Ask your vet. I work in rescue and many of the dogs have Heartworm it takes 6 weeks to 3 months of treatment and it is toxic and painful to the dog. The dogs have to be keep away from the other dogs and special foot coverings are use when cleaning their pens and the poop is cleaned up right away so the other dogs are not at risk. Even dogs on heartworm can get it if they eat poop with the eggs in it (yuck). Here in the South it is a big problem, in Southern California my vet did not recomend it. So it is best to ask your vet the preventive costs very little.