Dog Heartworms

Updated 2/10/2012

Author: VetDepot

Heartworms are long round parasites that live in the heart and lungs of dogs. Heartworms are transmitted from one individual to another when a mosquito feeds on the blood of an infected dog and then bites an uninfected dog later. It is impossible to completely protect against mosquito bites, so all dogs are at risk for heartworm disease. Heartworm has been diagnosed in dogs in all 50 states, plus associated territories such as Puerto Rico and also in Canada.

Why Should Pet Owners Be Concerned?

Adult heartworms are big. Some can grow to be 14 inches long, so when a large number of the parasites invade a dog's heart and the blood vessels leading into the lungs, a lot of damage can occur. The combination of the physical presence of the worms and inflammation from the dog's immune response makes it difficult for the heart to pump blood through the lungs. When the blockage becomes severe enough, heart failure develops. Adult worms breed and produce larvae called microfilariae that circulate in the dog's blood stream, causing organ damage as they travel around the body. Heartworm disease is often fatal if left untreated.

How Do I Know If My Pet Has Heartworms?

A dog that has a new or mild case of heartworms may show no clinical signs at all. This is why heartworm tests are so valuable; they can diagnose dogs before a lot of damage has occurred and when treatment is at its safest and most effective. Early diagnosis is not only more effective – early treatment is also generally less expensive. As heartworm disease progresses, most dogs begin to cough. They may also lose weight, tire easily, have difficulty breathing and develop a pot-bellied appearance if fluid accumulates in the abdomen. Some dogs may experience sudden death.

Heartworm Infection in the USA

Veterinary Care

If you suspect that your dog might have heartworms, see your veterinarian. He or she will run a heartworm test. A small amount of blood will be drawn for the test. Most tests look for an immune response by the dog's body against these parasites. In some cases, the larvae or microfilaria can be seen in the blood sample. If the test comes back positive, treatment should begin as soon as possible. If the test is negative, there is still a chance that your dog is in the early stages of developing a heartworm infection and might come up positive on a repeat test in a few months. Follow your veterinarian's recommendations when it comes to repeat testing.

A heartworm positive dog will need to undergo additional diagnostic testing, such as chest x-rays, blood work, and a urinalysis, to determine how much damage has occurred, the best form of treatment for him, and his prognosis. Treatment will be customized to your individual dog. Most dogs will receive medication to kill circulating microfilariae and a series of  injections over a month's time to kill adult worms in the heart and lungs. In some cases, oral medications may be used in place of the injections. Hospitalization at the time of these injections is necessary so that your veterinarian can watch closely for side effects. As the adult worms die off, they may form clots or stimulate severe inflammation that can be life threatening to your dog. Other treatments may also be given based on an individual dog's condition.

Home Care

While a dog is being treated for heartworms, he must rest. If your pet has been sent home, follow your veterinarian's instructions closely and call the clinic immediately if you notice a decline in your dog's condition.


The good news about heartworm disease is that prevention is very easy. Prevention is much safer and less expensive than treatment. Have your dog tested, and if he is negative, your vet will prescribe one of the many safe and effective heartworm preventative medications that are now available. Most of these products also offer some protection against intestinal parasites. Several heartworm medications, like Revolution, Sentinel and Advantage Multi control fleas as well. Your vet can help you pick the best product for your dog.

Most heartworm preventatives are a pill, chewable treat or topical solution that you give monthly. Year round treatment is best to provide the highest level of protection for your dog. Annual testing is still important even if you are very good about giving your dog his medication on schedule. No preventative is 100% effective, and if you can catch a heartworm infection in its early stages, treatment is much more likely to be successful.

The above is provided for information purposes only and should not be used for the diagnosis or treatment of any condition. This information does not cover all possible variables, conditions, reactions, or risks relating to any topic, medication, or product and should not be considered complete. Certain product or medications may have risks and you should always consult your local veterinarian concerning the treatment of your pet. Any trademarks are the property of their respective owners.

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