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Heartworm Disease and Prevention
by Linda Mar Veterinary Hospital

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Heartworms can cause a serious and sometimes fatal disease of the heart, lungs, and other organs. Heartworm disease can be cured with appropriate therapy. More important, it can be prevented with heartworm medication. This education sheet will help you learn more about heartworms.

Q. What causes heartworm disease? A. A nematode (worm) called Dirofilaria immitis.

Q. What causes the transmission of the disease?

A. Mosquitoes carry the larval stage of the worm. Please see the diagram on the other side of this page.

Q. What animals are affected?

A. Most commonly dogs; however, cats can occasionally contract the disease.

Q. What organs are affected? A. Mostly the heart, but the lungs, kidneys, liver, and the blood vessels to and from the lungs can also be affected.

Q. What are the symptoms in an animal with heartworm disease?

A. The disease is directly related to the number of worms residing in the heart, the duration of the infection, and each animal's individual response to the disease. Consequently, a dog with a low worm burden may show no clinical signs. Dogs with severe heartworm disease show signs of congestive heart failure (pronounced cough, exercise intolerance, "fainting-like" episodes.)

Q. What is Linda Mar Veterinary Hospital's heartworm prevention protocol?

A. We recommend your dog be placed on heartworm prevention medication that is given monthly.

Following a physical examination, puppies less than 6 months of age can be placed on the preventative medication without having a heartworm blood test. Puppies greater than 6 months of age, adult dogs that have never been on the preventative medication, and dogs that have been off the medication for a period of greater than 6 months require a physical examination and a heartworm test. A small sample of blood is taken for this test; if the test is negative the preventative medication should be immediately started.

If your dog has been off the medication for greater than 6 months, we also recommend repeating the heartworm test 6 months after restarting the medication.

Q. How often should a heartworm test be done?

A. If your dog is on monthly heartworm preventative all year round, we now only require a recheck test and examination every 3 years. However, an annual physical examination is strongly recommended for all dogs. If your dog is not on the preventative medication, we recommend an annual heartworm test and a physical examination.


The transmission cycle of the heartworm begins when a mosquito bites an infected dog and feeds on blood that contains tiny immature heartworms. As it feeds, the mosquito takes in the immature heartworms. During the next two to three weeks, these tiny worms develop into larvae (the infective stage of heartworms) within the mosquito. If the infected mosquito bites another dog or puppy that is not protected with a heartworm preventative, it can transmit infective larvae to that healthy animal.

Infective larvae enter the dog's body through the mosquito bite, and they continue to develop over the next few months. They migrate through the dog's tissues, eventually reaching the heart and lungs. Once in the dog's heart, the worms may grow to between 7 and 11 inches in length and can cause significant damage to the heart and lungs. If left untreated, heartworm disease may result in death. The life cycle of the heartworm is approximately 6 months.

The above is general veterinary information. Do not begin any course of treatment without consulting your regular veterinarian. All animals should be examined at least once every 12 months.

About the Author
Linda Mar Veterinary Hospital and its cat-only affiliate, Coastal Cat Clinic, are small animal practices located in Pacifica, California. To find a veterinarian or to learn more about the vet clinic and our staff, visit:[http://lindamarvet.com/]