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Heartworm Preventive FAQs

Drs. Foster & Smith Educational Staff
Dewormer Comparison Chart 
Dog Deworming Guidelines 
Heartworm Disease: Signs, Treatment, Prevention 
Heartworm Prevention in Dogs

1.) Why do I need a prescription for heartworm preventives? By federal law, a veterinarian-client-patient relationship must be established (your veterinarian-pet parent-and your dog) and a prescription is required for a heartworm preventive. Your veterinarian needs to determine your dog's heartworm status before she writes a prescription for heartworm preventives. If you give a dog with heartworm disease any heartworm preventive, this could be harmful to the dog.

2.) Can I use any natural preventives for heartworm? There are no natural preventives for heartworm that have been shown to be truly effective.

3.) How do monthly heartworm preventives work? Monthly medications are given every 30 days. Monthly heartworm preventives interfere with the development of the immature heartworm and prevent it from maturing into an adult. They have a retroactive effect, which means they will eliminate the immature heartworms that have infested the dog in the preceding month. We recommend that you give your dog his monthly heartworm preventives year round. There are three reasons we recommend this:

  • To ensure the medicine has been given properly by the pet owner.
  • Due to climate change, transmission of heartworm disease can occur nearly year round.
  • To treat and control intestinal parasites, since some monthly heartworm preventives have activity against intestinal parasites.

4.) Which heartworm preventives can be used to eliminate intestinal parasites? Are these products stronger than other heartworm preventives? Certain heartworm preventive products, such as Heartgard Plus and Tri-Heart Plus are also effective in removing specific intestinal parasites and are labeled for these uses. These products are no stronger, and have no more of the active drug (Ivermectin) than products that are not effective against parasites other than heartworm. The preventives that are effective against other parasites either contain an additional ingredient, such as Pyrantel Pamoate, effective against internal parasites, or contain an active ingredient that, by itself, is effective against a number of different parasites - an example is Selamectin in Revolution. Selamectin kills adult fleas, ear mites, American Dog ticks, and Sarcoptic mange mites in addition to preventing heartworm disease. These products meet the same safety requirements as the preventives that are effective against heartworm only.

Heartgard PlusHeartgard Plus, a preventive that contains Ivermectin and Pyrantel.
Sentinel ®Sentinel, contains Milbemycin Oxime to prevent heartworm and control some intestinal parasites.

5.) Do I need to worry if I have forgotten to give my dog his heartworm preventive? Monthly heartworm preventives are effective against the heartworm larvae (the immature heartworm) that accumulate in the dog's body in the month prior to giving the dose of preventive. If an owner forgets to give the monthly heartworm preventive, we recommend that the owner contact their veterinarian to determine if dosing should be resumed or if the dog needs to be tested for possible heartworm infection prior to restarting the preventive. In all cases of missed doses, the dog should be heartworm tested seven months after resumption. A heartworm test can only detect the presence of the heartworm parasite after it is at least 7 months old.

For additional information, view our "How to Manage Heartworm" video.


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