The disease can only be spread between pets by mosquitoes. When a mosquito bites and feeds on the blood of an infected pet it ingests microfilariae. The mosquito becomes a host while the microfilariae mature. When the same mosquito bites another pet it infects the healthy pet with heartworm larvae. The larvae migrate through the pet's tissues and circulatory system, eventually reaching the heart and lungs where adult worms grow and reproduce. - See more at: http://www.vetwest.com.au/pet-library/heartworm-disease#sthash.qaOSbj5P.dpuf