Striegel Animal Hospital

The following information is provided as a public service only, and is NOT intended to replace the advice or recommendations of your pet’s veterinarian.

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

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

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. They survive up to 5 years and, during this time, the female produces millions of young (microfilaria). These microfilaria live in the bloodstream, mainly in the small blood vessels. The immature heartworms cannot complete the entire life cycle in the dog; the mosquito is required for some stages of the heartworm life cycle. The microfilaria are therefore not infective (cannot grow to adulthood) in the dog--although they do cause problems.

As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog and ingests the microfilariae during a blood meal. The microfilariae develop further for 10 to 30 days in the mosquito by growing and molting twice, and then enter the mouthparts of the mosquito. The microfilariae are now called infective larvae because at this stage of development, they will grow to adulthood when they enter a dog. When the mosquito feeds again, these larvae enter the dog’s body. When fully developed, the infective larvae enter the bloodstream and move to the heart and adjacent vessels, where they grow to maturity in a few months and start reproducing, and begin to release microfilaria into the bloodstream, thereby completing the life cycle.

Geography

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. Since transmission is totally dependent on mosquitoes, heartworm disease is most common in regions (like Southern Illinois) where mosquitoes are present.

Contagion

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. . In Southern Illinois, the mosquito season frequently lasts all twelve months of the year, since some species of mosquitoes can survive and feed any time the environmental temperature is above freezing.

It takes a number of years before dogs show 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. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.

Most dogs infected with heartworms do not show any signs of disease for as long as 2 years. Unfortunately, by the time signs are seen, the disease is well advanced. The signs of heartworm disease depend on 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 microfilariae.

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

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

Microfilariae (Young worms): Microfilariae 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. The body cells being supplied by these vessels are deprived of the nutrients and oxygen normally supplied by the blood. The lungs and liver are primarily affected. Destruction of lung tissue leads to coughing. Cirrhosis of the liver causes jaundice, anemia, and general weakness because this organ is essential in maintaining a healthy animal. The kidneys may also be affected and allow poisons to accumulate in the body.

If given to a dog which already has microfilariae in its bloodstream, most heartworm preventatives can cause a shock reaction which can be very rapidly fatal. This fatality can occur within minutes of the time the reaction begins. Therefore, it is critical that your dog receive a blood test to rule out a heartworm infection before starting (or restarting) any heartworm preventative.

DIAGNOSIS

In most cases, diagnosis of heartworm disease can be made by a blood test that can be run in the veterinary hospital. Further diagnostic procedures are essential, in advanced cases particularly, to determine if the dog can tolerate heartworm treatment. Depending on the case, we will recommend some or all of the following procedures before treatment is started.

Serological test for antigens to adult heartworms: This is a test performed on a blood sample. It is the most widely used test because it detects antigens (proteins) produced by adult heartworms. It will be positive even if the dog does not have any microfilaria in the blood; this occurs about 20% of the time. Dogs with less than five adult heartworms will not have enough antigen to turn the test positive, so there may be some false negative results in early infections. Because the antigen detected is produced only by the female worm, a pure population of male heartworms will give a false negative, also. Therefore, there must be at least 5 female worms present for the most common test to be positive.

Blood test for microfilariae: A blood sample is examined under the microscope for the presence of microfilariae. If microfilariae are seen, the test is positive. The number of microfilariae seen gives us a general indication of the severity of the infection. However, the microfilariae are seen in greater numbers in the summer months and in the evening, so these variations must be considered. Many dogs do not test positive even though they have heartworms because of an acquired immunity to this stage of the heartworm, or due to interference caused by several commonly-used medications. Because of this, the antigen test is the preferred test. Also, there is another microfilarial parasite which is fairly common in dogs; on the blood smear, these can be hard to distinguish from heartworm microfilariae.

Blood chemistries: Complete blood counts and blood tests for kidney and liver function may give an indirect indication of the presence of heartworm disease. These tests are also performed on dogs diagnosed as heartworm-infected to determine how much damage the worms may have already caused to these organs.

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.

TREATMENT

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. 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.

Eliminating heartworms must be done in stages.

