Dogs and cats get heartworm disease when an infected mosquito bites them. The heartworm then gets inside of their body and can reproduce, which only worsens your pet’s symptoms. The illness is much more prevalent in dogs, but cat owners also need to know about the symptoms so they can prevent and treat it if necessary. Puppies can start on preventive heartworm medication at eight weeks old without any type of testing. As six months of age, a puppy needs to test negative for heartworm infection before a veterinarian can prescribe preventive medication.