Common questions and answers:
https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics

Do I need a prescription for my pet’s heartworm preventive medication? If so, why?:
Yes. The U.S. Food and Drug Administration (FDA) labeling on heartworm preventives states that the medication is to be used by or on the order of a licensed veterinarian. This means heartworm preventives must be purchased from your veterinarian or with a prescription through a pet pharmacy Prior to prescribing a heartworm preventive, the veterinarian typically performs a heartworm test to make sure your pet doesn’t already have adult heartworms, as giving preventives can lead to rare but possibly severe reactions that could be harmful or even fatal. It is not necessary to test very young puppies or kittens prior to starting preventives since it takes approximately 6 months for heartworms to develop to adulthood. If the heartworm testing is negative, prevention medication is prescribed.

How do monthly heartworm preventives work?:
Whether the preventive you choose is given as a pill, a spot-on topical medication or as an injection, all approved heartworm medications work by eliminating the immature (larval) stages of the heartworm parasite. This includes the infective heartworm larvae deposited by the mosquito as well as the following larval stage that develops inside the animal. Unfortunately, in as little as 51 days, immature heartworm larvae can molt into an adult stage, which cannot be effectively eliminated by preventives. Because heartworms must be eliminated before they reach this adult stage, it is extremely important that heartworm preventives be administered strictly on schedule (monthly for oral and topical products and every 6 months for the injectable). Administering prevention late can allow immature larvae to molt into the adult stage, which is poorly prevented.

At what age should puppies and kittens be started on heartworm prevention? What do I need to know about prevention in my new pet?:
The risk of puppies and kittens getting heartworm disease is equal to that of adult pets. The American Heartworm Society recommends that puppies and kittens be started on a heartworm preventive as early as the product label allows, and no later than 8 weeks of age.
The dosage of a heartworm medication is based on body weight, not age. Puppies and kittens grow rapidly in their first months of life, and the rate of growth—especially in dogs—varies widely from one breed to another. That means a young animal can gain enough weight to bump it from one dosage range to the next within a matter of weeks. Ask your veterinarian for advice about anticipating when a dosage change will be needed. If your pet is on a monthly preventive, you may want to buy just one or two doses at a time if a dosage change is anticipated (note that there is a sustained-release injectable preventive available for dogs 6 months of age or older). Also make sure to bring your pet in for every scheduled well-puppy or well-kitten exam, so that you stay on top of all health issues, including heartworm protection. Confirm that you are giving the right heartworm preventive dosage by having your pet weighed at every visit.

Are heartworms more common in certain areas of the United States?:
Heartworms have been found in all 50 states, although certain areas have a higher risk of heartworm than others. Some very high-risk areas include large regions, such as near the Atlantic and Gulf coasts, and along river tributaries. Most states have “hot spots” where the heartworm infection rate is very high compared with other areas in the same state. Factors affecting the level of risk of heartworm infection include the climate (temperature, humidity), the species of mosquitoes in the area, presence of mosquito breeding areas, and presence of animal “reservoirs” (such as infected dogs, foxes or coyotes).

More questions about heartworm disease are listed and HIGHLY recommend reading all of them at the above site. You MUST fully weigh prevention and treatment with our ME dogs but this is between you and your veterinarian. Make sure your veterinarian knows your dog has ME as many of our dogs can only use certain medications in treating this. Here is what Dr Kathy said: ” I am personally concerned w/ the injectable heartworm prevention, Proheart, because IF the dog has a reaction, the drug lasts 6 months, and there are not any treatments for that. I personally like Revolution or Advantage Multi topicals for those dogs who might have problems w/ the orals. ANY, and I repeat, ANY medication can cause side effects, however, we have to weigh the risks of NOT giving it, to the benefits.”