2. Academic Validation
  3. Standardization of larval bioassays for determination of fluralaner susceptibility in Rhipicephalus microplus

Standardization of larval bioassays for determination of fluralaner susceptibility in Rhipicephalus microplus

  • Vet Parasitol. 2025 Sep 4:340:110600. doi: 10.1016/j.vetpar.2025.110600.
Guilherme Klafke 1,  Jorgelina Torrents 2,  Priscila Teixeira Ferreira 3,  Macarena Sarli 4,  Ana Luiza Loch 5,  Barbara Rauta de Avelar 6,  Thais Ribeiro Correia Azevedo 7,  Fabio Barbour Scott 8,  Nicolás Morel 9,  Santiago Nava 10,  José Reck 11
Affiliations

Affiliations

  • 1 Instituto de Pesquisas Veterinárias Desidério Finamor, Eldorado do Sul, Rio Grande do Sul, Brazil. Electronic address: guilherme-klafke@agricultura.rs.gov.br.
  • 2 Instituto Nacional de Tecnología Agropecuaria, Estación Experimental Agropecuaria Rafaela (INTA E.E.A. Rafaela), e Instituto de Investigación de la Cadena Láctea (IDICAL, INTA-CONICET), Rafaela, Santa Fe, Argentina. Electronic address: torrents.jorgelina@inta.gob.ar.
  • 3 Instituto de Pesquisas Veterinárias Desidério Finamor, Eldorado do Sul, Rio Grande do Sul, Brazil; Universidade Federal Rural do Rio de Janeiro, Instituto de Veterinária, Seropédica, Rio de Janeiro, Brazil. Electronic address: priscilateixeiraferreira@gmail.com.
  • 4 Instituto Nacional de Tecnología Agropecuaria, Estación Experimental Agropecuaria Rafaela (INTA E.E.A. Rafaela), e Instituto de Investigación de la Cadena Láctea (IDICAL, INTA-CONICET), Rafaela, Santa Fe, Argentina. Electronic address: sarli.macarena@inta.gob.ar.
  • 5 Instituto de Pesquisas Veterinárias Desidério Finamor, Eldorado do Sul, Rio Grande do Sul, Brazil. Electronic address: analuizaloch29@gmail.com.
  • 6 Universidade Federal Rural do Rio de Janeiro, Instituto de Veterinária, Seropédica, Rio de Janeiro, Brazil. Electronic address: barbararauta@gmail.com.
  • 7 Universidade Federal Rural do Rio de Janeiro, Instituto de Veterinária, Seropédica, Rio de Janeiro, Brazil. Electronic address: thaisrc@ufrrj.br.
  • 8 Universidade Federal Rural do Rio de Janeiro, Instituto de Veterinária, Seropédica, Rio de Janeiro, Brazil. Electronic address: scott.fabio@gmail.com.
  • 9 Instituto Nacional de Tecnología Agropecuaria, Estación Experimental Agropecuaria Rafaela (INTA E.E.A. Rafaela), e Instituto de Investigación de la Cadena Láctea (IDICAL, INTA-CONICET), Rafaela, Santa Fe, Argentina. Electronic address: morel.nicolas@inta.gob.ar.
  • 10 Instituto Nacional de Tecnología Agropecuaria, Estación Experimental Agropecuaria Rafaela (INTA E.E.A. Rafaela), e Instituto de Investigación de la Cadena Láctea (IDICAL, INTA-CONICET), Rafaela, Santa Fe, Argentina. Electronic address: nava.santiago@inta.gob.ar.
  • 11 Instituto de Pesquisas Veterinárias Desidério Finamor, Eldorado do Sul, Rio Grande do Sul, Brazil. Electronic address: jose.reck@gmail.com.
PMID: 40915046 DOI: 10.1016/j.vetpar.2025.110600
Abstract

Fluralaner is an isoxazoline acaricide with potent activity against Rhipicephalus (Boophilus) microplus, a major tick species affecting cattle in South America. In this study, larval bioassays were performed to evaluate the baseline susceptibility of R. microplus populations from Brazil and Argentina to fluralaner. The larval packet test (LPT) was applied to Brazilian laboratory and field populations, while the larval immersion test (LIT) was used in Argentina. Both tests were carried with technical grade and a commercial formulation of fluralaner. LPT results showed consistent LC₅₀ values across two susceptible laboratory strains, with resistance ratios near unity in all field samples. Similar observations were obtained with the LIT, with resistance ratios also near 1 for all tested populations from Argentina. The LIT produced LC₅₀ values approximately 100 times lower than the LPT, a difference consistent with previous studies using systemic acaricides. Both technical and commercial formulations yielded comparable results in both tests. These findings confirm the high susceptibility of R. microplus to fluralaner and establish reliable bioassay parameters for future resistance monitoring. The choice between LPT and LIT and technical grade or commercial formulation of fluralaner should be guided by laboratory infrastructure and logistical needs, as both methods are valid for surveillance purposes.

Keywords

Acaricide; Cattle; Chaco; Diagnostics; Dose-response; Pampa; Resistance.

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