PREVENTION OF HEARTWORM INFECTION IN DOGS
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, 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, 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.
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.
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:
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.
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.
For more information on Trifexis, Elanco has a web site for this product at:
For more information on Interceptor, Elanco has put up:
For more information on Revolution (from the manufacturer), visit:
For more information on Proheart6® please visit:
For more information on Advantage Multi® visit:
WHEN TO START GIVING HEARTWORM PREVENTIVE EACH YEAR?
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.
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: 7/11/2017