May 2011

May 2011: Is Heartworm Prevention Really Necessary?

Dear Doctor: My Veterinarian recommends Heartworm medication for my Bichon, but he hardly spends any time outside. I do not want him to be sick, but I do not want to give him unnecessary medicine, either. What do you recommend?

Signed,

Jane in Plover

The official name of Heartworm is Dirofilaria immitis, which is an actual worm that prefers to live in the blood vessels of the heart and lung. By definition, Heartworm is a parasite. As veterinarians, we are always working to prevent parasite infections in the pet population. When pet owners worry about parasites, they are most familiar with the parasites that they can actually see on their pet, such as fleas and ticks, and various intestinal parasites that are visible in feces, such as roundworms and tapeworms. It seems that people have a difficult time imagining parasites that they cannot actually see, which would include the Heartworm parasite. If you could see into the heart and vessels, you would easily visualize adult Heartworms since they may be as long as 14 inches, and may be present in large numbers. As veterinarians, we know the awful problems that the Heartworm parasite can cause, potentially causing death in untreated pets.

If the Heartworm parasite is present in very low numbers in dogs, they may not cause any problems; however, in larger numbers, they can cause an inflammatory reaction within the blood vessels. This inflammatory change can weaken the blood vessels, and may cause blood clots. The immune system’s reaction may cause problems in other body systems, including the eyes and kidneys.  If the heart is forced to work harder, heart failure and arrhythmias may develop. In the early stages, you would not see any symptoms in your dog. As the disease progresses, the ability to exercise becomes compromised, patients will generally cough, and may have difficulty breathing. Sudden death may occur in pets that develop irregular heart beats and those that produce blood clots.

As you can see, Heartworm can become very serious, and can be a “silent killer” in affected pets. If we diagnose Heartworm disease in a dog, we can give appropriate treatment to kill off the parasite. Most pets will do well with treatment, depending on the stage of the disease. However, treatment is not without risk as the adult worms die off within the body. Since treatment also rather costly, we would much rather prevent disease from developing in the first place.

As the life cycle of the heartworm parasite was studied, it was found that the adult heartworm produce young microfilaria, which are released into the blood stream. Mosquitoes will “bite” an infected dog, taking in the microfilaria, which matures into a larval form. After time, the mosquito will transmit the larval parasite to the next dog. The larval form will migrate and mature into an adult worm, and the cycle continues. With knowledge of the life-cycle, researchers developed preventative medications that eliminate the larval forms, thus halting development of adult worms. I believe it is the preventative medication that you are referring to in your question.  There are several brands to choose from, so your veterinarian will be able to make a recommendation for your pet. As we have utilized preventative medications, the actual incidence of Heartworm disease has decreased dramatically through the years. The preventative is safe; the disease is not, so I always choose in favor of prevention.