Macrocyclic lactone resistance in Dirofilaria immitis

a b s t r a c t Microfilariae were isolated from a Katrina rescue dog that remained microfilariaemic despite successful adulticidal treatments and repeated treatment with high doses of macro- cyclic lactones (MLs). The microfilariae were genotyped at two P-glycoprotein single nucleotide polymorphic sites which had been found to correlate with reduced sensitiv- ity 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 sur- vived repeated treatments with high doses of ML anthelmintics. The data show phenotypic and genotypic evidence of ML resistance in Dirofilaria immitis.