What Is New In Heartworm Prevention, Diagnosis And Treatment?

Ron Hines DVM PhD

Has My Dog’s Risk Of Catching Heartworms Changed In The Last 10 years?

That depends on you and your pet’s lifestyle and where you live. Over the last 10 years, the general risk has remained about the same. On the positive side, the cost of monthly preventatives have come down and urbanization, pet owner education and standards of living have increased. But during the same period, changes in rainfall patterns and a warmer climate allowed heartworms to extend their range.

I wrote this website’s articles on heartworms some years ago. It is still as formidable an illness now as it was then. And over those years, heartworms have steadily extended their range in the United States. They are now present (at least spottily) in dogs in 49 states (all but Alaska). (ref1, ref2) In Europe, heartworms have gradually moved northward. (ref) In Australia, they have appeared inland from the subtropical coast. In the USA, areas of heavy infection mirror the areas where the 16 common species of mosquitoes that transmit heartworm larva are found. Those are areas with mild winters, high humidity, lots of rainfall and water impoundments. An unusually cold winter or period of drought is often followed by a temporary drop in the number of heartworm cases that veterinarians see. But other factors come into play too. Urban areas with high incomes have fewer cases than poorer economic areas – probably due to better mosquito control as well as a more informed citizenry able and willing to give their pets monthly heartworm preventatives. (ref)

Some folks associate the enlarging areas of heartworm disease with climate change (heartworm larvae do not become infectious in mosquitoes at temperatures below 57F=14C [ref]). Others suggest that its due to today’s greater willingness of pet owners to test their dogs for heartworms – as well as better and more reliable tests. Both are plausible; I cannot tell you that they are true.

Has My Cat’s Risk of Catching Heartworms Changed?

Whenever the chances of dogs in your area catching heartworms goes up or down, the chances of cats in the area becoming infected changes accordingly. Parasites periodically mutate and adapt to a new host. So one cannot guarantee that cats will remain a second-best host for heartworms forever. (ref)

Studies to determine how common heartworm infection in cats is tend to “under-discover” some infected cats. That’s because the in-office tests we veterinarians use to detect heartworms are ELISA-based. They look for a specific antigen protein that diffuses out from the reproductive tract of “pregnant” adult female worms. Infected cats rarely have more than a few adult heartworms. If they are all males or still juveniles, the test will be falsely negative. Even one mature female worm – capable of doing the cat considerable damage – can be missed by the test. (ref) Its been estimated that these tests pick up ~89% of infected cats versus 92% of infected dogs (ref) Others are less optimistic about the test’s value in cats.

Despite those limitations, a 2017 study used these antigen tests to looked for evidence of heartworm infection in the blood of cats. None of the Canadian samples were positive, but some cats from 35 American states were positive. The overall prevalence of positive samples in the United States was 0.4%; and had not changed much from what it was ten years earlier. As expected, the percent positive was considerably higher in the southern states where mosquitoes thrive – and three times the average when cats were allowed to go out of doors. Only 12.6% of the tested cats were receiving monthly heartworm preventatives. Cats with other health issues had a higher likelihood of being positive. That could have been due to their leading a more risky lifestyle or due to immunosupression that made it more likely for the heartworm parasite to survive. (ref) They do not say, but I assume the blood samples used were not heat-treated. Many believe that heat treatment of blood samples improves the sensitivity of heartworm tests. (ref)

When your cat has or is suspected of having heartworms, the most important thing to do is probably to place the cat on doxycycline. That is because doxycycline kills Wolbachia and, as you will read below, Wolbachia pipientis is responsible for much of the inflammation that accompanies heartworm disease. Topically applied Advantage Multi for cats is probably also a very good idea – both because it weakens mature heartworms that are present and because fleas and some of the mosquitoes that transmit heartworms also carry Wolbachia pipientis within their bodies. (ref1, ref2) We think that Wolbachia enters the heartworm ‘s eggs when they are incubating within the mature female heartworm ’s uterus to produce already-infected microfilaria. But Wolbachia is a very ingenious bacteria and whether its modes of transfer are plastic remains unknown. (ref) At the least, your cat will be protected from fleas and future heartworm infections.

