Abstract: Disclosed are nucleic acid molecules from the genome of Dirofilaria spp. nematodes that contain single nucleotide polymorphisms related to reduced responsiveness of the nematodes to macrocyclic lactones. In one example, the species of Dirofilaria is Dirofilaria immitis (the agent of heartworm in animals). Also disclosed are methods for determining the responsiveness of Dirofilaria spp. nematodes to macrocyclic lactones, methods for selecting a treatment to treat an animal infected with a Dirofilaria spp. nematode, and kits for determining the responsiveness of Dirofilaria spp. nematodes to macrocyclic lactones.