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Heartworm Testing


Why does my dog need a heartworm test every year? Can’t I just buy the preventive from you without a prescription? – ER, Vermont

Dear ER,

To help answer your question, I turned to the American Heartworm Society (, an excellent resource for questions about the diagnosis, treatment and prevention of heartworm disease. According to the AHS, there are a couple of reasons why an annual heartworm test is a good idea. First, there have been situations where dogs looked perfectly healthy on the outside but were actually harboring heartworms on the inside. Giving monthly preventative to some of these dogs caused severe reactions. Second, an undiagnosed heartworm infection causes permanent damage to your dog the longer it’s there so the quicker it’s diagnosed the quicker it can be treated. Giving monthly preventive doesn’t treat the adult heartworms living in the heart, lungs and blood vessels of your dog. And third-let’s admit it-there will be the occasional time your dog doesn’t get his full monthly dose (you forgot to give it, he threw it right back up, etc.) so an annual test just makes sure some heartworms didn’t slip through the gap.

Here are a couple of the American Heartworm Society’s questions and answers on this topic:

Q: Why do dogs need to be blood tested before starting heartworm medication?
A: Before starting a preventive program, all dogs should be tested for heartworms. Giving preventives to dogs that have adult heartworm infection can be harmful or even fatal to the pet.
Adult heartworms produce millions of microscopic “baby” heartworms (called microfilaria) into the bloodstream. When you give a monthly heartworm preventive to a dog with circulating microfilaria, this can cause the sudden death of microfilaria, triggering a shock-type reaction. Even if your dog does not have this type of reaction, heartworm preventives do not kill the adult heartworms (although they may shorten the worms’ life expectancy). This means an infected dog will remain infected with adult heartworms.
Unfortunately, as long as a pet remains infected, heartworm disease will progress and damage the heart and lungs, which can lead to life threatening problems. Giving heartworm preventives to heartworm-positive dogs can mislead an owner into thinking everything is all right, while within a pet, heartworm disease is worsening.

Q: How often should I have my dog tested for heartworm infection?
A: Annual testing is recommended for several important reasons. First, many of us do not take our own medications as directed let alone medicate our pets. We’re busy; we forget; we miss a dose here and there. Second, even if you never miss a dose there is nothing to prevent your dog from eating some grass and vomiting up the medication you just gave. Your pet would be without protection for an entire month. Third, if your pet accidentally became infected with heartworms, your veterinarian needs to detect it as soon as possible before irreversible heart and lung damage occur. Early detection and treatment are always best. Finally, some heartworm tests now come combined with tests that monitor other significant diseases (Lyme disease, Ehrlichiosis and Anaplasmosis). With annual testing, you know your pet is heartworm free and not infected with these serious tick-borne diseases, some of which could affect family members. The tests are quick and accurate and make sure your pet is free from infection. Annual testing provides peace of mind in knowing that your pet is free of heartworms, and should your pet be infected, it assures you of early diagnosis.

Q: Do you need a prescription for heartworm preventive medication? If so, why?
A: Yes, heartworm preventives must be purchased from your veterinarian or with a prescription through a pet pharmacy. The U.S. Food and Drug Administration (FDA) labeling on heartworm preventives indicates that the medication is to be used by or on the order of a licensed veterinarian. This means a veterinarian must have a doctor-client-patient relationship in order to write a prescription. Typically, prior to prescribing a heartworm preventive, the veterinarian will perform a simple heartworm test to make sure your dog doesn’t already have adult heartworms. It is not necessary to test very young puppies prior to starting preventives since it takes approximately six months for adult heartworms to develop to adulthood in a dog. If the pet is free of heartworms, prevention is prescribed. Giving preventives to dogs infected with heartworms can lead to rare but possibly severe reactions that could be harmful or even fatal to the dog.

Lydia F. Gray, DVM, MA SmartPak Staff Veterinarian and Medical Director Dr. Lydia Gray has earned a Bachelor of Science in agriculture, a Doctor of Veterinary Medicine (DVM), and a Master of Arts focusing on interpersonal and organizational communication. After “retiring” from private practice, she put her experience and education to work as the American Association of Equine Practitioner’s first-ever Director of Owner Education. Dr. Gray continues to provide health and nutrition information to horse owners through her position at SmartPak, through publication in more than a dozen general and trade publications, and through presentations around the country. She is the very proud owner of a Trakehner named Newman that she actively competes with in dressage and combined driving. In addition to memberships in the USDF and USEF, Dr. Gray is also a member of the Illinois Dressage and Combined Training Association (IDCTA). She is a USDF “L” Program Graduate and is currently working on her Bronze Medal. Find Dr. Gray on Google+

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4 comments on “Heartworm Testing
  1. Mark says:

    It’s so important to get your dog checked. Adult worms live in the right side of the heart, continues to be a major problem for animals.If he has the following symptoms, then take him to the vet immediately:
    reluctance to exercise/play
    rapid breathing
    weight loss

    Just take him to the vet for a regular check up…

    • Steve says:

      I think Heartworm testing in the Northern US in Feb or March is a scam!!!! Perhaps someone can clear this up for me. If my dog was infested in early September, when the last mosquito is alive, wouldn’t my dog be dead or seriously ill by April when mosquitoes reappear? Also, if my dog is given Heartworm meds in Sept, there are NO mosquitoes alive in the Winter in the northern US, why would our dogs have to be tested? The answer at the surface seems to indicate the only reason would be to make the Vet more money. If I am incorrect, please correct me with FACTS. Thanks

  2. Martin says:

    My dog Guy was diagnosed September 9th and he was pclead on the waiting list. He just received his first treatment November 18th. I just can not see how something that is in such high demand and vital have this problem. The dosage my dog received came in from Europe. I hope the 2 months do not make a difference as my dog was in the later stages of this.

  3. Jacky says:

    Most veterinarians will remenmocd a yearly test for blood parasites – not just heartworm but other parasites including lyme disease and other ‘nasties’ – a vet will want to be certain before your dog continues with the heartworm preventative, that he does not test positive for heartworm as continuing with preventatives under those circumstances could have very serious consequences.If you are getting your prescription for heartworm from your vet, he must be sure that your dog is not heartworm positive to satisfy the requirements of the drug manufacturers as the medications stipulate that they are only to be prescribed to dogs who have tested negative for heartworm.Some vets will allow you to only have a test every two years for your dog if you are buying your prescription medicine through them, have bought all the doses for a year from them and are confident that you have not missed giving a dose.Personally, I would prefer an annual test for the peace of mind.

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