I Read About Drug-Resistant Heartworms – How Common Is That?

In 2011, I told you that there were isolated areas of the USA where some heartworms appeared to have become resistant to ivermectin-based monthly heartworm preventatives like Heartgard®. (ref) Researchers now know that a mutation in a specific heartworm gene (the P-glycoprotein gene) appears responsible for that resistance. That mutation causes a decrease in effectiveness of most of the monthly heartworm preventative formulas (=monthly ivermectin, milbemycin, selamectin and twice-yearly moxidectin injections [Proheart 6]at least against the MP3 and JYD-34 resistant heartworm strain used for testing resistance at Auburn University [ref]).

This form of resistance has not been reported outside of the United States – yet. Although it is still rare, it usually occurs in dogs living in areas where the rain that falls drains into the Mississippi river. Parasites of all kinds have a tendency to mutate to avoid the effects of the products that veterinarians dispense to control them. Whether this will continue to occur with dog heartworms we do not know. But it is not a far-fetched concern. The human ivermectin tablet (Mectizan®) does not appear to be as effective in treating the heartworm ’s cousin, Onchocerca, in Africa as it once was. (ref1, ref2) Farmers use ivermectin and moxidectin to control intestinal parasites in their cows; and those parasites appear to be becoming resistant to those two drugs considerably faster than heartworms in dogs are. (ref1, ref2)

The rate at which parasites tend to become resistant to a drug depends on the size of the untreated pool of animals that do not receive the drug. The larger the untreated pool of animals, the less likely the parasite is to mutate to resist a drug. In the case of cattle, there are few that do not receive those drugs. But in dogs and the wild animals that heartworms live in, that pool of untreated animals is very large. That causes the parasite’s mutation to be much slower because there is no pressure on the parasite species to mutate in order to survive. Besides the strays and pet dogs that receive no heartworm preventative, this untreated pool includes coyotes and foxes. Up to 45.65% of adult Florida coyotes were found to have canine heartworms. (ref)

It is hard for veterinarians to get a good feel for the extent of the resistance problem because we do not have good tests to gauge resistance. (ref1, ref2) There are folks however who are attempting to develop such tests. (ref)

The monthly heartworm preventatives that your veterinarian has for sale are very effective – about as effective as human birth control pills – but with less side effects. Giving them to your dog all year long and beginning giving them at 8-9 weeks of age (when the parasite is still in its L4 stage and most susceptible monthly preventatives – see the graph above) is the best way to prevent heartworms or the possibility of resistance. (ref)

Most veterinarians have had a client or two who swore that their dog could not possibly have heartworms because they know for a fact that it received its preventative medication monthly. Yet their dog tested positive. Many jump to the conclusion that the heartworms in their dog must be ones that are resistant to monthly heartworm preventatives. But when those cases are closely examined, its usually found that it was owner error or misinformation that accounted for the infection. Perhaps it was a shelter dog and the information that it tested heartworm negative was incorrect. Perhaps at the time of the test it had heartworms too immature to be picked up by the test. Perhaps it was an oral medication that the pet later spit out. Perhaps the dose was too small for the dog’s weight. All our lives have periodic stresses – their ups and downs. Perhaps you or your family were distracted for a time by urgent matters. Compliance with anything as boring as a monthly pill or topical drops is rarely 100% year after year. And it only takes a small window of opportunity for heartworms to sneak in.

The risk increases substantially if you scrimp and only give preventatives during the “mosquito season”. Sure, the odds will still be quite high that you dog will not catch heartworms. But why play those odds? A yearly antigen test is not that expensive either – particularly if you shop around.

Are Snap Heartworm Tests Always Accurate And Are There Ways To Make Them Even More Reliable?

