Despite the widespread availability of heartworm preventatives, an unfortunate number of dogs do still develop heartworm infections. While many of us adopt heartworm-positive dogs, in which case heartworm treatment is simply part of the rescue process, many owned dogs also have infections, often because owners are unaware of the risks or because owners are hesitant to spend money on a monthly preventative medication, the benefits of which (if it works correctly) are not obvious. There are many stages of heartworm disease, from sub-clinical infections, where the dog tests positive but shows no signs of illness, to congestive heart failure, where the dog is truly dying from his disease. There are many downsides to treating heartworm infections rather than preventing against them.
Heartworm treatment is expensive; at the average neighborhood veterinary clinic, the total costs (from that heartworm-positive snap test to your heartworm-free dog) will often approach $1000. Heartworms cause secondary changes in the dog's heart and blood vessels, changes which are not reversed by treatment of the heartworms and which will, therefore, affect the dog's lifelong health. Treatment of heartworms is not a benign process and carries its own set of risks. Immiticide, the drug we administer to dogs to treat heartworms, is a very carefully regulated poison. It has been shown to be safe when administered correctly and at appropriate doses; however, its use is not something to be undertaken lightly. This Immiticide, administered as a series of intramuscular injections, kills the heartworms.
Unlike intestinal parasites, however, which, once killed, can simply be passed in the animal's stool, heartworms, once dead, do not have an easy way to be eliminated from the body. The dog's immune system must break down the dead worms, an elimination process which is very effective but does take some time. While that immune process is taking place, fragments of dead heartworms are circulating in the blood stream. These fragments can cause a multitude of problems, the most common of which is physical obstruction of blood flow to the lungs. Because of this and other risks, the dog must be kept calm and quiet, without any rigorous exercise or boisterous play, for six to eight weeks, until we can be certain all of the heartworms are completely eliminated.
Because of the significant strain heartworm treatment places on the dog's body, if you go to your regular veterinarian seeking heartworm treatment, he will first want to assess whether or not your dog is healthy enough to undergo treatment. Standard pre-treatment evaluation includes a complete physical exam, confirmatory heartworm test, a microfiliaria check (to see if the heartworms are reproducing), complete blood work, and x-rays. Other tests may also be indicated depending on your particular situation. This diagnostic workup allows your veterinarian to determine the overall health of your dog, to have some indication of the severity of the heartworm infection, and to have a knowledge of changes that may already have occured in the heart and lungs secondary to the heartworm infection. While this knowledge is unlikely to prevent the vet from treating your dog, it will allow her to tailor the treatment and post-treatment care to each dog's specific needs. Current heartworm treatment consists of three treatments with Immiticide, given over the course of a month. Due to the possibility of adverse events after these treatments, your dog will need to be hospitalized - perhaps for as little as a day, and perhaps for several weeks. Heartworm treatment is something veterinarians perform every day, however, for the sake of the dog's health, the owner's stress, and the owner's wallet, it is a procedure we prefer to avoid whenever possible.
Another heartworm treatment option, the slow-kill method, is also available. This method is often chosen when finances are an overwhelming concern or when a dog is not considered healthy enough to undergo Immiticide treatment. This treatment involves administration of ivermectin (in the form of monthly heartworm preventatives such as Heartgard and Iverhart) to a heartworm positive dog. This treatment is not without risks and should not be undertaken without consulting a veterinarian. Monthly administration of Iverhart prevents the heartworms in your dog from reproducing, preventing your dog from being a source of infection to other dogs. It also prevents your dog from acquiring more adult heartworms. This treatment does not actually kill the worms, however it does decrease their lifespan; keep in mind, however, that the average heartworm can live six years, so shortening that lifespan could still mean your dog having a heartworm infection for four more years. While this slow-kill method prevents a worsening of infection, it does not prevent any of the secondary heart and lung changes that may eventually lead to heart failure.
Heartworm treatment is a carefully performed medical procedure. It is undertaken when its benefits are believed to outweigh its risks, a decision which is made on a case-by-case basis for each individual dog. While in many cases it is the best option, it should not be undertaken lightly. It also bears mentioning that veterinary medicine does not currently have a treatment for heartworm infections in cats. It has been found that cats generally do not tolerate the sudden release of dead heartworm debris into their bloodstream; therefore worm death also leads to the death of the cat. For this reason, heartworm positive cats generally live the rest of their life with heartworms.
Dr. Danielle Irving, DVM, is an associate veterinarian at the Richmond SPCA. To read her biography or that of our other bloggers, please click here. Before posting a comment, please review our comment guidelines. Please note that our comment policy requires a first and last name to be used as your screen name.
Editor's note: The dogs pictured in this post have recently undergone heartworm treatment while in the care of the Richmond SPCA. Some of them are currently available for adoption. Contact the adoption center at 804-521-1307 for more information.