|He, Louis - Haiqi|
Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2007
Publication Date: 9/4/2007
Citation: Chen, A.C., He, H., Davey, R.B. 2007. Mutations in a putative octopamine receptor gene in amitraz-resistant cattle ticks. Veterinary Parasitology. 148:379-383. Interpretive Summary: The southern cattle tick (SCT) and the disease it transmits, babesiosis, were eradicated from the US after a 50-year eradication program of dipping cattle and temporarily vacating tick-infested pastures. SCT is still widespread in Mexico and other parts of America. Reintroduction of the tick to the US either unintentionally or by active agro-terrorism could prove devastating to cattle industry. Quarantine by vat dipping with chemical acaricides has prevented reinfestation in the US. But SCT has developed resistance to many acaricides. One acaricide that is still efficacious against SCT is amitraz, but SCT is showing some resistance to amitraz in Australia, Brazil and Mexico. It is imperative that we know the resistance mechanism to an acaricide so that we can design effective tests for assessing resistance. In this report we have found mutations, in amitraz-resistant ticks, in a cellular receptor that is believed to be the target of amitraz. Further work will demonstrate whether these mutations are responsible for amitraz resistance. If confirmed to be resistance mechanism, these mutations can be developed into accurate and sensitive tests for amitraz resistance. Having accurate tests will lead to better pest management strategies and prolong the usefulness of the currently available pesticides.
Technical Abstract: The mode of action of amitraz is thought to be its toxic effects on a receptor for a neuromodulator, octopamine. Resistance could arise from modifications of this receptor so that it would not be affected by amitraz. A putative octopamine receptor cDNA was cloned and sequenced from a cattle tick in Australia. However, when the sequence was compared between Australian strains of amitraz-susceptible and resistant ticks, no differences were detected. We have sequenced this putative octopamine receptor gene in tick strains from America. The American ticks have a sequence almost identical to that of the Australian ticks. In a Brazilian strain, the Santa Luiza strain that is very resistant to amitraz, there are two nucleotide substitutions that result in amino acids different from the susceptible strain, thus raising the possibility of alternative mechanisms of resistance.