The Idexx Snap® test, or its competitor, Zoetis’ Witness®, that veterinarians use are quite accurate in detecting if you dog is infected with heartworms – considerably more so than the earlier tests we had (eg Difil Test) that just looked for heartworm larva circulating in your pet’s blood. But no test as complex as a Snap® or Witness® test is free from errors and certain situations trick the test. Both look for tiny protein fragments that mature female heartworms release into your pet’s blood. So dogs with few or no mature female heartworms can give false-negative results. Some of those false-negative dogs had only become infected with male heartworms. In others, the female parasites are not old enough yet to release these uterine proteins that the test is designed to detect. In still others, the worm’s ability to produce uterine proteins may have been decreased due to prior ivermectin and/or doxycycline administration (ref) There are some vets that believe that extremely heavy burdens of mature female heartworms also occasionally produced negative tests as well (perhaps because that dog’s immune system is producing large amounts of anti-heartworm antibody that ties up the heartworm protein fragments that normally circulate in the pet’s blood stream – but, if something like that can actually occur, it is quite rare). Some consider an alternative, but more laborious test, DiroCHEK®, to be the least likely to be in error. (ref)

False positive heartworm tests occasionally occur as well. The Snap test has been designed to be as sensitive as possible – it has a “hair trigger”. In most of those false-positive cases, a blood sample sent off to a central commercial or university laboratory comes back negative for heartworms (in the USA Antech or Idexx). I am told that central commercial veterinary laboratories almost always confirm a positive initial test with a second more accurate confirmatory test – although that may not be noted on the final report the lab sends to your vet.

Most, but not all, heartworm-infected dogs that are not receiving monthly heartworm preventative medications or doxycycline (now or in the past) have immature heartworm larva circulating in their blood stream. Occasionally (very rarely), your veterinarian will see those larva in the blood of the dog (the old Difil test  or a drop of blood) that just had a negative Snap test. There are a few reasons that might occur: Adult heartworms only live a few years (remember, the larval worms need to go through a mosquito before they can become adults). So many vets believe that in those pets, the mature female(s) that gave birth to the larva have already died and the adult’s remnants were absorbed by the pet’s body. There are also a number of worms similar to heartworms that also pass living larva into the pet’s blood stream. Some can pass larva into the dog’s blood stream that look quite similar to heartworm larva. Most have subtle differences in their shape and movement that let veterinarians tell them apart and several veterinary colleges have developed sophisticated tests to tell them apart. (ref) (Theoretically possible- but highly unlikely – a transfusion with infected blood might pass dead-end heartworm larva to the recipient dog.)

In my opinion, it would be wise for any dog that has symptoms compatible with heartworm disease and a negative Snap/Witness test and any dog that has an in-office positive test and no symptoms or indications of heartworm disease to have a blood sample sent off to a sophisticated central veterinary laboratory for confirmation before contemplating the best treatment plan. That is particularly so when the pet’s blood globulin level is elevated, ultrasounds (echocardiograms) show suspicious heart objects or right side, heart flow abnormalities (ref) , the vet sees X-ray heart and lung pattern changes, the dog is having breathing difficulties, tires easily, is aging rapidly, or has increased inflammatory blood markers. (ref) Even more so when the patient is a cat. (ref)


Heat Treatment Of The Pet’s Blood Sample For A More Sensitive Heartworm Test

For Snap-negative dogs with any of the suspicious symptoms I mentioned earlier, something else can be done. A recently study showed that “hidden” heartworm antigen is sometimes liberated by heating the pet’s blood sample before running the test. Doing so has been found by some to increase the number of heartworm cases that are discovered. That is particularly true in cats since they rarely have more than one or two adult heartworms (ref) and in dogs that have produced high levels of anti-heartworm antibodies. That antibody binds to heartworm antigen and can make it invisible to the test. One study also identified another unidentified “blocking” agent that caused positive test results to become negative. (ref1, ref2, ref3, ref4) The results of a Romanian study that used heat treatment (using DiroCHECK®, Zoetis) to discover hidden positive dogs were even more dramatic. (ref)

But others dispute the value of heat-treating your pet’s serum prior to running the test. (ref) Whatever the case, heat treatment has never been reported to produce a false positive. Idexx will run the heat treatment test if your veterinarian requests it. (ref) I do not know of any vets attempting to do serum heat treatments in their office. They send it to a national lab to have that procedure performed (Idexx #7232 @ $30 or $14.95 as add on, Antech # T613 @ $21.20).


