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Heartworm Disease in Dogs

Updated on March 14, 2009

Heartworm Disese in Dogs

Heartworm disease in dogs and cats is still a major health threat and can cause death when not treated in the early stages of the disease. This week I diagnosed 2 dogs with severe heartworm disease. They were both in right-sided heart failure from the mechanical presence of a handful of heartworms inside the right ventricle of the heart. The mechanical prescence of the heartworms caused the heart chamber to increase 3 times it's normal size and the main valve in the heart could no longer close. The blood acutally flowed "backwards" with the blood backing into the right atrium and increasing the pressure in the abdomen. This caused fluid building up in the abdomen resulting in a bloated appearance called ascites. The ascites combined large numbers of heartworms in their hearts, these dogs were literally gasping for breath. At this point in the disease progression, the damage to the heart is irreversible and successful treatment is unlikely. the disease progression takes 1 to three years for damage to develop to this magnitude.

Transmission of heartworms to the dog or cat begins with the bite of a mosquito. The mosquito must have previously bitten a dog with heartworms that had circulating microfilaria (baby heartworms). The microfilaria then live in the mosquito for about 2 weeks and then they are injected into the dog or cat when the mosquito feeds on the blood of the next victim. The microfilaria molt and change into larvae migrating through the dog's body until it finally finds it's way into the right side of the dog's heart about 6 months later. During this migration, the dog can have some signs of allergic reaction.

Once the heartworms migrate to the heart, they cause damage to the heart and impair the blood flow to the lungs. The dog may not exhibit clinical signs of disease until it is more advanced making treatment riskier. Signs of heartworms included couhing, exercise intolerance, weight loss and some times vomiting. Sometimes, the tests may not show positive results in the earlier stages. It is recommended to do annual testing to find those dogs that may have been incubating the heartworms prior to starting the heartworm prevention.

The heartworms can live in the dog for up to 7 years. When the disease is diagnosed before major complications occur, it can be treated with a drug called an adulticide that is injected into the deep muscles of the back. The heartworms then slowly die and are deposited into the ends of the arteries in the lungs causing intense reactions as these heartworms are slowly broken down by the body. If there is a very heavy worm burden, the treatment is split into two treatments a month apart to give the lungs a chance to heal. Because of the damage occurring in the lungs, it is imperative that the dog is rested for several months. Even a run down the fence line can stress the dog enough to cause severe complications or even death.

The best treatment is prevention. In most of the United States the prevention must be given once a month all year round. The prevention does not kill the adult heartworms already in the heart so testing on a yearly basis is necessary to detect the adults that may have migrated to the heart. Because there is a small window between exposure and detection, it may take a year after exposure before the test becomes positive .Veterinarians across the country have noted in increase in severe heart worm disease and this may be due to the purchasing of heartworm prevention from the internet without prior heartworm testing. Newer preventions are now combined with parasciticides that also kill fleas and intestinal parasites.Your veterinarian is the most qualified to aid in diagnosis, treatment and recommendations for the care of your dog.



Photos of a heart with heartworms

Click thumbnail to view full-size
Following from the right ventricle into the pulmonary artery, you can see the heartworms.Heart with enlarged Right ventricle and atrium resulting in heart failure.Right ventricle reveals heartworms. After a dog dies, the heartworms leave the heart chamber and can be found in the major arteries of the lungs.More heartworms in another lung lobe.Another lung lobe.Dissection reveals greatly enlarged right ventricle and atrium
Following from the right ventricle into the pulmonary artery, you can see the heartworms.
Following from the right ventricle into the pulmonary artery, you can see the heartworms.
Heart with enlarged Right ventricle and atrium resulting in heart failure.
Heart with enlarged Right ventricle and atrium resulting in heart failure.
Right ventricle reveals heartworms. After a dog dies, the heartworms leave the heart chamber and can be found in the major arteries of the lungs.
Right ventricle reveals heartworms. After a dog dies, the heartworms leave the heart chamber and can be found in the major arteries of the lungs.
More heartworms in another lung lobe.
More heartworms in another lung lobe.
Another lung lobe.
Another lung lobe.
Dissection reveals greatly enlarged right ventricle and atrium
Dissection reveals greatly enlarged right ventricle and atrium

Dog with heart failure from heartworm disease

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