What Is New In Heartworm Prevention, Diagnosis And Treatment?

Ron Hines DVM PhD

This Is About Drug Resistant Heartworms Too 

The most effective medication to prevent me from catching the JYD-34 resistant heartworm is the moxidectin found in Advantage Multi®  monthly topical & Proheart®12 yearly injection. I prefer you give me the monthly topical form. No need for me to get shots and jabs.” Read about that here

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). (read here ) In Europe, heartworms have gradually moved northward. 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 areas used to be those 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. In the last few years dog heartworms have extended their North American range into southern Canada. Most attribute that to climate change. (read here) Most parasitologists a believe that heartworm larvae do not become infectious in mosquitoes at temperatures below 57 F/14 C. ( read here ). But others suggest that today’s greater willingness of pet owners to test their dogs for heartworms and better and more reliable tests account for some of the increase in statistics. 

Has My Cat’s Risk of Catching Heartworms Changed Too?

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. (read here)

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. (read here) Its been estimated that these tests pick up ~89% of infected cats versus 92% of infected dogs. Others are less optimistic about the test’s value in cats although heat treating the blood sample before running the test is thought to improve accuracy. ( read here )

Despite those limitations, a 2017 study used current antigen tests to looked for evidence of heartworm infection in the blood of North American cats. None of the Canadian samples were positive, but heartworm-positive cats were found in 35 American states. The overall prevalence of positive samples in the United States then was 0.4%; and had not changed much from what it was ten years before that. 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. ( read here ) 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. No similar studies have been performed in North American cats since then. But a 2020 study of 1000 stray cats in Florida animal shelters found that 4% of them were positive for heartworms. (read here)

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. ( read here & here ) We think that Wolbachia bacteria enters the heartworm ‘s eggs when they are incubating within the mature female heartworm ’s uterus to produce already-infected microfilaria larva. But Wolbachia is a very ingenious bacteria and whether its modes of transfer are plastic (ingenious) remains unknown. ( read here ) At the least, your cat will be protected from fleas and future heartworm infections.

 

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

I mentioned earlier that there are pockets in the United States where some heartworms have become resistant to monthly ivermectin-based heartworm preventatives such as Heartgard® . ( read here ) Researchers now know that mutations in a specific heartworm gene (the P-glycoprotein gene) appears responsible for that drug 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 that has been used to testing resistance at Auburn University. ( read here )  This form of drug resistance has not been verified outside of the United States – yet. (read here) And even in the US it is still rare to encounter dogs with medication-resistant heartworms. But when dogs that received monthly heartworm preventative and still caught heartworms are identified, they are usually dogs living in areas where the rain that falls eventually drains into the Mississippi river.

Parasites of all kinds have long been known 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®) is not as effective in treating the heartworm ’s cousin, Onchocerca, in Africa as it once was. ( read here & here) Farmers worldwide use ivermectin and moxidectin to control intestinal parasites in their cows. Those cattle parasites appear to be becoming resistant to those two drugs even faster than heartworms in dogs are. (read here & here)

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  and remain infected. The larger the untreated pool of non-treated infected 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. (read hereIt is hard for veterinarians to get a good feel for the extent of the resistance problem because we still do not have good laboratory tests to measure heartworm resistance. ( read here & here) Currently all that veterinarians and research scientists can do is to give ivermectin to a heartworm-infected dog and note how effective the drug is in killing the heartworm larva. The less microfilaria killed, the more likely that the dog harbors an ivermectin-resistant strain of heartworms.

The monthly heartworm preventatives that your veterinarian has for sale are still 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. 

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.

Which Is The Best Monthly Heartworm Preventative On The Market?

I believe that currently it Bayer’s monthly moxidectin drops, Advantage®Multi. That is what I use on my dog Maxx. Moxidectin is also available from your veterinarian as a ProHeart 12® once-a-year Injection. the Advantage® line are all Bayer/Elanco products. ProHeart® is sold by Zoetis. I never give injectable medicines to dogs when topical products are just as effective. You can wash topical medications off but once you give drugs by injection, you are faced with the consequences should things not go well. The second point is that the concentration of moxidectin in your dogs system will not be as high in the 11th month as it was in the 2nd month. There is no getting around that. Both ivermectin and moxidectin are in the same family of anti parasitic drugs,  avermectins. However moxidectin is considerably more lipophilic (fat loving) than ivermectin. So much of the moxidectin is absorbed by your pet’s fat. Stored in the fat, the drug is slower to leave the body or be degraded. That means that moxidectin is available longer to kill the heartworm microfilaria (larva) than ivermectin or the other common monthly heartworm prevention medications. We call that a longer drug half-life. (read here)    

 

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 (The 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 (read here) 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. 

