Microfilariae were isolated from a Katrina rescue dog that remained microfilariaemic despite successful adulticidal treatments and repeated treatment with high doses of macrocyclic lactones (MLs). The microfilariae were genotyped at two P-glycoprotein single nucleotide polymorphic sites which had been found to correlate with reduced sensitivity to MLs. The genetic polymorphism (GG-GG), previously found to be associated with insensitivity to MLs in vitro, was present at a frequency of 45.3% in microfilariae that survived repeated treatments with high doses of ML anthelmintics. The data show phenotypic and genotypic evidence of ML resistance in Dirofilaria immitis.
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