Wednesday, March 31, 2010

Heartworm Disease: Getting To The Heart Of The Matter

Heartworm Facts:
-Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs caused by a worm called Dirofilaria immitis.

-The female worm is 6 to 14 inches (15 to 36 cm) long and 1/8 inch (5 mm) wide; the male is about half the size of the female.

-Heartworms are found in the heart and large adjacent vessels of infected dogs. One dog may have as many as 300 worms.

Read on to find out how this potentially deadly parasite is spread and what you can do to prevent it from affecting your dog.

How Heartworms Get into the Heart

Adult heartworms live in the heart and pulmonary arteries of infected dogs. They have been found in other areas of the body, but this is unusual. In a dog they can survive up to 5 years and, during this time, a female worm can produce millions of young (microfilaria). These microfilaria live in the bloodstream, mainly in the small blood vessels, where they wait for a mosquito to bite the infected dog so they can complete their life cycle. Microfilaria in this stage are not conidered to be infective as they are unable to fully mature without the intermediate mosquito host.  However, even though they are not considered "infective" they can still cause other problems for the infected dog.

Once a female mosquito bites an infected dog and ingests the microfilaria during a blood meal the microfilaria continue developing for another 10 to 30 days before entering the mouthparts of the mosquito. After entering the mouthparts of a mosquito the microfilaria are now considered "infective." If the microfilaria are transmitted to a dog during this stage they will complete their life cycle and grow into an adult worm.


Canine heartworm disease occurs all over the world. In the United States, it was once limited to the south and southeast regions. However, the disease is spreading and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent.


The disease is not spread directly from dog to dog. An intermediate host, the mosquito, is required for transmission. Spread of the disease therefore coincides with the mosquito season. The number of dogs infected and the length of the mosquito season are directly correlated with the incidence of heartworm disease in any given area.

It takes a number of years before a dog can show any outward signs of infection. Consequently, the disease is diagnosed mostly in 4 to 8 year old dogs. The disease is seldom diagnosed in a dog under 1 year of age because the young worms (larvae) take up to 7 months to mature following establishment of infection in a dog.

Effects on the Dog

Adult worms: Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the valve action in the heart and by clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly in the lungs, liver and kidneys, leading to malfunction of these organs.

Dogs infected with heartworms may not show any signs of the disease for as long as 2 years. Unfortunately, this means by the time signs are seen, the disease is well advanced. The severity of the signs of heartworm disease depend on several factors including the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys from the adult worms and the microfilaria.

The most commons symptoms are: a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. All of these signs are usually most noticeable following exercise, and may cause some dogs to even faint.

Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds and in advanced cases, congestive heart failure may be apparent or the abdomen and legs may swell from fluid accumulation. There may also be evidence of weight loss, poor condition, and anemia.

In extreme cases severely infected dogs may die suddenly during exercise or excitement.

Microfilaria (Young worms): Microfilaria circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, they may block blood flow in these vessels causing damage to the cells and tissues beyond. The lungs and liver are two organs commonly damaged in this way.


In most cases, diagnosis of heartworm disease can be made by a simple blood test that can be run in the veterinary hospital. If a postivie result comes back further diagnostic procedures are essential, in advanced cases particularly, to determine if the dog can tolerate heartworm treatment. Depending on the case, some or all of the following procedures may be recommended before treatment is started.

Serological test for antigens to adult heartworms: This is a test performed on a blood sample and works by detecting antigens (proteins) produced by adult heartworms.

Blood test for microfilariae: A blood sample could also be examined under the microscope for the presence of microfilaria. If microfilariae are seen, the test is positive. The number of microfilaria is then counted to give a general indication of the severity of the infection.

Blood chemistries: Complete blood counts and blood tests for kidney and liver function may give an indirect indication of the presence of heartworm disease if they show abormal values. These tests are also often performed on dogs diagnosed as heartworm-infected to determine the function of the dog's organs prior to treatment.

Radiographs (X-rays): A radiograph of a dog with heartworms will usually show heart enlargement and swelling of the large artery leading to the lungs from the heart. These signs are considered presumptive evidence of heartworm disease. Radiographs may also reveal the condition of the heart, lungs, and vessels. This information allows us to predict an increased possibility of complications related to treatment.

Electrocardiogram: An electrocardiogram (EKG) is a tracing of the electric currents generated by the heart. It is most useful to determine the presence of abnormal heart rhythms.

Echocardiography (Sonogram): An echocardiogram allows us to see into the heart chambers and even visualize the heartworms themselves. Although somewhat expensive, this procedure can diagnose heartworms when other tests fail.


There is some risk involved in treating dogs with heartworms, although fatalities are rare. In the past, the drug used to treat heartworms contained arsenic so toxic effects and reactions occurred somewhat frequently. Now a newer drug is available that does not have the toxic side effects of the old one. Today we are able to successfully treat more than 95% of dogs with heartworms.

We see some dogs with advanced heartworm disease. This means that the heartworms have been present long enough to cause substantial damage to the heart, lungs, blood vessels, kidneys, and liver. A few of these cases will be so far advanced that it will be safer to just treat the organ damage rather than risk treatment to kill the worms. Dogs in this condition are not likely to live more than a few weeks or months.

Treatment to kill adult worms: An injectable drug to kill adult heartworms is drug is given for two days. It kills the adult heartworms in the heart and adjacent vessels.

Complete rest is essential after treatment: The adult worms will die in a few days and then start to decompose. As they break up, they are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This is a dangerous period, and it is absolutely essential that the dog be kept quiet and not be allowed to exercise for 1 month following treatment. The first week after the injections is very critical because the worms are dying. A cough can be noticeable for 7 to 8 weeks after treatment in many heavily infected dogs.

Prompt treatment is essential if the dog has a significant reaction in the weeks following the initial treatment, although such reactions are not common. If a dog shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, and/or depression, you should notify us. Response to antibiotics, cage rest, and supportive care, such as intravenous fluids, is usually good in these cases.

Treatment to kill microfilaria: Approximately 1 month following treatment to kill the adults, the dog is returned to the hospital for administration of a drug to kill microfilaria. Your dog needs to stay in the hospital for the day. Seven to ten days later a test is performed to determine if microfilaria are present. If they have been all killed, the treatment is complete. If there are still some present in the blood, treatment for microfilaria is repeated.

In some cases, the heartworm infection is "occult," meaning that no microfilaria were present. In this case, a follow-up treatment at one month is not needed.

Other treatments: In dogs with severe heartworm disease, it may be necessary to treat them with antibiotics, special diets, diuretics to remove fluid accumulations, and drugs to improve heart function prior to treatment for the heartworms.

Dogs with severe heart disease may need lifetime treatment for the failing heart, even after the heartworms have been killed. This includes the use of diuretics, heart drugs, aspirin, and special low salt, low protein diets.

Response to treatment: Dog owners are usually pleasantly surprised at the change in their dog following treatment for heartworms, especially if the dog had been showing signs of heartworm disease. The dog has a renewed vigor and vitality, improved appetite, and weight gain.


When a dog has been successfully treated for heartworms, you cannot sit back and relax because dogs can be reinfected. Therefore, it is essential to begin a heartworm prevention program. Three commonly available products for heartworm prevention are HeartGard*, Interceptor*, and Revolution*. Heartgard* and Interceptor* are chewable tablets that are given only once a month while Revolution* is a topical product that is also applied once monthly. All three products are very safe and very effective and one of these should be started immediately after treatment is completed.

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