Is The FDA-Approved Heartworm Treatment The Best Option For Every Heartworm-Infected Dog?

No, not always.

For dogs that are still in relatively good health, I still believe that the traditional 3-dose melarsomine treatment is usually the best option. There were periods of time when Immiticide®, the Merial melarsomine product, was unobtainable. (ref) But since Zoetis entered the market in 2017 with their competitor formulation, Diroban® , that is no longer the case.

But there are situations where neither Immiticide® or Diroban™ is a good or safe fit. Both are melarsomine. Melarsomine has what is called a narrow spectrum of safety – more so than any other drug veterinarians administer to dogs. The difference between the treatment dose and a toxic dose is very small. Younger dogs, discovered before heartworms have caused serious damage, generally tolerate Immiticide/Diroban (melarsomine) well – when given at the correct dose for their body weight and when clients strictly control their dog’s activities after the drug is administered. But once the damage heartworms cause becomes extensive, the chances of the drug being toxic are greatly increased. Those are what we call class3 and class4 stage dogs (the sickest) and, like any classification system, which class of heartworm disease a pet falls into is somewhat subjective and arbitrary. You can read about the stages of heartworm disease here.

High-risk treatment dogs generally do have warning flags. They are low in energy, tire and pant easily. They are often thin – although their potbelly may disguise that. They often have a dry cough – particularly when they are lying down relaxed. Dogs over 4-5 years that are high risk often show premature signs of aging – grey hairs on their face, paws and muzzle. Their pulse may be visible on their neck. Heart murmurs are common. If those dogs are examined by x-ray or ultrasound, there are telltale signs of heart failure. Blood work often shows that they are anemic and tests that measure the efficiency with which they move blood through their bodies are reduced. (CRT) Dogs that have heartworms discovered later in life often have elevated liver and kidney tests (serum AST, ALT, alkaline phosphatase, bilirubin, and urea nitrogen) Those dogs are also high-risk candidates for the use of melarsomine. Both manufacturers of melarsomine suggest a 3-dose, spread out administration protocol and strict post-treatment exercise restriction for high-risk dogs (4-6 wks of low-stress cage confinement). But the high risk remains (blood clots = thromboembolisms ). Also, some dogs and some owners just do not tolerate the long term caging and restraint of their pet. Those are issues that your veterinarian has little control over.

There is another major issue with melarsomine treatment. It’s expensive. Particularly when pre and post-treatment monitoring follows the recommendations of the American Heartworm Society to the letter. (ref) That can put proper melarsomine treatment beyond the capacity of many pet owners, most humane societies and most in the developing world to pay for it. (ref)

Even when these guidelines are followed to the letter, some dogs are not entirely cleared of heartworms by the two-dose or three-dose melarsomine treatment. No one really knows how many that might be (heat treating blood samples is a relatively new procedure and doxycycline can give mature female heartworms a cloak of invisibility). It may also occur when the younger stages of heartworms, those not readily killed by melarsomine, went on to mature in the pet. But even some mature worms appear to resist the killing-effects of the drug. Traditionally, vets told clients that, perhaps 2-5% of dogs might still have a few heartworms after melarsomine treatment (its probably closer to 10% [ref]) There are very few studies where the melarsomine and test manufacturers, the Heartworm Society (an industry-funded organization) or all of them were not involved. But in one independent, real-world study that I know of, the treatments risks and deficiencies were more apparent than in the group-funded studies. Even from that one study, we really do not know how many dogs were truly free of heartworms – since male worm, immature female worms or female worms sterilized by the doxycycline or the monthly heartworm preventatives that were given would all test negative as well. (ref) The same applies when heartworm test manufacturers and the laboratories that run them are involved in evaluating the reliability of their products. (ref)


Doxycycline Wolbachia And Heartworms

Veterinary school parasitologists do not have the funds, or the personnel that medical school scientists and international organizations have to conduct ground-breaking research. But they do know that another filarial parasite of humans, Onchocerca volvulus, is very similar to the dog heartworm. The difference is that the adult human parasite resides in “boils” under the skin and is transmitted by biting flies, not mosquitoes. It causes a great deal of human suffering (ref); so Medical schools and international agencies spend a great deal of time looking for ways to control it.

