Amitraz is a formamidine pesticide that has been shown to be efficacious against ticks. However, there are signs that the southern cattle tick, Boophilus microplus has developed resistance to amitraz in Australia, South America and Mexico. The mode of action of amitraz is thought to be its toxic effects on a receptor for a neuromodulator, octopamine. The tick would become resistant if this receptor is modified so that it would not be affected by amitraz. A putative octopamine receptor cDNA has been cloned and sequenced in Australia. However, when the sequence is compared between Australian strains of amitraz-susceptible and resistant ticks, no differences are detected. We have studied this putative octopamine sequence in tick strains from the Americas. We found that the American ticks have a sequence almost identical to that of the Australian ticks. A significant finding in our investigation is that in a Brazilian strain, the Santa Luiza strain that is very resistant to amitraz, there are two nucleotide substitutions which result in amino acids different form the susceptible strains. Further study is necessary to show that these substitutions are responsible for the amitraz resistance. Studies with synergists have shown that mechanisms of resistance other than target site modification, such as increased detoxification may be involved in amitraz resistance. These might account for the amitraz resistance in Australian strains.
Species 1: Acari Ixodidae Boophilus microplus (southern cattle tick)
Keywords: octopamine, receptor
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