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Vernon, Connecticut 06066

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Pet Health Care Articles

Heartworm Disease in Dogs
What is Heartworm Disease?

November 29, 2011

At present, Heartworm disease is seen throughout much of the nation.  Due to ever changing climate and enviornmental factors, it is nearly impossible to determine when mosquito season starts and stops.  Even in areas of the country where season changes seem definitive, there can still be risk for off-season transmission. 

Town & Country strongly encourages pet owners to continue heartworm prevention year round.  Because a single dose of prevention is not 100% effective, and the effectiveness declines with increasing time from possible infection via mosquito bite, we believe it is imperative to continue prevention 12 months out of the year.  Additionally for dogs who travel, year round prevention will greatly minimize risk in other parts of the country, where transmission timeframes may be different.

Canine heartworm disease develops when a dog is bitten by a mosquito carrying microscopic heartworm larvae (juvenile worms) of a parasite called Dirofilaria immitis. As a mosquito feeds, these microscopic larvae are deposited on the dog and quickly penetrate the skin to begin their migration into the dog's bloodstream. Adult heartworms can grow 10 to 12 inches in length and make their home in the right side of the heart and pulmonary (lung) arteries, often causing lung disease and heart failure. Although easy to prevent, heartworm disease continues to be a major health problem for dogs living in the United States and throughout the temperate and tropical regions of the world.

When a dog or other suitable host has a mature heartworm infection, female worms release their young directly into the animal's bloodstream. When a susceptible mosquito bites an animal with microfilariae in the blood, it ingests the microfilariae along with the blood. Over the following 10 to 14 days, these microfilariae develop and mature into infective larvae inside the mosquito. When the mosquito bites another dog, the larvae are left behind to enter the fresh wound. In six to seven months, these microscopic larvae migrate to the heart and vessels of the lungs where they can grow up to a foot long. The larvae also become sexually mature and produce microfilariae of their own, which are available in the dog's blood to other mosquitoes. Because heartworms may live for five to seven years in the dog, each mosquito season can lead to an ever-increasing number of worms in unprotected dogs. 

Since some dogs can be infected for many years before symptoms develop, heartworm disease in dogs may not be obvious. But, as heartworms slowly cause damage to the pulmonary arteries of the lungs, signs of disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. Eventually, as blood flow through the diseased lungs becomes more restricted, some dogs can develop heart failure. This is commonly recognized by a buildup of fluid in the abdomen and the appearance of a "swollen belly".
Although less common, a large number of heartworms can lead to a sudden obstruction of blood flow through the heart and lungs. This blockage often becomes a life-threatening form of cardiovascular collapse and is referred to as "caval syndrome". Symptoms of caval syndrome often include a sudden onset of labored breathing, pale gums, dark red or "coffee-colored" urine, and an inability or unwillingness to move. Without prompt surgical removal of the heartworm blockage, few dogs suffering from caval syndrome survive.

Numerous blood tests are available for detecting heartworm infections in dogs, and your veterinarian will perform the test most appropriate for your dog. Keep in mind that no diagnostic test can accurately detect all heartworm infections. Tests cannot consistently detect infection until heartworms are at least seven months old. Moreover, tests are unable to detect infections if only male worms are present and may miss infections with only one or two female worms. At times, your veterinarian may recommend the use of X-ray or ultrasound imaging to help in the diagnosis of heartworm disease. They may also repeat the blood test at suggested intervals. 

Annual testing is recommended for monitoring the success of any heartworm prevention program. Sometimes, individual risk factors, clinical signs, preventive changes and testing limitations might warrant more frequent testing.The frequency of testing should be discussed with your veterinarian, but all dogs more than six months of age should be tested for heartworms prior to initiating a preventive program. At a minimum, retesting is recommended six months following initiation of preventive or any lapse in administration, and annually thereafter.

There are several things to consider once a dog is diagnosed with heartworm disease. Without treatment, heartworm disease will worsen and may lead to more serious illness. Unless medical reasons identify a dog as a poor candidate, heartworm-positive dogs should be treated. However, treating dogs for heartworms can also lead to serious health concerns, as these dead parasites may cause further injury to the lungs and pulmonary arteries. A thorough physical examination, radiographs, and blood and urine tests may be needed prior to treatment to assess your dog's level of risk. 
While heartworm treatments currently available are extremely effective in eliminating adult worms, some dogs may not be completely cleared with a single course of treatment. Testing is recommended six months after treatment to ensure all heartworms were killed. If tests are positive, additional treatment may be indicated.

Heartworm preventives are effective when given properly and on a timely schedule. It is important to monitor your pet's weight to insure your pet falls within the weight range listed on the package. All approved heartworm preventives are highly effective, safe, easy to use, relatively inexpensive, and often provide treatment for additional parasites. Prevention is always more safe and affordable than treating dogs with adult heartworm infections.  The best way to eliminate the risk of heartworm infection in your dog is to institute a year-round prevention program. Be certain to have all pets tested prior to initiating or restarting any heartworm prevention program, as administration of some preventives can cause life-threatening reactions when given to heartworm-infected pets.