Its been known since 1977 that a peculiar bacteria, Wolbachia lives within all stages of the dog heartworm ’s cousin, Onchocerca (ref) Actually, that author saw them in dog heartworms in 1970 but thought they were just parts of the heartworm – not independently living things. In 1997, it was noticed that a cow, infected with another relative of the heartworm, recovered when it was given tetracycline. (ref) and on that basis, doxycycline was proposed to sterilize Onchocerca and prevent river blindness. In 2008, it was suggested that killing these bacteria, would eventually kill the onchocerca worms themselves. (ref1, ref2) As time went by, it also became apparent that the Wolbachia within those parasitic diseases were responsible for much of the inflammation they produced. That included heartworm-infected dogs and cats as well. (ref1, ref2, ref3)

The inflammation that Wolbachia stimulates is not confined to the dog or cat’s heart and lungs. Through complex processes, even the pet’s kidneys can be affected. (ref) It is a stress on the the entire body.

There is some evidence that pulsing the dose of doxycycline, rather than giving it continuously, kills Wolbachia more efficiently. (ref) There are also studies in progress at the vet school in Georgia to see if minocycline is an acceptable alternative drug to doxycycline.

Things rarely stay the same forever and Wolbachia might conceivably become resistant to doxycycline (and minocycline as well). But for now, it can be very helpful for dogs and cats that have become infected with heartworms.


Slow Kill

For extended periods of time, Merial, the only maker of Immiticide® (melarsomine) at the time, did not make their product available. (ref) The company never gave an explanation for those outages; but the situation spurred interest throughout the dog heartworm research community in finding alternative treatment options.

Perhaps with that in mind, in 2008, a team of veterinary researchers based at the Veterinary schools of Athens Georgia and the University of Pennsylvania and others working in Milan and Parma Italy published results of the first detailed studies I know of that looked for an effective alternative to melarsomine therapy for heartworm disease in dogs.

When examined 9 months after beginning treatment with ivermectin and doxycycline, the group of dogs in that study that received doxycycline (the same drug class as tetracycline) and ivermectin had a 78.3% reduction in their mature heartworm burden. Those receiving melarsomine plus doxycycline plus ivermectin had a 92.8% reduction and those receiving only melarsomine no longer had any heartworms. Those figures lead me to conclude that the 5 dogs in each group in this study were not enough to draw broad conclusions as to which of their treatment option might be most effective. How heartworm disease progresses in a dog is just too variable. But sustained treatment with doxycycline and ivermectin was definitely quite beneficial. You can read the results of that study here.

Very quickly thereafter, a number of articles appeared that were highly critical of what came to be know as the doxycycline/ivermectin “slow kill” method. The Heartworm Society was dead set against it. Some members suggested (without hard evidence) that this slower kill method produced more false-negative snap tests after the treatment ended and that the dogs were probably still infected. They proposed, again without evidence, that people using this method to treat their pets would be sloppy in giving their pets the monthly preventative they require after the treatment. They proposed that using the slow kill method would produce a super-strain of drug-resistant heartworms. (ref) Reading through those articles, they appear to be based more on the rigid dogma of an entrenched organization than a contemplative review of new evidence. Others articles echo those or similar opinions. (ref1, ref2, ref3) Zoetis, the second manufacturer of melarsomine, still uses those points in its marketing strategy today. (ref)

Others cited a better documented ramification (consequence) for veterinarians who suggest the slow kill treatment: Melarsomine is FDA approved for adult heartworm treatment; doxycycline and ivermectin/moxidectin are not. If a dog does not do well subsequent to properly-administered melarsomine, the FDA stands behind that veterinarian. With a non-FDA-approved therapy, they do not (Like the Walendas, there is no net).

