are respectively presented in Tables 1, 2 and 3
The lethal concentration for 50% (LC50) of amitraz of the Mozo strain calculated for the LPT, LIT and SIT were 2.9, 27.3 and 52.7, respectively

Summary

Introduction

Parasitism by Rhipicephalus (Boophilus) microplus has a high impact on cattle production in tropical and subtropical areas around the world This is aggravated by the increasing numbers of acaricide-resistant tick populations. Amitraz is the acaricide most used in the southern region of the state of Rio Grande do Sul, Brazil (VARGAS et al, 2003; FARIAS et al, 2008; SANTOS et al, 2009). It has some advantages over other acaricides, such as low toxicity to cattle and short withdrawal periods for milk (one day) and meat (14 days). Reduced in vitro efficacy of amitraz against R. (B.) microplus field populations has been noticed since 2000 (FARIAS et al, 2008)

Objectives

Methods

Results

Conclusion