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Giving Heartgard to a Heartworm Positive Dog

Giving Heartgard to a Dog

Veterinarians often warn dog owners about the dangers of giving heartguard to a heartworm positive dog. If you ever questioned why heartworm pills are only given by prescription and why veterinarians recommend yearly heartworm testing before refilling your dog's heartworm prescription, this article should help you understand why. Interestingly, heartguard has been used as part of a treatment plan for dogs suffering from heartworm, but the American Heartworm Society warns about its dangers and does not recommend it.

Gray muzzles need good dental care

Gray muzzles need good dental care

Heartgard is Not an Adulticide

Heartgard is a monthly medication that is given to dogs so to prevent heartworm larvae from developing into adults. When you give your dog his monthly Heartgard pill, it is killing all the microfiliaria (the immature forms of the worm) that have been growing in your dog's system in the past 30 days. These larvae are therefore killed before they have a chance to develop and travel to the dog's heart.

It's important to recognize that Heartguard cannot and will not kill any adult worms. To kill adult worms, you will need an adulticide drug such as immiticide (melarsomine dihydrochloride).

So to recap, heartgard will kill all the potential young heartworm larvae (microfiliaria) who may have been living in your dog's tissues in the past 30 days, preventing them from developing into adults. In order to keep the heartwom population in check, it's therefore important that Heartgard is given monthly. Heartgard is a microfilariocidal drug, meaning that it kills microfilaria. Heartgard is also considered a chemoprophylactic drug, meaning that it's a medication administered for the purpose of preventing disease.

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If you forgot to give your dog's heartworm pill on time, there are chances that if your dog has heartworm larvae, they will be able to develop and travel to your dog's bloodstream and reach your dog's heart. At this point, Heartgard is no longer capable of killing them. A series of immiticide injections will be needed to kill the worms before they have a chance to wreck havoc to the dog's body. Immiticide is therefore an adulticide drug, meaning that it's for the most part a pesticide designed to kill adult insects rather than their larvae.

Dangers to Dogs with Heartworm

Heartgard is a medication that can only be obtained by veterinary prescription, and in order to obtain the prescription, veterinarians are required (by law!) to have a valid "veterinary-client-patient relationship" before dispensing any medications. On top of that, the American Heartworm Society recommends that the dog must test negative for heartworms before being placed on heartworm preventative. It's therefore important for dog owners to understand that if the vet is refusing to fill up a prescription without seeing the dog, he or she is not being stubborn or trying to rip the owner off, but is abiding to the law.

If Heartgard was available over the counter without a prescription, there would be risks that dog owners would skip the heartworm test and would give a heartworm pill to a dog who is potentially heartworm positive. Even if dog owners are certain that they have religiously given their dogs their heartworm pill every single month, it's important to consider there have also been cases of heartworm resistance to preventative medications and sometimes, even with the best effort, it is possible that a dog vomits the heartworm pill up or spits it out without the owner ever knowing.

What's the problem with giving Heartgard to a dog that has heartworm disease ? If a dog is infected with a large quantity of microfilaria, their sudden death can trigger a shock-type reaction in the bloodstream that can be life threatening. On top of that, since Heartgard cannot kill the adult worms, these would continue to thrive causing the disease to worsen, explains veterinarian Dr. Christine M.

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"Annual testing is necessary, even when dogs are on heartworm prevention year-round, to ensure that the prevention program is working.. Heartworm preventives are highly effective, but not 100 percent effective. If you don’t get your dog tested, you won’t know your dog needs treatment." American Heartworm Society 

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Treating Heartworms with Heartguard

The standard way to treat dogs with heartworms is for veterinarians to give a series of immiticide injections for the purpose of killing the adult worms in the heart. For mild heartworm disease, two injections are usually given 24 hours apart followed by a period of rest, while for more serious cases, one injection is given, followed by a period of rest, and then, a month later 2 injections are given 24 hours apart.

Dogs often undergo concomitant doxycycline therapy for the purpose of eliminating wolbachia, a bacteria known for playing an aggravating role in heartworm disease producing inflammation.

Another treatment option known as the "slow kill" protocol consists of putting the dog on monthly Heartgard (ivermectin) which will kill only the larvae in the blood stream, while the adults will be killed over the course of 2 to 3 years or even more. While this may seem like a cheaper and most cost-effective option, it's important understanding that, in the meanwhile, the adult heartworms will continue to cause damage to the dog's heart and lungs with possibly long-term repercussions.

Did you know? Studies have shown that a quarter of all pets fail to get all their heartworm doses as they should and this gap leaves room for the development of heartworm disease.

did you know

A Risky Treatment Option

life expectancy of addison disease in dogs

For sake of comparison, both heartworm treatment options can be compared to the methods employed by smokers to quit smoking. On one hand, you have those who quit smoking cold turkey and therefore prevent further damage to their heart and lungs, and on the other hand, you have those who gradually lower their amount of cigarettes smoked every day over the course of several years.

In this latter case, yes, the cigarette amount is lowered gradually, but the cigarettes will continue to cause damage to the smoker's heart and lungs, explains veterinarian Dr. Christian K in this excellent example.

On top of worms being allowed to continue to cause damage and potential scarring to the dog's heart and lungs, the slow kill method is very long, and even after 18 months, there are risks that it may not eliminate all adult worms. Because of these risks, the American Heartworm Society does not recommend any slow-kill methods that entail the use of continuous monthly administration of heartworm preventives.

" I am not a big believer in the "slow kill" approach to treating heartworms, because during the entire multi-year process, the adult heartworms are causing damage to the heart and lungs, damage which may be irreversible and life-threatening." Dr. Drew


  • American Heartworm Society, Prevention, Diagnosis, and Management of Heartworm Infection in Dogs
  • McCall JW, Guerrero J, Roberts RE, et al. Further evidence of clinical prophylactic, retroactive (reach back) and adulticidal activity of monthly administrations of ivermectin (Heartgard PlusTM) in dogs experimentally infected with heartworms. In Seward RL (ed): Recent Advances in Heartworm Disease: Symposium 2001. Batavia, IL: American Heartworm Society, 2001, pp 189-200.

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