title: In vitro diagnosis of the first case of amitraz resistance in Rhipicephalus microplus in Santo Tomé (Corrientes), Argentina.
abstract: In Argentina, the cattle tick Rhipicephalus microplus has already developed resistance to organophosphates and synthetic pyrethroids. However, no cases of amitraz resistance have ever been recorded in this country despite its heavy use. A recent failure of amitraz to control ticks in a farm located in Santo Tomé, province of Corrientes, resulted in the collection of samples for acaricide resistance diagnosis. The modified Drummond adult immersion test (AIT) and the larval tarsal test (LTT) were performed separately in Argentina and Switzerland to evaluate efficacy of amitraz and other acaricides. The AIT showed that oviposition in the Santo Tomé field isolate was not inhibited when it was challenged to 250 and 500 ppm amitraz, and 50 ppm deltamethrin. However, oviposition was reduced by 90.6% when this field isolate was challenged to a combination of 400 ppm ethion and 100 ppm cypermethrin. To confirm the results obtained with the AIT, 2 additional tick samples were collected and shipped to Switzerland for resistance diagnosis of amitraz, cypermethrin and flumethrin, using the LTT. With this bioassay, the resistance ratios of the 2 field isolates were 32.5 and 57.0 for amitraz and between 5.9 and 27.2 for the synthetic pyrethroids. Both in vitro bioassays confirmed amitraz and synthetic pyrethroid resistance in the Santo Tomé samples. These results account for the first evidence of amitraz resistance in R. microplus in Argentina.
authors.affiliation: Instituto Nacional de Tecnología Agropecuaria (INTA), Instituto de Patobiología, CICVyA, Área de Parasitología, Dr. Nicolás Repetto y De los Reseros S/N, Castelar, Buenos Aires, CC25 (B1712WAA), Argentina. ccutulle@cnia.inta.gov.ar
authors.forename: Christian
authors.initials: C
authors.lastname: Cutullé
authors.forename: Léonore
authors.initials: L
authors.lastname: Lovis
authors.forename: Beatriz Inés
authors.initials: BI
authors.lastname: D'Agostino
authors.forename: Gabriel Gerardo
authors.initials: GG
authors.lastname: Balbiani
authors.forename: Gabriel
authors.initials: G
authors.lastname: Morici
authors.forename: Daniel
authors.initials: D
authors.lastname: Citroni
authors.forename: Julio
authors.initials: J
authors.lastname: Reggi
authors.forename: Jorge Luis
authors.initials: JL
authors.lastname: Caracostantogolo
mesh_terms: D056810:Acaricides / Q000494:pharmacology*; D000818:Animals; D001118:Argentina; D002417:Cattle; D002418:Cattle Diseases / Q000175:diagnosis* / Q000469:parasitology; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance; D005260:Female; D007814:Larva; D010058:Oviposition; D010270:Parasite Egg Count / Q000662:veterinary; D048494:Rhipicephalus / Q000187:drug effects* / Q000502:physiology; D013984:Tick Infestations / Q000175:diagnosis / Q000469:parasitology / Q000662:veterinary*; D014052:Toluidines / Q000494:pharmacology*
chemical_list: D056810:Acaricides; D014052:Toluidines; C014983:amitraz