What are heartworms?
Heartworms are parasites that live in a dog’s heart or pulmonary arteries. They are nine to 11 inches long and look like angel hair pasta. Heartworm infection, also known as dirofilariasis, is a serious and potentially fatal disease in dogs.
How are heartworms transmitted?
Heartworms are transmitted by mosquitoes. When an infected mosquito bites a dog, it deposits heartworm larvae into the body. The larvae migrate for several months before ending up in the right side of the heart or the pulmonary arteries. Once the parasites mature (about six months from the time they enter the dog's body), they begin to release immature heartworms, known as microfilaria. Microfilaria live in the dog’s blood for about one month and may be ingested by mosquitoes feeding on the dog. 
Because of the parasites' life cycle, it is necessary for a dog to be bitten by a mosquito to be infected with heartworms. Heartworms are not transmitted directly from one dog to another nor from a cat directly to a dog. 
Where are heartworms found?
Canine heartworm infection occurs all over the world. In the United States, it was once limited to the South and Southeast regions; however, the disease has spread and is now found in most U.S. regions and Canada, particularly in areas where mosquitoes are prevalent.
It can take a number of years before dogs show outward signs of infection. Consequently, the disease is diagnosed mostly in four- to eight-year-old dogs. Heartworms are seldom diagnosed in a dog under one year of age because the larvae take up to seven months to mature following establishment of infection.
What do heartworms do to the dog?
Adult worms cause disease by clogging the heart and major blood vessels. They also interfere with the valve action in the heart. By clogging primary vessels, the blood supply to other organs—particularly the lungs, liver and kidneys—is reduced, often resulting in malfunction.
Most dogs infected with heartworms do not show any signs of disease for as long as two years, but this is entirely dependent on the severity of infection. Unfortunately, by the time signs are seen, the disease is usually 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. The most obvious symptoms 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.
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. Evidence of weight loss, poor condition and anemia may also be noted. Severely infected dogs may die suddenly during a period of exercise or excitement.
Microfilariae circulate throughout the body but remain primarily in the small blood vessels. Because they are as wide as the small vessels, microfilariae can easily block the blood flow and deprive cells of nutrients and oxygen. As a result, the lungs and liver are primarily affected.
How is heartworm infection diagnosed?
In most cases, diagnosis of heartworm infection can be made by a blood test. Further diagnostic procedures are essential, particularly in advanced cases, 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: This test is performed on a blood sample. It is the most widely used test for canine heartworm infection because it can detect antigens produced by adult heartworms. This test will be positive even if the dog does not have any microfilaria in the blood, which occurs about 20 percent of the time. Dogs harboring less than five adult heartworms will not have enough antigens 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 also give a false negative. Therefore, there must be at least five female worms present for the test to be positive.
  • Blood test for microfilariae: This blood sample is examined under a 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. Approximately 20 percent of infected dogs do not test positive because of an acquired immunity to this stage of the heartworm. For this reason, the antigen test is the preferred.  
  • Blood chemistries: Complete blood counts and blood tests for kidney and liver function may give an indirect indication of the presence of heartworm infection. These tests are also performed on heartworm-positive dogs 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 connecting to the lungs. These signs are considered presumptive evidence of heartworm disease. Radiographs may also reveal the condition of the heart, lungs and vessels in infected dogs. This information allows us to predict an increased possibility of complications related to treatment.
  • Electrocardiogram (EKG): An electrocardiogram (EKG) traces 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 visualize the heartworms. Although somewhat expensive, this procedure can diagnose heartworms when other tests fail.
How are dogs treated for heartworms?
There is some risk involved in treating dogs with heartworms, although fatalities are rare. In the past, the drug most commonly used to treat heartworm infection contained arsenic. While this treatment was effective for killing heartworms, it often led to toxic reactions and illness in the dog. Today, newer drugs are available that do not have these toxic side effects, and as a result, we are able to successfully treat more than 95 percent of canine heartworm infections.
Some dogs with advanced heartworm disease are more difficult to treat. An advanced heartworm infection 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 are so far advanced that it is safer to simply 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 given for two days. It kills the adult heartworms in the heart and adjacent vessels. Complete rest is essential after treatment as the adult worms die and start to decompose. As the worms break up, they are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This can be a dangerous period, so it is absolutely essential that the dog refrains from exercise for one month following treatment. A cough may be noticeable for seven to eight 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 uncommon. If a dog shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, and/or depression, you should notify a veterinarian immediately. Response to antibiotics, cage rest and supportive care, such as intravenous fluids, is usually good in these cases.
  • Treatment to kill microfilaria: Approximately one month following treatment to kill the adult worms, an infected dog will need to be returned to the hospital for administration of a drug to kill the microfilariae. Seven to 10 days after this treatment, a test will be performed to determine if microfilariae are present. If all the microfilariae have been killed, the treatment is complete. If there are still some present in the blood, treatment for microfilariae is repeated. In some cases, the heartworm infection is "occult," meaning that no microfilariae were present. In this case, a follow-up treatment is not needed.
  • Other treatments: In dogs with severe heartworm infections, it may be necessary to treat with antibiotics, special diets, diuretics to remove fluid accumulations and drugs to improve heart function prior to treatment for the heartworms themselves. 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 medication and special low-salt, low-protein diets.
What is the prognosis?
Dog owners are usually pleasantly surprised at the change in their dog following treatment for heartworms, especially if the dog had been showing symptoms. In most cases, the dog has a renewed vigor and vitality as well as improved appetite and weight gain.
Are changes made in the treatment protocol for dogs who have severe heartworm infection?
Yes. The state of heart failure is taken into consideration and treated as described above. In these cases, we also treat adult heartworms in two stages rather than one. Only one treatment to kill the worms is given initially. This causes the death of only some of the worms. One month later, the full treatment is given to kill the remaining worms. By killing the heartworms in two stages, severe effects on the lungs are much less likely to occur.
How can I prevent this from happening again?
When a dog has been successfully treated for heartworms, he/she is still at risk for reinfection. Therefore, it is essential to begin a heartworm prevention program. Monthly chewable tablets can be used to prevent heartworm infection. These products are very safe and effective and should be started immediately after treatment is completed.
See Feline Heartworm Infection