Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




(also called “Chemoprophylaxis”)

IVERMECTIN Based Products (Heartgard, Heartgard Plus, Iverhart Plus, Iverhart Max, Tri-Heart Plus, Pet Trust Plus)

MILBEMYCIN Based Products (Sentinel, Trifexis, Interceptor, Interceptor Plus)

SELAMECTIN Based Products (Revolution)

MOXIDECTIN Based Products (Advantage Multi, Coraxis, Proheart6, Proheart12)

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 biannual injection. This means that they kill all the heartworm larvae (stage “L3” and “L4”) that have accumulated in the past month each time they are given. Some products offer the ability to kill older larvae which helps keep the pet protected in case someone is late giving the heartworm preventive medication at some point. There are presently many choices, topical, oral and even injectable, plus, while the subject of this page is canine heartworm prevention, many of the products discussed have feline formulations as well.


  • HEARTGARD®, HEARTGARD PLUS® made by Merial
  • IVERHART PLUS®, IVERHART MAX® made by Virbac
  • TRI-HEART PLUS® made by Merck
  • PET TRUST PLUS® made by FidoPharm







The approval of ivermectin in 1987 represented a huge breakthrough in heartworm prevention. Preventive medication for the first time could be given once a month instead of daily. These medications utilize an extremely low dose of ivermectin which is adequate to kill any L3 and L4 larval stages which 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 administered.


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 and clears the circulating microfilariae thus rendering the dog unable to spread its infection and minimizing the number of adult worms to be killed in the second phase of treatment when the adult worms are specifically addressed.

If the larval worms die too quickly, a shock-like circulatory reaction can occur so for this reason the American Heartworm Society recommends that the first dose of ivermectin be given under veterinary supervision. This allows the dog to be observed for several hours following the oral dose in case of trouble. That said, in most cases no reaction of any kind occurs and the larval worms are cleared without event. 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.


The Reach Back Effect

There is also a phenomenon called the “Reach back effect.” This means that if a dog goes off heartworm preventive medication for a prolonged period (four months was the time tested), re-starting preventive could still preclude adult heartworm infection in the heart and pulmonary arteries. In the 1988 experiment by Atwell, dogs who went off heartworm preventive for four months and then restarted with ivermectin had 95% fewer adult heartworms than dogs who went without ivermectin (though it should be noted that some heartworms were still able to establish infection). This means that if one skips several doses of ivermectin accidentally, it is still worth picking up where one left off.


Other Parasites Covered

Ivermectin at the heartworm preventive dose is not strong enough to kill common intestinal parasites. Because of this fact, pyrantel pamoate, a dewormer, was added to cover hookworms and roundworms in the original Heartgard product. As other ivermectin-based products have entered the market, these have also added pyrantel pamoate to extend the spectrum of protection. Whipworms are not covered by any of the ivermectin containing products at this time but, in order to remain competitive in the market, manufacturers may pay for treatment for whipworm infections acquired while their product is administered. The products containing both ivermectin and pyrantel pamoate are Heartgard Plus®, Iverhart Plus®, and Tri-Heart Plus®. Iverhart Max® includes both pyrantel pamoate and praziquantel so as to cover tapeworms as well.


Breed Sensitivity

There are breed-related sensitivities with ivermectin (i.e. collie-related breeds have some difficulties) though at the very low doses used in the prevention of heartworm disease are not a problem for any breed.


Use of Large Animal Products

It is neither safe nor legal to obtain large animal ivermectin products for use in dogs for heartworm prevention. An assortment of doses have circulated around on the internet and in other sources advocating the use of highly concentrated ivermectin formulas for heartworm prevention in dogs. These doses are not comparable to the miniscule doses in licensed heartworm preventive products and using them represents an element of gambling. Large animal ivermectin products are vastly more concentrated than those meant for dogs and it becomes problematic to dilute them properly. Even small doses of these products are unnecessarily high and if they are inadvertently given to a sensitive individual death can result.

For information on these products from their manufacturers visit:



  • SENTINEL® made by Virbac

This product is also given monthly, also clears microfilariae, acts by killing all L3’s and L4’s accumulated in the month prior to administration, and will suppress female worm’s ability to reproduce. There are a few important differences to note between this product and the ivermectin-based products, though.

If given to a heartworm positive dog by accident

If milbemycin is inadvertently given to a dog with active heartworm infection, the microfilariae are killed much faster than with the ivermectin products. This might sound like a good thing but in fact it increases the likelihood of the previously mentioned shock-like reaction when all the first stage larvae die all at once. In a dog with a light infection this might not be important but in a heavily infected dog it is safer not to use milbemycin to clear the microfilariae.


Of course, heartworm preventives are meant to be used in heartworm negative dogs. If these products are used according to their labeled instructions, this issue should never arise. Milbemycin-based preventives are safe and highly effective in preventing heartworms in dogs that are heartworm negative to begin with.

The Reach Back Effect

When milbemycin is given to a dog after a prolonged period without heartworm preventive (the Atwood experiment), the dog can be expected to have 41% fewer heartworms than if heartworm prevention was not resumed. This was not as good a result as with the ivermectin products because ivermectin is better at killing older heartworm larvae. If one finds oneself in the situation of having skipped several months of heartworm prevention in the middle of heartworm season, one might do better to restart an ivermectin-based product rather than a milbemycin-based product.


Other Parasites Covered

Milbemycin, however, does not require the addition of other dewormers in order to provide a broad spectrum of parasite control The milbemycin products control roundworms, hookworms, and whipworms without the addition of a second parasiticide. Milbemycin is also available combined with lufenuron for the control of fleas in the form of Sentinel®. Lufenuron is an oral flea sterilizer which prevents any fleas feeding on the dog from laying viable eggs. It is also available as Sentinel Spectrum® which adds praziquantel to regularly kill any tapeworms the dog has contracted from its flea infestation.

