Heartworm Preventative

Article written by Jessica L. Marsh.
In memory of her sweet Samuel Adams, who lost his battle to Heartworm Disease in 2007.

Heartworm Preventative, Testing and Treatment come up frequently as the warm summer months come upon us I figured I'd put together a little fact sheet for everyone.

All information is from the American Heartworm Society www.heartwormsociety.org or from personal experience in rescuing, treating and saying goodbye to a HW+ dog.

What is Heartworm Disease? Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are but one of many species of roundworms. The specific roundworm causing heartworm in dogs and cats is known as Dirofilaria immitis.

How does a dog get HW? By being bitten by a mosquito that is carrying heartworm larvae called microfilariae. The mosquitoes become infected with these microscopic worms after biting into an infected adult animal for a blood meal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. When the mosquito bites another host (dog, cat or other susceptible animal), the infective larvae enters a new host through the bite wound.

How do you know if your dog has HW? Currently, the only way to test for the presence of heartworms in a dog is a blood test. Your DVM can draw blood and then test the blood for the presence of microfilariae.

How big are HWs? Male adult heartworms are 4 to 6 inches long. Female adult heartworms are 10 to 12 inches long.

If my dog has been infected, when will he test positive? It takes five to seven months from the time a dog is bitten by infected mosquitoes until a blood test can accurately detect the presence of adult female worms.

My dog has HW, will they just die off on their own? No. Adult heartworms can and will reproduce rapidly. They will produce offspring and those offspring will then mature to adulthood to produce more offspring. The life span of an adult heartworm is 5 to 7 years.

Is Heartworm preventable? It is the most preventable canine disease. Ask your DVM which brand of preventative he recommends. Two popular brands are Heartguard and Interceptor. A six month supply of Heartguard for a dog that is under 25lbs. is approximately $25-30 dollars.

What if my dog has HW and I choose not to get it treated? Heartworm disease may cause a combination of medical problems within the same dog including dysfunction of the lungs, heart, liver and kidneys. The worms are found in the right side of the heart and in the major vessels that bring blood to and from the right chambers, where they cause inflammation and interfere with blood flow. This primarily causes pulmonary thromboembolisms (clots in the lungs) and congestive heart failure. It can also lead to liver or kidney failure. Death can be caused by one or a combination of these problems.

How young can a dog contract HW? A dog can contract HW immediately after birth if it is bitten by a mosquito carrying microfilariae. The mother does not produce antibodies for the puppies to protect them from heartworm.

How young can preventative be started? A dog as young as 8 weeks can be placed on HW preventative. However, consult your DVM for his/her recommendation.

My dog doesn't have any worms in his feces. Does that mean he doesn't have Heartworm? No. Unlike intestinal parasites heartworms are not excreted. They live in the bloodstream - mainly in the right side of the heart and the lungs. The only way to be sure that your dog does not have heartworm is to have him tested by your DVM.

I live in a state without many mosquitoes, can my dog get HW? Heartworm has been found in all 50 states. However, The highest infection rates in dogs are observed within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River and its major tributaries.

Is HW contagious? No. Your dog cannot pass HW to your other pets or your children by licking them or playing with them. HW can only be passed through a mosquito.

What does a positive test mean? That means your dog actually has heartworms in the right chambers of its heart or pulmonary arteries. If left untreated, they could begin to cause heartworm disease if the process has not already started. Almost all dogs can be successfully treated and rid of heartworms if they are detected prior to causing severe heartworm disease. In asymptomatic dogs, less than 1% will likely have any significant problem from treatment.

What does the treatment entail? Adult heartworms are killed using a drug called an adulticide that is injected into the muscle. A series of injections is given. Treatment may be administered on an outpatient basis, but ordinarily hospitalization is recommended. When the dog is sent home, exercise should be limited to leash walking for the duration of the treatment period, which can last from one to two months. This will decrease the risk of blocked blood flow through the lungs by dead worms. The expense of treatment is affected by several factors, such as the cost of the drug, the pre-treatment tests and multiple office visits.

How much does treatment cost & what happens if I cannot afford treatment? Treatment is often quite expensive. There is only one drug (melarsomine - Immiticide®) approved by the FDA for the treatment of heartworms. Research has shown that ivermectin, the active ingredient in several heartworm preventives, has the ability to slowly kill young adult heartworms. This generally requires more than a year of continuous monthly administrations and may take more than two years before heartworms are eliminated completely. The older the worms are when first exposed to ivermectin, the slower they are to die. In the meantime, the infection persists and continues to cause disease. Therefore, long-term continuous administration of ivermectin generally is not a substitute for conventional treatment.

Can a mother dog pass HW to her puppies? Yes. The mother can pass microfilariae to her puppies. However, these microfilariae miss a crucial development stage that can only occur in the mosquito. So, microfilariae passed from mother to puppy cannot mature into adult worms. These microfilariae would be eliminated when you place your pet on a HW preventative.

How often should I have my dog tested for HW? Annual testing for heartworm infection is now highly recommended. Even though heartworm preventives, particularly monthly oral and topical formulations, are essentially 100% effective in preventing infection when administered according to instructions on the label, animals on heartworm prevention occasionally test positive for heartworms. This apparent lack of efficacy is usually due to owner compliance failure, travel or relocation of the animal to an area of active heartworm transmission, or unknown (or misdiagnosed) prior infection. Annual testing gives owners peace of mind in knowing that their pet is free of heartworms, and in cases where the animal is infected, it assures them of early diagnosis of infection and maximal benefits from heartworm adulticide therapy.

Thank you so much for reading and I hope you choose to keep your pets on preventative! ~Jessica


Rest in peace sweet Sam...