by Ben J. Character, DVM
Every few years the American Heartworm Society issues and update to their guidelines for the prevention, diagnosis, and treatment of heartworm disease in dogs and cats.
The American Heartworm Society is one of the few disease specific para-veterinary organizations available. Unlike human medicine that seems to have a “taskforce” or society of every major disease, having a resource like the American Heartworm Society (AHS) is a luxury in veterinary medicine. Disease specific society’s and taskforces are important because they have medical professionals whose sole job is to review the most recent research related to that specific disease and then consolidate that information into a highly evidence-based protocol for the understanding and treatment of the disease. In other words, they take a lot of the guess work out of how the general practitioner should go about dealing with the disease. Because of this, the AHS guidelines are my go to source for keeping my patients healthy and safe regarding heartworm disease.
Heartworm disease is still one of the most commonly found preventable diseases of dogs. It affects hundreds of thousands of dogs every year. Heartworm infected dogs are found in all the continental United States and across the globe. Heartworm’s vectors include several different species of mosquitoes found in every environmental sphere.
The tragedy of heartworm disease is that it is an easily preventable problem that when not prevented can lead to irreversible heart damage and death for the dog. When it comes to prevention, efforts to reduce mosquito populations have some but little effect on the problem and products marketed to “repel mosquitoes” as a way to prevent heartworms, while helpful, are not reliable enough to use on their own.
Today there are a wide variety of modalities available for the prevention of heartworms. The classic orally administered products are still available and come in many affordable forms. For dogs that are difficult to give a pill there are also topical and injectable forms. For maximum convenience the injectable form is even available in a 12-month formulation, making it possible to protect your dog all year with a single injection.
In the past few updates of the AHS guidelines, the most important new advancement in the treatment has been the use of antibiotics as a way to weaken the heartworms prior to treatment. Researchers discovered that heartworms have a symbiotic relationship with a bacterium that lives on its outer surface. This bacterium protects the worms and can itself harm the dog. The latest research suggests that products from the bacteria can cause damage to the linings of the lungs and kidneys increasing the likelihood of complications during heartworm treatment. Proper antibiotic therapy is fatal to these bacteria, and when they are eradicated from the worms, it weakens the heartworm making successful treatment more likely.
Because of this, the latest heartworm treatment recommendations call for a 28 day therapy of antibiotics to kill the bacteria before starting the final medications that actually kill the heartworms. After this antibiotic therapy, a rest period is completed which allows the dog to rid itself of the toxic products of the bacteria and allow the heartworms to grow weaker and weaker. The result is fewer post-treatment complications and greater treatment success rates.
Another important change in the AHS recommendations is that all dogs receiving heartworm treatment be given three injections of adulticide medications. In the past, the standard protocol for heartworm treatment was a two injection course of medications to kill the adult heartworm. These injections were given two days in a row and 24 hours apart. Initially this protocol was following in all dogs no matter what their clinical signs/status. Unfortunately, this lead to severe complications in dogs with advanced heartworm disease.
To combat this unacceptable level of complications, further research was performed that showed that a delay in the treatment could be utilized without compromising the outcome. After this, the AHS revised its guidelines such that for severely effected dogs a single injection of adulticide was given, followed by a month delay before then giving the same two injection protocol as before. This change was much better tolerated by dogs that had more advanced heartworm disease.
Now, this three injection protocol is has become the standard AHS protocol for all dogs receiving adulticide heartworm treatment and has a higher than 99% success rate for eradicating the adult heartworms.
Even though the American Heartworm Society has issued clear and safe guidelines for the treatment of heartworms there is still autonomy within the veterinary community and other ways of trying to kill heartworms have been suggested. One of these ways is to use the monthly preventative more frequently and for a longer duration. Another way is to use the prevention in combination with the antibiotics. And there have been many other suggestions.
The problem with all of these other ways is that they take significantly longer – sometimes over two years – to finally kill the adult heartworms and in some cases they do not fully eradicate the worms. Why is this a big deal?
The big deal is that from the day your dog has a heartworm – or worse several hundred – the size of angel-hair spaghetti living in its pulmonary artery and/or heart damage is being incurred. Heartworms usually kill a dog because they result in cardiopulmonary failure. This happens secondary to inflammatory damage to the heart and vessels in combination with flow disruption causing heart muscle failure. These changes happen over time building to the point of complete dysfunction.
Research into these “alternative” treatment protocols has shown that dogs receiving these show the exact same damage changes as dogs not receiving any treatment, and had a much higher rate of potentially life threatening arterial clot formation. So, even while the alternative treatments are being employed the dog is still dying from heartworms. Conversely, dog’s receiving the AHS guideline treatments not only are heartworm free in a matter of only a few months, but had almost no arterial clot formation.
This last advancement in heartworm treatment understanding might just be the most important. The idea that alternative therapy protocols are actually putting the dog at more risk than simply doing nothing is recent information of which your veterinarian might not be aware. The good news is that with proper diagnosis and treatment most dogs do well with todays advanced heartworm treatments.