PREVENTION OF HEARTWORM INFECTION IN DOGS
(also called “Chemoprophylaxis”
IVERMECTIN Based Products (Heartgard, Heartgard Plus, Iverhart Plus, Iverhart Max, Tri-Heart Plus,
Pet Trust Plus)
MILBEMYCIN Based Products (Sentinel, Trifexis)
SELAMECTIN Based Products (Revolution)
MOXIDECTIN Based Products (Advantage Multi, Proheart6)
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. 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, both topical and oral, plus, while the subject of this page is canine heartworm prevention, all the products discussed have feline formulations.
HEARTGARD®, HEARTGARD PLUS® made by Merial
IVERHART PLUS®, IVERHART MAX® made by Virbac
TRI-HEART PLUS® made by Schering Plough
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.
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.
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.
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.
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.
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:
INTERCEPTOR® (discontinued 2013) & SENTINEL® made by Novartis
TRIFEXIS® made by Elanco
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 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.
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.
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.
Milbemycin can also be used in the treatment of demodectic mange. A special dosing schedule is needed to accomplish this; heartworm preventive doses are not adequate but milbemycin does offer a convenient treatment option for collie-type breeds. Click here for more information.
There are no breed-related sensitivities for milbemycin.
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:
(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® made by Zoetis
Moxidectin is another relative of ivermectin. There are presently two products that use moxidectin to prevent heartworm infection: Advantage Multi® which is available for both dogs and cats as a topical and Proheart6® which is available only for dogs as a injection.
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. As with selamectin, it can be given to heartworm positive dogs and it will decrease the number of circulating microfilariae but it is not a good choice in the treatment of active heartworm disease.
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 the Australian version of the same product, a larger volume is given and it lasts 12 months). In other countries, Proheart6 rapidly captured 40-50% of the entire heartworm prevention market but in this country, it was voluntarily withdrawn from the market in 2004 after a number of adverse reactions were reported. There has been great deal of controversy regarding these adverse reaction reports, especially since similar reactions have not been reported in the international market using the identical product made in the same manufacturing plant as the U.S. product. In June of 2008, Proheart6® returned to the U.S. market with some restrictions as the FDA studies the situation. The present restrictions are up for review in one year. To review the restrictions, click here.
Proheart6® is also effective in controlling hookworm infection.
For more information on Proheart6® please visit:
For more information on Advantage Multi® 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.
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.
Page last updated: 11/22/2013