But in 2014, another Italian study documented the effectiveness of ivermectin and doxycycline given as an alternative to melarsomine. (ref) Although considerable lung pathology (damage) still resulted from the dying worms, one of the authors reported that the extent of lung damage following melarsomine treatment was even greater. (ref)

The veterinary school in Auburn, Alabama has, on staff, a research scientist who studies heartworm drug resistance. In 2011 he and his colleagues reported that of the four monthly heartworm preventative medications studies, moxidectin/imidacloprid (Advantage Multi®) topical drops were the most effective in preventing heartworms. (ref1, ref2) A few years later, he told me that he believed that was because moxidectin persisted in the dog’s body longer than ivermectin did (because it is more lipophilic than ivermectin, that is, more soluble and persistent in the fatty tissues of your pet’s body). By mid-month, little of the ivermectin from the first-of-the-month dose of ivermectin you gave remains in the dog’s system. Others confirmed that. (ref) Both dog studies were paid for by Bayer, the manufacturer of Advantage Multi ®. But other studies not paid for by Bayer that used moxidectin in cattle support their conclusions.

That knowledge – that moxidectin might be more efficient in killing heartworms – led veterinarians at Colorado State University College of Veterinary Medicine, North Carolina State University, University of Milano Veterinary Sciences and Università di Parma to begin trials to see if using Advantage Multi and doxycycline, rather than ivermectin and doxycycline might be more effective in treating heartworm-infected dogs.

It is August of 2017. Some of those studies are still in progress. Others are finished but have not been published or widely distributed yet. They call it the MOXY protocol. You can read an early announcement here. It is tentative. Some preliminary Georgia data was presented at the August 2016 AAVP meeting as well. You can read that here.

Emails from the European Groups:

April 3, 2017 : ………………………………………………………………………………………….. In my experience, the combination moxidectin/doxycycline showed a good efficacy and was able to decrease the risk of thromboembolic reactions. However, my data were obtained in field studies. Recently, in an experimental study (not from my group; it will be published soon) dogs were treated with moxi/imidacloprid per labeled dosage for 10 months and with 10 mg of doxycycline BID orally for 30 days. Throughout the study, dogs were not exercise-restricted. The efficacy in eliminating adult heartworms was 95.9%. One dog of 8 treated showed adverse events suspected to be a consequence of thromboembolism self-resolved. In my opinion, moxi/doxy is a valid alternative to classics melarsomine therapy.

April 4, 2017 : …………………………………………………………………………………………. We are also evaluating the efficacy of doxycycline combined with topical moxidectin (Advantage multi) and results are very promising. We currently recommend 10mg/kg/bid of doxycycline for one month and Advantage multi once a month for nine months. We observe: 1. Very fast elimination of microfilaria (30-60 days from the beginning of treatment); 2. negative antigen tests within several months from the beginning of treatment. The moxy/doxy combination appears to clear infection faster [than ivermectin], with antigen tests going negative at about 6 months. The ivermectin/doxy combo takes about 12 months to work. We have observed some coughing at around 2-3 months into the regime, but this is managed well with pred [prednisone].

Just a word of advice; I believe that It would be exceedingly foolish to begin or attempt any of these treatments without the close supervision of your local veterinarian – much like attempting to do your own maintenance on the 747 airliner you plan to be on, on your next flight. I also cannot speak to the safety of using moxidectin in herding dogs with the MDR1/ ABCB1 mutation. But there is some evidence it might be a bit safer than ivermectin in those susceptible dogs. (ref)

Should I Feel Responsible For Causing Heartworms To Mutate And Become Resistant To Monthly Preventatives When I Agree To Use The Slow Kill Treatment Method For My Dog?

I think you should not.

