Ivermectin in Heartgard for Dogs

Heartworm in dogs is a potentially deadly disease, where parasitic worms of the species Dirofilaria immitis infest the heart and associated blood vessels. Luckily, it is also an easily preventable with monthly tablets, chews or topical liquids.

The ingredients most commonly used to prevent heartworm are from a group of chemicals called "macrocyclic lactones" (MLs). One of the most common of these is ivermectin, which is found in Heartgard and generic alternatives such as Nuheart. Ivermectin is derived from a sugar produced by a soil bacterium called Streptomyces avermitilis.¹

Ivermectin, like all MLs, works at different dose rates against different parasites, both internal and external. It is used in a wide range of animals, including cattle, horses, pigs and birds, as well as cats and dogs. For heartworm prevention in dogs, only very low doses are used, so its only use is in killing heartworm larvae, which are extremely sensitive to the chemical. Some preparations combine ivermectin with other active ingredients to target intestinal worms as well.

Ivermectin acts by interfering with nerve transmission in the parasite, causing a flaccid (floppy) paralysis and subsequent death.^1,2 Ivermectin effectively kills the young larval parasites which are migrating through the dog's body. The younger the parasites, the more effective the product. Ivermectin is not registered as being effective in killing older worms which have matured to adulthood and migrated to the dog's heart.

Ivermectin is known to be effective in killing microfilaria, the offspring produced by adult worms in the heart, however this is not currently a label indication of any products containing the chemical. It has also been suggested that it may sterilize adult female parasites.¹ Once-per-month treatment clears any larvae which have been contracted from a mosquito in the past four weeks.

Heartgard® also has registered a 'reach-back' effect. This is also likely to carry through to the generic brands. Thus if a dog has contracted heartworm in the previous four months, the medication will be effective in clearing the infection if it is continued monthly for at least 12 months afterwards.³ If doses are missed, you should contact your veterinarian for advice, as they may wish to schedule heartworm tests to confirm that the infection has successfully been cleared. (These tests cannot be done until approximately six months after infection has occurred).

All of the ML products have very wide safety margins in mammals, with adverse reactions being uncommon. Allergic reactions to the drug bases are more likely than reactions to the toxicity of the drugs themselves, when used as recommended.

The safety margin for ivermectin in most breeds of dog is well over 100 times the usual dose used for heartworm prevention. However in Collies it is about 16 times the usual dose, so a large overdose is required to evoke an adverse reaction even in this breed.² Some breeding lines of Collies are missing a particular protein carrier in their blood-brain barrier which regulates the entry of ivermectin and other MLs, so the drug can move freely into the brain and cause nervous problems.

In very young animals, the blood-brain barrier has not yet completely closed, so the drug can enter freely and cause nervous symptoms. This is one reason that ivermectin-based products are not recommended for newborn pups. They can be safely administered from six weeks of age.

At the doses needed for heartworm prevention, it is unlikely that any of these signs will occur, but if you do notice any of them, contact your veterinarian immediately.

Prevention is definitely better than cure in the case of heartworm. Ensure that your dog is on preventative medication throughout the heartworm transmission season. This may mean year-round treatment in some parts of the US. Consult your veterinarian for further advice and to discuss an appropriate treatment for your dog.

Heartgard® is a registered trademark of Merial
Nuheart® is a registered trademark of Pharmachem