Milbemycin can also be used to treat demodectic mange. A specific dosing schedule is needed to accomplish this; heartworm preventive doses are not adequate. That said, the isoxazoline class of flea and tick products has largely eclipsed other medications in the treatment of demodectic mange. For more information on the treatment of demodectic mange, click the link above.


For more information on Sentinel®, Novartis Animal Health has a Heartworm section on their web site at:


For more information on Trifexis, Elanco has a web site for this product at:

For more information on Interceptor, Elanco has put up:


  • (REVOLUTION® made by Zoetis (formerly Pfizer Animal Health)

Ivermectin’s entrance onto the anti-parasite warfront represented a culmination in the trend for broader and broader spectrum parasite control. Selamectin is a closely related cousin of ivermectin. It is designed for broad coverage of small animal parasites and will protect dogs not only against heartworm but also against ear mites, sarcoptic mange mites, ticks, and fleas. Cats are protected against heartworm, fleas, ear mites, roundworms, and hookworms. The product is topical, applied monthly and is fully approved for safe use in heartworm infected animals. Selamectin is not as effective at clearing microfilariae as other products and thus is not generally used in the treatment of active heartworm infections.


For more information on Revolution (from the manufacturer), visit:


  • ADVANTAGE MULTI® made by Bayer
  • PROHEART6® and PROHEART12 made by Zoetis
  • CORAXIS made by Bayer

Moxidectin is another relative of ivermectin. There are presently four products that use moxidectin to prevent heartworm infection: Advantage Multi® which is available for both dogs and cats as a topical, Coraxis® which available for dogs as a topical, and Proheart6® and Proheart12 which are available for dogs as long-acting injections.

Advantage Multi® represents the combination of moxidectin with imidocloprid, the flea-killing ingredient in Advantage®, to create a broad spectrum anti-parasite product for both dogs and cats. Obviously both Advantage® and Advantage Multi® are made by the same manufacturer (Bayer). Advantage Multi® prevents heartworm infection, kills roundworms, hookworms, and whipworms. The imidocloprid present will kill the pet’s fleas. Advantage Multi® actually carries approval for use in heartworm positive dogs, meaning that it can be used to kill microfilaria in an active heartworm infection. Coraxis® is a similar product except that it consists only of moxidectin and has no flea-killing properties; it is a topical dewormer and heartworm preventive only. Coraxis® is made only for dogs.

Proheart6® is an injection given once every six months, obviating the need for the owner to remember to use a monthly product. The moxidectin is contained in special "microspheres" enabling the drug to last a full six months (or in the case of Proheart12, a larger volume is given and it lasts 12 months). Veterinarians and their technical staff must undergo a special certification in order to administer the injection. It is not recommended for sick or debilitated dogs nor for dogs positive for heartworm infection and puppies must be at least 6 months old to receive the injection of Proheart6 and at least 12 months old for the injection of Proheart12.

Proheart6® is also effective in controlling hookworm infection.

Advantage Multi       Coraxis


For more information on Proheart6® please visit:

For more information on Advantage Multi® visit:

 For more information about Coraxis visit:


Obviously the answer to this question is regional. Indeed it may be simplest to just use preventive medications all year round or to see what your regular veterinarian recommends for your area.

There is more to transmission than simply the presence of mosquitoes; it must also be warm enough for a long enough time period to allow the development of microfilariae to infective L3’s within the mosquito’s body. A simple formula involves counting the degrees above 57 degrees F reached each day. Each degree is called a “heartworm development unit” and when 234 heartworm development units have accumulated within a 30-day period, conditions have been reached to allow the transmission of L3 heartworm to new hosts. A monthly heartworm pill, chewable, or topical must be given at the end of a month in which 234-heartworm development units has accumulated.

Feeding Mosquito
(Photo Credit: CDC Public Health Image Library)

When 30 days pass and 234 heartworm development have not accumulated, mosquitoes will be dying from the cold before any microfilariae they carry can develop to the infective stage. Monthly heartworm preventive needs not be given after a month under these conditions.

If all this sounds complicated, it is. In addition, most of us have better things to do besides monitoring average weather temperatures. It may be simpler to use the product all year round or just go by the recommendations of a practicing veterinarian in the region in question.



Strains of heartworm that are resistant to the preventives currently on the market (all those listed above), have been documented in the Mississippi River delta area. Resistance has emerged because of inappropriate use of preventives (i.e. the "slow kill" treatment of heartworm infection). It is particularly important in this geographic area to treat known heartworm infection definitively and promptly and not to skip doses of preventives. At this time only this limited area seems affected and not all heartworm strains are resistant. Be sure to include avoiding mosquito contact in the preventive regimen for dogs in this area.



Traditionally heartworm prevention has centered on killing heartworm larvae in the first month of infection. Infection is not prevented per se because the worms actually do transmit into the new host's body but they are killed long before they are able to develop and achieve significance. Indeed, prevention centers on the use of heartworm preventive products of this nature as were reviewed above; however, more recently the prevention of actual mosquito bites has become a goal as well. Use of products that repel mosquitoes in combination with products that kill young heartworms is called the "Double Defense Heartworm Protocol." Research has shown that better prevention is achieved this way. What products repel mosquitoes? Basically, any flea or tick products that contains permethrin will repel flying insects including mosquitoes. To view the flea product comparison chart click here. To view the tick product comparison chart click here.


For a side-by-side detailed comparison of how the current commercially available heartworm preventive products above work, take a look at our Heartworm Preventive Comparison:



  Page last updated: 6/26/2020