Heartworm Prevention


Heartworm disease (dirofilariasis), transmitted by mosquitoes and primarily affecting domestic dogs and wild canids, remains the most serious parasitic disease affecting dogs in North America.1


A heartworm preventative comes as a flavored chewable tablet, non-flavored tablet, or topical liquid. Such preventatives are effective against the larval stage of the heartworm, prohibiting development to the adult stage. Because heartworm preventatives are effective for 30 days following a potential infection, each month’s dose is essentially being administered to prevent infection from the bite of any heartworm-infected mosquito a dog may have come in contact with during the previous month. Therefore, it is important that if a dose is missed, one be administered immediately and a monthly regimen resumed. Diagnostic testing should be performed six (6) months following the resumption of a heartworm preventative.2

Furthermore, because mosquitoes can be present in the environment all year, including indoors; it is important a heartworm preventative be administered year-round. This recommendation is also maintained by the US Food and Drug Administration (FDA)3 and the American Heartworm Society.2

Refilling Your Dog’s Heartworm Preventative Prescription

If your dog is up-to-date for an annual examination and heartworm test, it is not necessary to call ahead before coming in to refill a heartworm preventative prescription.

Important Information Regarding Manufacturer’s Guarantees & Rebates

Manufacturer’s guarantees and rebates are only applicable if the heartworm preventative is purchased at a veterinary hospital.

In the rare event one’s dog develops heartworm disease while regularly receiving their preventative, the guarantee typically includes compensation for adulticide treatment along with free heartworm preventative. Annual heartworm disease testing is required for eligibility, and depending on the frequency at which a dog was tested as a puppy; the guarantee may also cover veterinary services related to treatment.

If you would like more information regarding this guarantee, please ask.

A manufacturer’s rebate is available when purchasing a year supply (12 doses) of heartworm preventative.

Additionally, a higher-value rebate is available when purchasing heartworm preventative along with oral the Flea & Tick Preventative from the same manufacturer.


Heartworm disease is transmitted when a mosquito ingests microfilariae, or pre-larvae,4 of Dirofilaria immitis nematodes (a general classification for roundworms) in the superfamily Filarioidae, while feeding from a heartworm-infected mammal.5 In addition to domestic dogs; wild canids microfilariae can reside within include coyotes, foxes, and wolves; while other potential hosts include cats, ferrets, sea lions,68 horses, beavers;8 and very rarely, humans.56

Microfilariae ingested by the mosquito migrate from the midgut (intestine) to the malpighian tubule system (part of the mosquito’s digestive system) within 48 hours, where further development into first-stage larvae occurs in roughly four (4) days.7 Four (8) to six (6) days later, first-stage larvae molt to the second-stage, and in another four to six days molt into third-stage juvenile larvae. These larvae migrate to the mosquito’s mouthparts where they are deposited into a host’s skin during the mosquito’s next blood meal.257

Third-stage larval worms molt to fourth-stage larva within three (3) days after entering the host and remain in connective tissue for roughly five (5) months before a final molt to fifth-stage immature adult worms, at which point they migrate to the pulmonary arteries of the heart.9 Mature adult female worms can grow to around 12 inches long (males, somewhat shorter),8 and begin producing microfilariae six (6) to nine (9) months following infection.29


Symptoms of heartworm disease include cough, shortness of breath (dyspnea), unwillingness to exercise, fatigue after moderate activity, decreased appetite, and weight loss.26 Additionally, due to increased blood flow during periods of activity, moderate activity can accelerate progression of the disease or exacerbate other symptoms.2

Mature adult worms can physically obstruct the vessels, chambers, and valves of the heart, impairing both circulatory and respiratory function and causing permanent heart damage; while microfilariae can obstruct capillaries and further impede circulatory function,9 potentially leading to heart failure and death.2

Furthermore, prolonged mature adult worm burdens can lead to permanent damage to the heart.9


Testing for heartworm disease, as part of our in-house Laboratory service, indicates the presence of heartworm antigen, secreted by the mature adult female heartworm.

We recommend testing for heartworm disease at the same time as tick-borne diseases, using a single test for heartworm, Lyme, Anaplasmosis, and Erlichiosis.

Testing Requirements for Heartworm Disease

Testing for heartworm disease in dogs is required yearly, if a previous test result is unknown or can not be verified, and/or if heartworm preventative is halted for any reason. This also ensures a negative test result can be confirmed prior to a dog undergoing an anesthetic procedure.

Proof of annual testing is also required by the manufacturer of heartworm preventative to obtain the most comprehensive guarantee in the rare event one’s dog develops heartworm disease while regularly receiving their heartworm preventative. This guarantee typically includes compensation for veterinary services, adulticide treatment, and free heartworm preventative; however, eligibility may require additional testing six (6) months following the administration of the first dose dependent on your dog’s age. Please ask if you would like more information regarding this manufacturer guarantee.


The American Heartworm Society (AHS) publishes guidelines for the prevention, diagnosis, and management of heartworm disease; revised regularly as knowledge of heartworm infection increases based on new research and clinical experience.10

Nevertheless; management of heartworm disease depends on a variety of factors, including severity of infection and progression of the heartworm life cycle, varying in scope and duration as a result.29 Therapy for dogs infected with mature adult heartworms relies primarily on the injectable arsenical (arsenic-containing compound), melarsomine dihydrochloride, but requires confirmation of effectiveness and may require follow-up management.2

Additional Resources

View additional heartworm-related Pet Owner Resources provided by the American Heartworm Society.

Read more information about Heartworms from the Companion Animal Parasite Council (CAPC).


  1. ^ Bowman DD, Atkins CE. “Heartworm Biology, Treatment, and Control.” Vet Clin N Am-Small 2009; 39: 1127-1158. doi:10.1016/j.cvsm.2009.06.003.
  2. ^ a b c d e f g h i Nelson CT, McCall JW, Carithers D, eds. 2014 Canine Heartworm Guidelines. Wilmington, DE: American Heartworm Society; 2014. https://heartwormsociety.org/images/pdf/2014-AHS-Canine-Guidelines.pdf.
  3. ^ US Food and Drug Administration. Prevent Heartworms in Pets Year-Round. http://www.fda.gov/AnimalVeterinary/ResourcesforYou/UCM371377. Published October 23, 2013.
  4. ^ Bogitsh BJ, Carter CE, Oeltmann TN. Human Parasitology 3rd ed. Burlington, MA: Elsevier; 2005: 364.
  5. ^ a b c Roberts LS, Janovy J Jr. Foundations in Parasitology 8th ed. New York, NY: McGraw-Hill; 2010: 474.
  6. ^ a b c Heartworm Basics. American Heartworm Society. https://heartwormsociety.org/pet-owner-resources/heartworm-basics.
  7. ^ a b Foster WA, Walker ED. Mosquitoes (Culicidae). In: Mullen GR, Durden LA, eds. Medical and Veterinary Entomology 2nd ed. St. Louis, MO: Elsevier; 2009: 244.
  8. ^ a b c Life Cycle of D. immitis. Centers for Disease Control and Prevention. http://www.cdc.gov/parasites/dirofilariasis/biology_d_immitis.html. Updated February, 2012.
  9. ^ a b c d e Bowman DD. Georgis’ Parasitology for Veterinarians 9th ed. St. Louis, MO: Elsevier; 2009: 214-215.
  10. ^ Support AHS. American Heartworm Society. https://heartwormsociety.org/support-ahs.