Heartworm preventive medications are used to kill larval heartworms that have managed to gain access to the pet’s body. There are monthly products, either oral or topical, and there are long-acting injectable products. The monthly products wipe out any heartworm larvae that have entered the body in the last 30 days. (Note, they work retroactively). The injectables are simply working all the time and kill any larvae as they enter. 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.

Illustration by VIN.

Both canine and feline products are available in the topical and oral formats. The injectables are only for dogs. We have organized them here based on their active ingredients:

IVERMECTIN Based Products (Heartgard Plus, Iverhart Plus, Iverhart Max, Tri-Heart Plus, and other brands)
MILBEMYCIN Based Products (Sentinel, Sentinal Spectrum, Trifexis, Interceptor, Interceptor Plus, Comboguard)
SELAMECTIN Based Products (Revolution)
MOXIDECTIN Based Products (Advantage Multi, Coraxis, Proheart6, Proheart12, Simparica Trio)
EPRINOMECTIN Based Products (Nexgard Combo, Centragard - cats only)

Ivermectin-based Products: 
HEARTGARD^®, HEARTGARD PLUS^® made by Boehringer Ingelheim
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. 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 clears the circulating microfilariae (the newborn larvae born to the established adult worms), 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 if only microfilariae testing is done. (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 it could still prevent 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 you skip several doses of ivermectin accidentally, it is still worth picking up where you 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, has been added creating the "plus" which most ivermectin products include today.

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 used. The products containing both ivermectin and pyrantel pamoate are Heartgard Plus^®, Iverhart Plus^®, Pet Trust 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 as collie-related breeds have some difficulties, though the low doses used in heartworm prevention 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:

Iverhart
Triheart Plus

Milbemycin Oxime-based Products

INTERCEPTOR^® made by Elanco
INTERCEPTOR PLUS^® made by Elanco
SENTINEL^® made by Virbac
SENTINEL SPECTRUM by Virbac
TRIFEXIS^® made by Elanco
MILBEGUARD^® made by Ceva

These products are also given monthly as a treat. As with ivermectin, they kill the L3 and L4 larvae that have entered the body in the past month. They also interfere with the female worm's ability to reproduce so microfilariae production will be hampered. 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 inadvertantly 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 you find you have skipped several months of heartworm prevention in the middle of heartworm season, you 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. The Sentinel products combine milbemycin with lufenuron for flea sterilization and the Trifexis line combines milbemycin with spinosad for out-right flea killing. Sentinel Spectrum and Interceptor Plus have added praziquantel for tapeworm deworming. Note: Interceptor chewables are available for both dogs and cats. Interceptor Plus combines milbemycin and praziquantel so as to control tapeworms along with all the other worms listed for milbemycin alone.

There are no breed-related sensitivities for milbemycin.

See more information on Sentinel^®  from the manufacturer.

See more information on Trifexis from the manufacturer.

Selamectin-based Products:

Revolution and REVOLUTION Plus^® made by Zoetis

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. (Revolution Plus,which is for cats only, adds sarolaner for a second flea killing ingredient). 

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.

See more information on Revolution from the manufacturer.

Moxidectin-based Products:

ADVANTAGE MULTI^® made by Elanco
PROHEART6^® and PROHEART12 made by Zoetis
PARASEDGE by Virbac

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

Advantage Multi^® and Parasedge represent 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 (Elanco). Advantage Multi^® and Parasedge prevent heartworm infection, kill roundworms, hookworms, and whipworms. The imidocloprid present will kill the pet’s fleas. Both products actually carry approval for use in heartworm positive dogs, meaning that it can be used to kill microfilaria in an active heartworm infection. Both canine and feline products are available.

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; with Proheart12, an injection lasts 12 full months. Veterinarians and their technical staff must get a specific certification in order to give the injection. It is not recommended for sick or debilitated dogs nor for dogs positive for heartworm infection. Puppies must be at least 6 months old to receive Proheart6 and at least 12 months old for Proheart12.

See more information on Advantage Multi.

Learn more about Proheart6 made by Zoetis.

Eprinomectin-based Products:

Centragard made by Boehringer Ingelheim
Nexgard Combo made by Boehringer Ingelheim  
                                                                 
The two eprinomectin products are for monthly topical use in cats only. Centragard is basically a deworming covering roundworm, hookworms, and heartworm prevention. Praziquantel has been added to include tapeworm coverage. Nexgard combo adds esafoxalaner for flea and tick coverage.

See more information on Centragard from the manufactuer.

See more information on Nexgard Combo from the manufacturer.                                                              

When to Start Giving Heartworm Preventive Each Year?

There is more to transmission than just 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°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.The answer to this question is regional. Ask what your regular veterinarian recommends for your area. It may be simplest to just use preventive all year round.

When 30 days pass and 234 heartworm development units 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 go by the recommendations of a veterinarian in the region.

Resistant Heartworms

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.

Repelling Mosquitoes

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 using heartworm preventive products of this nature, as were reviewed above; however, more recently preventing 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. See a flea product comparison chart.