Think in terms of the best interests of your dog and you, not some nebulous, unconfirmed theory about the future. Might an asteroid strike the earth and end life as we know it ? Perhaps. Might something I do for the benefit of my dog today have negative consequences for some dog in the future ? Perhaps. But by that time, pharmaceutical companies will probably have other medications than macrocyclic lactones to prevent and treat heartworms. That is because they are so similar to human parasites like onchocerca. Many (including the Gates Foundation which, to date, has invested $2.5 million dollars) are looking for those options (ref1, ref2) and trials with a vaccine to protect humans against this parasites, so similar to the heartworm, are scheduled to begin in 2020 (ref1 ref2) A great deal of veterinary effort is also being spent looking for different classes of drugs that might control canine heartworms; if what we presently use becomes ineffective. (ref1, ref2, ref3)

Besides, if you take steps to keep your dog heartworm larva (microfilaria) free with monthly preventatives during and after slow kill treatment, what you do will have no effect on how things develop in the rest of the dog population. (ref) In 2017 There are many brands of monthly preventative that will kill heartworms before they take up residence in your dog’s heart; but only one, Advantage Multi® (because it contains moxidectin), is FDA-approved for the elimination of the microfilaria that could conceivably allow the parasite larva to mutated and then transfer to another dog. (ref)

But there is another important factor at work here against the spread of heartworm resistance. The larger the number of heartworm infected dogs and other canids not receiving monthly preventatives, the less likely a parasite species is to mutate. It’s estimated that half of out-of-door, minimally maintained dogs have heartworms. (rptref) That’s about 19.5 million+ dogs in the USA not receiving heartworm preventatives. It takes strong drug pressure on a parasite species to encourage it to develop drug resistance. That large untreated pool of dogs makes it considerably less necessary for heartworm to make that mutation to survive. A large number of coyotes carry and spread dog heartworm disease as well. (ref1, ref2) That means there is even less pressure on heartworms to become resistant to monthly heartworm preventatives or the “Slow Kill” method of treatment.

There might be another advantage when one uses Advantage Multi as part of the “Slow Kill” process. The moxidectin in Advantage Multi helps kill the heartworms. But the imidacloprid that is also in the product kills fleas. Fleas are carriers of the same Wolbachia (W. pipientis) that we are trying to kill within the heartworms with doxycycline. (ref) No one knows if Wolbachia is plastic in the ways it can move between hosts but other bacteria with an intracellular lifestyle are. (ref) Until we know more, providing a fresh source of Wolbachia bacteria from fleas is not something I would encourage.

Are There Alternative Ways To Remove Heartworms From Dogs?

Advanced imaging techniques and specialized surgical skills are making it possible for veterinarians (generally at large teaching institutions) to extract heartworms from the heart and pulmonary arteries successfully. The technique requires superb judgment and hand-eye coordination. In most veterinary centers, folks with that talent dwell in the school’s interventional radiology department. (ref) A vet school professor emeritus at, North Carolina State University told me they perform quite a few of those surgeries there. He mentioned other institutions with good veterinary interventional programs were usually located in areas of high heartworm prevalence that bring them a lot of candidate dogs. Standing out among them were the veterinary schools at the University of Georgia, University of Florida, Texas A&M University, Auburn and Colorado State. At those institutions there may be no more than one highly experienced individual. Since those folks may come, develop their skills and move on, the institution’s success rate can fluctuate.

It is quite rare to successfully remove all of the heartworms from a dog with these techniques. But critically ill dogs can often be stabilized and sufficient time gained for medical treatment to kill the worms that remain.

I rarely mention products by name. In this article, I mention Bayer’s Advantage Multi frequently. That’s because there is presently no topical competing moxidectin containing product for dogs. I believe that the patent on moxidectin expired in 2011; so I am not sure why that is. Zoetis markets Proheart®6, a twice-a-year moxidectin injection to prevent heartworms, as well as some moxidectin-containing farm animal formulations. I personally avoid using ProHeart® 6. (ref)

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