title: Applicability of in vitro bioassays for the diagnosis of ivermectin resistance in Rhipicephalus microplus (Acari: Ixodidae).
abstract: The applicability of laboratory bioassays to diagnose ivermectin (IVM) resistance in Rhipicephalus microplus was evaluated. Adult immersion tests (AITs), larval immersion tests (LITs) and larval packet tests (LPTs) were performed to characterise the effects of ivermectin toxicity on adults and larvae of a susceptible reference strain. The AIT was determined to be a reasonable assay but requires a large number of individuals to attain interpretable results. The LIT and LPT were validated with an IVM resistant strain, revealing resistance ratios (RRs) of 6.73 and 1.49, respectively. In a field survey, nine different populations of cattle tick from the states of São Paulo and Mato Grosso do Sul, Brazil, were analysed with the LIT. Populations without previous exposure to ivermectin exhibited RRs between 0.87 and 1.01. Populations previously exposed to IVM showed RRs between 1.83 and 4.62. The LIT was more effective at discriminating between resistant and susceptible populations than the LPT. The use of the LIT is recommended for the diagnosis of ivermectin resistance in R. microplus.
authors: Departamento de Parasitologia, Instituto de Ciências Biomédicas, Universidade de São Paulo, Av. Prof. Lineu Prestes, 1374, 05508-900 São Paulo, Brazil. gmklafke@usp.br
authors: G M
authors: GM
authors: Klafke
authors: E
authors: E
authors: Castro-Janer
authors: M C
authors: MC
authors: Mendes
authors: A
authors: A
authors: Namindome
authors: T T S
authors: TT
authors: Schumaker
mesh_terms: D000818:Animals; D000977:Antiparasitic Agents / Q000494:pharmacology*; D001681:Biological Assay / Q000592:standards*; D001938:Brazil; D004351:Drug Resistance; D005260:Female; D007559:Ivermectin / Q000494:pharmacology*; D007814:Larva / Q000187:drug effects; D007928:Lethal Dose 50; D015203:Reproducibility of Results; D048494:Rhipicephalus / Q000187:drug effects*
chemical_list: D000977:Antiparasitic Agents; D007559:Ivermectin