Comparison of three larval bioassays to evaluate susceptibility of Rhipicephalus (Boophilus) microplus to amitraz

Abstract

Field samples of Rhipicephalus (Boophilus) microplus from the state of Rio Grande do Sul, Brazil, were assessed using the following methods: larval packet test (LPT), larval immersion test (LIT) and syringe immersion test (SIT). The following parameters were determined for each population and for the Mozo susceptible reference strain: lethal concentration for 50% (LC 50 ) with its 95% confidence interval (95% CI), regression line slope and resistance ratio (RR). Using the LPT, only one population was susceptible to amitraz, presenting a RR of 1.9. Using the same technique, the other populations presented RRs of between 92.9 and 3445.8 and were considered resistant. The LC 50 of the Mozo strain calculated using the LPT, LIT and SIT was 2.9, 27.3, and 52.7 µg/mL, respectively. In general, a good fit to the probit statistical model was only achieved using the LPT. The results obtained in this study impair recommendations for using the LIT and SIT to diagnose amitraz resistance in R. (B.) microplus populations. Additional studies are required to improve the sensitivity of these tests in relation to the LPT.Keywords: Rhipicephalus (Boophilus) microplus, amitraz, bioassays, resistance diagnosis. ResumoAmostras de Rhipicephalus (Boophilus) microplus coletadas à campo no Rio Grande do Sul, Brasil, foram analisadas pelos seguintes métodos: teste do pacote de larvas (TPL), teste de imersão de larvas (TIL) e teste de imersão em seringas (TIS). Os seguintes parâmetros foram determinados para cada população e para a amostra referência suscetível Mozo: concentração letal para 50% (CL 50 ) e seu intervalo de confiança de 95% (IC95%), inclinação da reta de regressão e os fatores de resistência (FR). Pelo TPL, apenas uma população foi sensível ao amitraz, com FR de 1,9. Utilizando a mesma técnica, as outras amostras apresentaram FR entre 92,9 e 3445,8 sendo consideradas resistentes. As CL 50 da cepa Mozo calculadas por meio do TPL, TIL e TIS foram 2,9, 27,3 e 52,7 µg/mL, respectivamente. De forma geral, a adequação ao modelo estatístico de probitos só foi alcançada com o uso do TPL. Os resultados obtidos no presente estudo limitam a recomendação de uso do TIL e TIS para diagnóstico de resistência ao amitraz em populações de R. (B.) microplus. Estudos adicionais são necessários para aprimorar a sensibilidade destes testes em relação ao LPT.Palavras-chave: Rhipicephalus (Boophilus) microplus, amitraz, bioensaio, diagnóstico de resistência.

Ten years later: Evaluation of the effectiveness of 12.5% amitraz against a field population of Rhipicephalus (Boophilus) microplus using field studies, artificial infestation (Stall tests) and adult immersion tests

“…In Brazil, this compound was introduced in 1987 and the first case of resistance was diagnosed in 1994 by Francisco Branco (reported by Martins et al, 2003). Resistance to amitraz is now a problem for cattle producers around the world and has been reported in various locations including Australia (Jonsson and Hope, 2007), Mexico (Rodriguez-Vivas et al, 2006), Brazil (Li et al, 2005;Martins et al, 2003;Santos et al, 2013) and New Caledonia (Chevillon et al, 2007). The prevalence of resistance to amitraz was estimated to be 11% in Australia (Jonsson and Hope, 2007) and 19.4% in Mexico (Rodriguez-Vivas et al, 2006).…”

Histone Methyltransferase DOT1L Is Involved in Larval Molting and Second Stage Nymphal Feeding in Ornithodoros moubata

“…The inhibitor SGC0946 was used in a larval immersion test (LIT) using a protocol established for testing acaricides [ 32 , 33 , 34 ]. Four groups of 60 O. moubata unfed larvae each were included in this assay, one control group treated with DMSO and three experimental groups treated with different concentrations of SGC0946, 0.1 μM, 1 μM, and 10 μM.…”

The current status of resistance to alpha-cypermethrin, ivermectin, and amitraz of the cattle tick (Rhipicephalus microplus) in Ecuador

“…Larvae were carefully placed, using brushes, in different concentrations of acaricides, for 10 minutes. Additionally, a control group was submerged in distilled water [10]. Then they were dried on absorbent paper for 30 minutes, and subsequently packaged in individual envelopes, sealed and placed in the hatchery at 27°C and 85% relative humidity.…”

“…Then they were dried on absorbent paper for 30 minutes, and subsequently packaged in individual envelopes, sealed and placed in the hatchery at 27°C and 85% relative humidity. The mortality rate was read 72 hours later [10,21]. …”

“…In Brazil, Mexico, and Colombia, several studies have been undertaken, showing the resistance and/or vulnerability to organ-phosphorated and organ-chlorinated compounds, pyrethroids (deltamethrin, cypermethrin, flumethrin, alpha-cypermethrin, and lambda-cyhalothrin), methyl carbamate and recently ivermectin [10–12]. According to observations of the "National Survey of Brucellosis, Bovine Tuberculosis and Ticks" carried out by the Public Health and Zoonosis Research Institute (unpublished data), amitraz, ivermectin and a recent alternative, alpha-cypermethrin, [2] are the acaricides extensively used in Ecuador in about 42%, 39%, 24% of farms, respectively.…”