title: Susceptibility of Rhipicephalus (Boophilus) microplus to ivermectin (200, 500 and 630 μg/kg) in field studies in Brazil.
abstract: The present study aimed to determine the susceptibility of 17 Rhipicephalus (Boophilus) microplus populations, originating in the Southeast and Southern regions of Brazil, to different ivermectin concentrations (200, 500 and 630 μg/kg), administered through subcutaneous or topical (pour-on) routes. R. (B.) microplus populations from the states of Minas Gerais (seven populations), São Paulo (seven populations) and Paraná (three populations) were chosen for the tests. The selected cattle were allocated to treatment groups on day 0, and block formation was based on the arithmetic mean of female ticks (4.5-8.0 mm long) counted on three consecutive days (-3, -2 and -1). To evaluate the therapeutic and residual efficacies of these formulations, tick counts (females ranging from 4.5 to 8.0 mm long) were performed on days 3, 7 and 14 post-treatment, and continued on a weekly basis thereafter until the end of each experiment. The results obtained throughout this study, utilizing field efficacy studies, allowed us to conclude that the resistance of R. (B.) microplus against 200 and 500 μg/kg ivermectin is widely disseminated because all tick populations that had contact with these specific concentrations were diagnosed as resistant. However, it is possible to infer that R. (B.) microplus resistance against 630 μg/kg ivermectin was also widespread, diagnosed at six of ten analyzed properties. Resistance of these ectoparasites to 630 μg/kg ivermectin is most likely emerging in three other populations of R. (B.) microplus. Strategies of resistance management need to be quickly determined to keep the selection pressure at a minimum level in Brazil.
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil.
authors.forename: Breno Cayeiro
authors.initials: BC
authors.lastname: Cruz
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil; Universidade Federal de Goiás, Regional de Jataí, BR 364, Km 192, n° 3800, Parque Industrial Jataí, Goiás, Brazil. Electronic address: wdzlopes@hotmail.com.
authors.forename: Welber Daniel Zanetti
authors.initials: WD
authors.lastname: Lopes
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil.
authors.forename: Willian Giquelin
authors.initials: WG
authors.lastname: Maciel
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil.
authors.forename: Gustavo
authors.initials: G
authors.lastname: Felippelli
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil.
authors.forename: Flávia Carolina
authors.initials: FC
authors.lastname: Fávero
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil.
authors.forename: Weslen Fabrício Pires
authors.initials: WF
authors.lastname: Teixeira
authors.affiliation: Universidade Estadual de Maringá, Campus Regional de Umuarama, Paraná, Brazil.
authors.forename: Rafael Silveira
authors.initials: RS
authors.lastname: Carvalho
authors.affiliation: Universidade Estadual de Maringá, Campus Regional de Umuarama, Paraná, Brazil.
authors.forename: Maycon Araújo
authors.initials: MA
authors.lastname: Ruivo
authors.affiliation: Universidade Estadual de Maringá, Campus Regional de Umuarama, Paraná, Brazil.
authors.forename: Marcos Henrique Alcantara
authors.initials: MH
authors.lastname: Colli
authors.affiliation: Universidade Estadual de Maringá, Campus Regional de Umuarama, Paraná, Brazil.
authors.forename: Cláudio Alessandro Massamitsu
authors.initials: CA
authors.lastname: Sakamoto
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil.
authors.forename: Alvimar José
authors.initials: AJ
authors.lastname: da Costa
authors.affiliation: CPPAR - Animal Health Research Center, Faculdade de Ciências Agrárias e Veterinárias, UNESP, Campus de Jaboticabal, Via de acesso Prof. Paulo Donatto Castellane, s/n°, CEP: 14884-900, Jaboticabal, São Paulo, Brazil.
authors.forename: Gilson Pereira
authors.initials: GP
authors.lastname: De Oliveira
mesh_terms: D056810:Acaricides / Q000494:pharmacology*; D000818:Animals; D001938:Brazil; D004351:Drug Resistance; D007559:Ivermectin / Q000494:pharmacology*; D048494:Rhipicephalus / Q000187:drug effects*
chemical_list: D056810:Acaricides; D007559:Ivermectin
keywords: Avermectins; Cattle; Resistance; Ticks