There are a lot of misconceptions about heartworm treatment in dogs. One of the most common misconceptions is that using ivermectin-based heartworm preventive medications for a "slow kill" is the preferred way to treat canine heartworm disease.
Immiticide Treatment Versus Ivermectin Treatment for Heartworm Disease in Dogs:
Essentially, there are two distinctly different methods of treating heartworm disease in dogs.
- Treatment with melarsomine (Immiticide®), which kills the adult heartworms, is one method of treatment. There are different protocols that are used under different circumstances, but the bottom line with this method of treatment is that the adult heartworms are killed in a relatively short timeframe. With this method of treatment, ivermectin-based preventives are also administered concurrently on a monthly basis to prevent new infections.
- Monthly administration of ivermectin-based heartworm preventive medications alone is sometimes used as a second method of heartworm treatment. This is referred to as the "slow kill" or "soft kill" method.
There is a risk of complications occurring with both treatment methods.
The Advantages of the Ivermectin Method of Canine Heartworm Treatment:
Often, the "slow kill" method of heartworm treatment is used because of financial considerations. Unfortunately, the melarsomine treatment method is quite expensive. Monthly ivermectin treatment, on the other hand, is affordable.
There are situations in which melarsomine treatment cannot be pursued due to other health issues. In these situations, in addition to monthly ivermectin administration functioning as a "slow kill" way to rid the infected dog of heartworms, it also clears the infected dog's bloodstream of the larval form of heartworms (microfilaria).
These microfilariae have the ability to infect mosquitoes which feed on the infected dog. The infected mosquito can then spread heartworms to other dogs. Monthly ivermectin administration stops this from happening and helps to protect other dogs in the area.
The Disadvantages of the Ivermectin Method of Heartworm Treatment:
The American Heartworm Society does not recommend the use of monthly ivermectin products to treat dogs infected with heartworm disease. There are several reasons that using melarsomine to kill the adult heartworms is safer and more effective for your dog than using ivermectin monthly.
- The adult heartworm is responsible for the damage to heart and lungs that causes the symptoms of heartworm disease in dogs.
- Melarsomine is the only medication we have available that can kill these adult worms. Ivermectin kills the larval stages but not the adult worms. It also does not shorten their lifespan or render them sterile.
- With time, as long the larval stages do not survive and no new infections occur, the adult heartworms will die of "natural causes." However, this may take as long as two years to occur.
- As long as there are adult heartworms living in the heart and pulmonary arteries, the damage to these organs will continue. That means that while your dog is receiving only the monthly ivermectin medication, his heartworm disease will continue to progress and his heart and lungs can suffer severe damage.
- Another reason that monthly ivermectin treatment is not recommended for heartworm-infected dogs is that some parasitologists believe that the "slow kill" method has contributed to the development of strains of heartworms that are resistant to heartworm preventive medications. (Dr. Byron Blagburn, webinar, Emerging Issues in Heartworm Prevention, presented by DVM360, 4/20/2011)
In cases where melarsomine treatment is not practical for a dog with heartworms, monthly ivermectin is preferable to no treatment. However, it should be remembered that this method of heartworm treatment has serious shortcomings and is not the preferred method of heartworm treatment.