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 more sophisticated tests to tell them apart. ( read here ) Theoretically possible- but highly unlikely – a transfusion with infected blood might pass dead-end heartworm larva to the blood 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 ( read here ) , 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. (read here) Even more so when the patient is a cat. (read here)

   

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 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. The results of a Romanian study that used heat treatment (using DiroCHECK®, Zoetis) to discover hidden positive dogs were even more dramatic. (read here)

But others dispute the value of heat-treating your pet’s blood serum prior to running the test. 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. I do not know of any veterinarians 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. 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 class 3 and class 4 stage dogs (the sickest) and, like any classification system, which class of heartworm disease a pet falls into is somewhat subjective and arbitrary. 

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. 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. 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 (others believe  that its probably closer to 10%. 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. (read here) The same applies when heartworm test manufacturers and the laboratories that run them are involved in evaluating the reliability of their products. (read here)

 

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 ( read here ); 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. ( read here ) 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. ( read here ) 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. As time went by, it also became apparent that the Wolbachia within those parasitic diseases were responsible for much of the inflammation and damage that they produced. That included heartworm-infected dogs and cats as well. 

The inflammation that Wolbachia stimulates is not confined to your dog or cat’s heart and lungs. Through complex processes, even your pet’s kidneys can be affected. (read here) 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. (read here) Some veterinarians believe that  minocycline is an acceptable alternative drug when doxycycline is not available. Others believe it might even be preferable. ( read here ) Things rarely stay the same forever. 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.

 

The “Slow Kill” Method Of Treatment

For extended periods of time, Merial/Boehringer Ingelheim, the sole producer of Immiticide® (melarsomine) at the time had none to sell. The company would never give veterinarians an explanation for those outages. But the situation spurred interest throughout the desperate dog heartworm treatment community to finding alternative treatment options. Perhaps with that in mind, a team of veterinary researchers based at the Veterinary schools of Athens Georgia and at 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 for heartworm disease treatment in dogs.

This is what they reported: When examined 9 months after beginning treatment with ivermectin and doxycycline antibiotic, the group of dogs in a study that received doxycycline (it is in the same drug class as tetracycline) and ivermectin had a 78.3% reduction in the number of mature heartworm in their heart and pulmonary blood vessels. Those receiving melarsomine plus doxycycline plus ivermectin had a 92.8% reduction and those received only melarsomine no longer had any heartworms. The 5 dogs in each group were really not enough to draw broad conclusions as to which of their treatment option was most effective. How heartworm disease progresses in a dog is just too variable. But sustained treatment with doxycycline and ivermectin was definitely quite beneficial. 

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 Drug Industry-supported Heartworm Society was dead set against it. Some members suggested (without much in 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. (read here) Reading through those articles, they appear to be based more on the rigid dogma of an entrenched political organization than a contemplative review of new evidence. Others cited a better documented ramification (consequence) for veterinarians who suggest or administer 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. If a non-FDA-approved therapy was used, they do not. That can have severe malpractice ramifications for veterinarians.

In 2014, another Italian study documented the effectiveness of ivermectin and doxycycline given as an alternative to melarsomine. (read here) 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. ( read here )

I mentioned earlier that my choice in monthly heartworm preventatives was topical moxidectin. The veterinary school in Auburn, Alabama has on its staff a research scientist whose prime interest is heartworm drug resistance. In 2011 he and his colleagues reported that of the four monthly heartworm preventative medications studied, moxidectin/imidacloprid (Advantage Multi®) topical drops were the most effective in preventing heartworms. ( read here & here ) 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). He explained to me that by mid-month, little of the ivermectin from the first-of-the-month dose of ivermectin you gave remains in your dog’s system. Others confirmed that. (read here) It is true that all of those dog studies were paid for by Bayer, the manufacturer of Advantage Multi ® who had a financial stake in their findings. But other studies not paid for exclusively by Bayer that used moxidectin to remove parasites from cattle support their conclusions. (read here)  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. Although to the best of my knowledge administering moxidectin and doxycycline to a heartworm-infected dog is not an approved method of ridding the dog of heartworms in any municipality, the results that have been reported are favorable. (read here & here

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. ( read here )

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?

No.

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. 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.

Are There Alternative Mechanical 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. (read here ) A vet school professor at North Carolina State University once 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 fluctuates. It is quite rare to successfully remove all of the heartworms from your dog with these techniques. But critically ill dogs can often be stabilized and sufficient time gained for medications to be given to kill any worms that remain.

I rarely mention products by name in my articles. But in this article I mentioned Bayer’s Advantage®Multi frequently. That’s because there is presently no other topical or oral competing moxidectin-containing product for dogs. I believe that the patent on moxidectin expired several years ago; so I am not sure why no generic moxidectin products labeled for heartworms are available . Perhaps packaging is a challenge. Zoetis markets Proheart®12, as a once-a-year moxidectin injection to prevent heartworms. It also sells other moxidectin-containing farm animal formulations. I personally avoided suggesting ProHeart®12 because you can wash off a topical product when unexpected problems occur, but you can’t wash off an injection. Merck markets monthly moxidectin/fluralaner chews for cats called Bravecto®Plus. What effect that product would have on heartworms I do not know.

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