Heartworm preventive medications are used to periodically kill larval heartworms that have managed to gain access to the dog’s body. At this point, the products available are intended for monthly use, with the exception of Proheart6 which is a biannual injection. This means each time they are given they kill all the heartworm larvae (stage L3 and L4) that have accumulated in the past month. Some products offer the ability to kill older larvae, which helps keep the pet protected in case the heartworm preventive medication is given late. There are many topical and oral choices.

There are currently many choices, topical, oral and even injectable; plus, while the subject is canine heartworm prevention, many of the products discussed have feline formulations as well. We have organized them here based on their active ingredients:

IVERMECTIN Based Products – oral (Heartgard, Heartgard Plus, Iverhart Plus, Iverhart Max, Tri-Heart Plus, Pet Trust Plus)
MILBEMYCIN Based Products – oral (Sentinel, Sentinal Spectrum, Trifexis, Interceptor, Interceptor Plus)
SELAMECTIN Based Products – Topical (Revolution)
MOXIDECTIN Based Products – Topical and injectable (Advantage Multi, Proheart6)

Whichever product you use, begin your heartworm season with a heartworm test to be sure your dog is negative and prevention can begin.

The approval of ivermectin in 1987 represented a huge breakthrough in heartworm prevention. For the first time, preventive medication could be given once a month instead of daily. These medications use an extremely low dose of ivermectin, which is adequate to kill any L3 and L4 larval stages (baby heartworms) that are inhabiting the pet’s skin tissues at the time the medication is given.  In other words, infection takes place but is halted every month when the medication is given.

If Given to a Heartworm Positive Dog by Accident

In most cases, no reaction of any kind occurs when an ivermectin-based heartworm preventive is given to a heartworm-positive dog.

In fact, giving an ivermectin-based heartworm preventive to an infected dog is the first step in heartworm infection treatment. Ivermectin kills the developing larval worms (the incoming baby heartworms) and in some instances clears the circulating microfilariae (the newborn larvae born to the established adult worms).

While usually infected dogs do well when started on Ivermectin products, problems can arise if the larval worms die too quickly. If this happens, a life-threatening shock-like circulatory reaction can occur. For this reason the American Heartworm Society recommends that the first dose of ivermectin be given under veterinary supervision. 

This does mean, however, that giving this product to a dog with heartworm will kill all circulating microfilariae and the dog will test erroneously heartworm negative by Difil or Knott’s testing. (ELISA test kits should still be accurate.) In addition to killing microfilariae, ivermectin will also suppress reproduction in the adult female worms and shorten the overall life span of adult worms. Ivermectin does not kill adult heartworms (just the immature ones) though, as said, it cuts their life expectancy.

Because just a single missed dose of a heartworm prevention can result in anew infection, annual heartworm testing with a microscopic check for microfilariae is the recommendation of the American Heartworm Society to ensure that pets remain heartworm negative and are not accidentally given medication when they have ‘quiety’ become positive due to a missed dose, product failure or an animal spitting out the drug unbeknownst to the owner.