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United States Department of Agriculture

Agricultural Research Service


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Title: Adaptation of in vitro bioassays for the diagnosis of resistance to Fipronil and Ivermectin in R. (B.) microplus

item Castro Janer, Eleonor
item Schumaker, Teresinha Tizu
item Klafke, Guilherme
item Rifran, Laura
item Gonzalez, Patricia
item Niell, Carlos
item Namindome, Andre
item Gil, Andres
item Piaggio, Jose
item Martins, Joao Ricardo
item Mendes, Marcia Cristina
item Miller, Robert

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 6/1/2012
Publication Date: 8/1/2012
Citation: Castro Janer, E., Schumaker, T.S., Klafke, G.M., Rifran, L., Gonzalez, P., Niell, C., Namindome, A., Gil, A., Piaggio, J., Martins, J., Mendes, M., Miller, R. 2012. Adaptation of in vitro bioassays for the diagnosis of resistance to Fipronil and Ivermectin in R. (B.) microplus. In: Janer, C., Garrapata, E. Resistencia a fipronil e ivermectina en rodeos vacunos de Uruguay y Brasil. Instituto Nacional de Inbestigación Agropecuaria. 1-69.

Interpretive Summary: In Uruguay and Brazil, the southern cattle tick represents the most important pathological constraint to livestock production. In order to control these ticks the acaricides fipronil and ivermectin are used widely. However, treatment failures have occurred. It has not been possible to test ticks for resistance to ivermectin and fipronil in these countries because no standardized bioassay tests were in place. This research was conducted in Brazil and Uruguay to determine the best technique to use to detect resistance to ivermectin and fipronil in these countries. This study found that the larval immersion test was the best for the detection of resistance to ivermectin and fipronil. Tests on susceptible laboratory strains in Uruguay and Brazil produced predictable and repeatable results. The testing of many populations of field-collected ticks from both countries showed that many were susceptible to ivermectin and fipronil. However, at least 4 populations tested were found to be resistant to ivermectin and 5 populations were resistant to fipronil. This research benefited Uruguay and Brazil by isolating populations of Fipronil and Ivermectin-resistant ticks so that other treatment options could be used. This strategy has the potential to save the Brazilian and Uruguayan tick control programs money since time and effort will be spent using compounds that will actually control the tick population infesting cattle. Additionally, proper treatment of cattle leads to lower tick burdens and healthier cattle. These factors work together to increase the profits of producers.

Technical Abstract: R. microplus represents the most important pathological constraint to livestock production in Brazil and Uruguay. The infestation of cattle by ticks is controlled by chemical applications on a regular basis. Fipronil and ivermectin have been widely used in recent years to the benefit of cattle producers. Because of treatment failure, there is a concern that resistance to these chemicals has emerged in these countries. New bioassay techniques have been developed for the estimation of susceptibility to fipronil and ivermectin in R. microplus. These tests were evaluated with 15 strains of ticks collected in Uruguay and Brazil. The larval immersion test was generally more sensitive than the larval packet test to detect resistance in the strains assayed where resistance ratios 2 to 3 times greater were observed. Implications of ivermectin and fipronil resistance on the control of R. microplus in Brazil and Uruguay are discussed.

Last Modified: 09/23/2017
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