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Title: Selection of an ivermectin-resistant strain of Rhipicephalus microplus (Acari: Ixodidae) in Brazil

item KLAFKE, GUILHERME - Sao Paulo State University (UNESP)
item AGUIAR DE ALBUQUERQU, THAIS - Sao Paulo State University (UNESP)
item Miller, Robert
item SATO-SCHUMAKER, TERESINHA - Universidad De Sao Paulo

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2009
Publication Date: 2/26/2010
Citation: Klafke, G.M., Aguiar De Albuquerqu, T., Miller, R., Sato-Schumaker, T.T. 2010. Selection of an ivermectin-resistant strain of Rhipicephalus microplus (Acari: Ixodidae) in Brazil. Veterinary Parasitology. 168(1-2):97-104.

Interpretive Summary: The cattle fever tick is a blood-feeding pest of cattle throughout the tropical and sub-tropical regions of the world. These ticks lower cattle production potential directly through reduced weight gain, due to blood loss from feeding, and indirectly through the transmission of diseases this tick carries. Ivermectin is a pesticide used commonly to control ticks on cattle. However, the development of resistance to ivermectin exposure in Brazilian ticks has reduced the effectiveness of this pesticide. In this study we determined the most effective method to increase the resistance level of these ticks under laboratory conditions. This will allow for further studies into the biological causes of ivermectin resistance and could lead to the development of new technologies to reduce or manage this resistance in the future.

Technical Abstract: Resistance to ivermectin (IVM) in field populations of Rhipicephalus microplus of Brazil has been observed since 2001. In this work, four selection methods (infestations with: (1) IVM-treated larvae; (2) larvae from IVM-treated adult female ticks; (3) larvae from IVM-treated adult female ticks on an IVM-treated host; and (4) larvae obtained from IVM-treated females that produced eggs with a high eclosion rate) were used on a field population with an initial ivermectin (IVM) resistance ratio at LC50 (RR50) of 1.37 with the objective to obtain experimentally a highly-resistant strain. After ten generations, using these methods combined, the final RR50 was 8.06. This work shows for the first time that it was possible to increase IVM resistance in R. microplus in laboratory conditions. The establishment of a drug resistant R. microplus strain is a fundamental first step for further research into the mechanisms of ivermectin-resistance in R. microplus and potentially methods to control this resistance.