1. Treatment to kill microfilaria: The first step in treating heartworms is eliminating the micofilaria. Your dog needs to stay in the hospital for the day. A single dose of medication is administered by mouth and your dog is observed closely all day. This same medication that eliminates the microfilaria also doubles as the first dose of heartworm preventative. Heartworm preventative is then safe to give each following month to prevent additional heartworm infestations. In some cases, the heartworm infection is "occult," meaning that no microfilariae were present. In this case, this step is not needed and heartworm preventative can be started immediately.

2. Doxycyline treatment: It was recently discovered that heartworms almost always have another parasite, called Wolbachia, on the surface of the heartworms. Wolbachia is symbiotic with heartworms, meaning that each parasite requires the other to remain healthy. Wolbachia are protective and beneficial to the heartworms. Treating your dog with an antibiotic called Doxycyline kills the Wolhachia and weakens the heartworms, reducing the heartworms’ ability to reproduce and making them easier to kill. Wolbachia is also thought to worsen the inflammation which occurs in the lungs when adult heartworms die during the next step in the treatment. Typically, Doxycycline is administered beginning three months after the microfilaria treatment and 1 month prior to the adulticide treatment. (If, for whatever reason, you choose not to have the adult heartworms in the dog’s heart eliminated, this step can be done at any time after the microfilarial treatment.)

3. Staging: This is where we “stage” the disease, meaning that we determine the amount of damage that the heartworms have already done to the dog’s heart, lungs and other major organs. This step can be done at any time prior to the adulticide treatment. Ideally, a full blood chemistry panel and X-rays of the chest (heart and lungs) are performed to check for changes. When a patient is in a more advanced stage of Heartworm Disease, it is best to change the adulticide treatment to a “slower kill” treatment; meaning that we split up the treatment and kill the adult heartworms more slowly, so it is safer for the patient.

4. Treatment to kill adult worms: This part of the treatment is done, ideally, 4 months after the microfilarial treatment and one month after the doxycyline is started. An injectable drug to kill adult heartworms is drug is given for two days. It kills the adult heartworms in the heart and adjacent blood vessels. Dogs are hospitalized for approximately the first week of this treatment.

Complete rest is essential after the treatment to kill the adult worms: The adult worms die in a few days and 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 is 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. Sometimes cage rest, antibiotics, anti-inflammation medication, intravenous fluids, or other supportive care becomes necessary to manage the dog’s body’s response to the dying worms.

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.

Follow-up testing: Four months after the treatment to kill the adult heartworms, a blood test is performed to confirm that the medications have eliminated all of the worms.

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.

PREVENTION

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 continue a heartworm prevention program.

There are several medications that can be used to prevent heartworm infection. Some of the more common ones are listed here. All of these are very safe and effective heartworm preventatives. All also do other good things for your dog. The additional advantages are listed here in increasing order of their spectrum of activity:

1) HeartGard Plus^TM is a chewable tablet given once each month. This tablet also eliminates 2 kinds of intestinal worms.

2) Interceptor^TM is a chewable tablet given once each month. This tablet also eliminates 3 kinds of intestinal worms.

3) Sentinel^TM is a chewable tablet given once each month. This tablet also eliminates 3 kinds of intestinal worms. In addition, this medication helps prevent flea infestation in your home by killing flea eggs and larvae.

4) Trifexis^TM is a chewable tablet given once each month. This tablet also eliminates 3 kinds of intestinal worms. In addition, this medication helps prevent flea infestation by killing the adult stage of fleas.

Interceptor^TM and HeartGard Plus^TM are similar in cost. Sentinel^TM and Trifexis^TM costs are also similar to each other, but due to their added flea protection, they both cost a bit more than the heartworm preventatives, alone. However, both Trifexis^TM and Sentinel^TM cost less than giving a separate heartworm preventative and a flea medication each month. A heartworm preventative should be started during the course of the heartworm treatment as soon as your veterinarian says that it is safe to do so, most commonly right after the microfilarial treatment.

Better yet, any dog living in or visiting geographic areas where heartworms are found should be placed on a regular schedule of Heartworm preventative before the dog’s first risk of exposure. In endemic areas, like southern Illinois, this means starting puppies on preventative as early as 4 weeks of age.

For an appointment at Striegel Animal Hospital, please call (618) 457-4133. For more information on pet care, we invite you to peruse the articles in the Medical Watch section of our site.