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Title: Larval immersion tests with ivermectin in populations of the cattle tick Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) from State of Sao Paulo, Brazil

item Miller, Robert

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2006
Publication Date: 12/20/2006
Citation: Klafke, G.M., Sabatini, G.A., de Albuquerque, T.A., Martins, J.R., Kemp, D.H., Miller, R.J., Schumaker, T.T.S. 2006. Larval immersion tests with ivermectin in populations of the cattle tick Rhipicephalus (Boophilus microplus) (Acari: Ixodidae) from State of Sao Paulo, Brazil. Veterinary Parasitology. 142(3-4):386-390.

Interpretive Summary: In Brazil, the southern cattle tick represents the most important pathological constraint to livestock production in the country. In order to control these ticks, producers use chemical pesticides. Ivermectin is a pesticide that has been used in Brazil for many years and some treatment failures have occurred. This study is the fist documentation of ivermectin resistance in the Sao Paulo region of Brazil. This research has benefitted Brazil by isolating populations of ivermectin resistant ticks so that producers could change to a better chemical to control these ticks. The proper treatment of cattle leads to lower tick burdens and healthier cattle. The information obtained from this research will benefit the U.S. too. The southern cattle tick is endemic in Mexico and we are unsure if ivermectin resistance has developed there. This work in Brazil will help us to monitor the quarantine zone between Texas and Mexico for ticks resistant to ivermectin so that the proper adjustments can be made to keep this damaging tick out of the United States.

Technical Abstract: Larval immersion tests (LIT) with commercial formulation of ivermectin were carried out with larvae of two field populations of Rhipicephalus (Boophilus) microplus from commercial dairy farms of the State of Sao Paulo, Brazil and a susceptible strain (Porto Alegre) to differentiate resistant suspected and susceptible strains. One of the populations tested (Barra Alegre) showed a LC50 value significantly higher than the susceptible strain and a resistance ratio (CI95%) of 3.78 (3.47B4.12), leading to suspect that this population shows traces of resistance to ivermectin. Population Sao Francisco, with no records of ivermectin injections on cattle, showed no difference on the ivermectin response in relation to Porto Alegre strain characterizing it as susceptible. Although LIT is not yet recommended by FAO to diagnose resistance to acaricides, this technique was successful on the differentiation of resistance suspected population and a susceptible strain and can be used to detect populations of R. (B.) microplus resistant to ivermectin. This is the first report of R. (B.) microplus resistant to ivermectin detected by an in vitro bioassay.