Heartworms are found in the heart and large adjacent vessels of infected dogs and cats. 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 and cats. 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/cat; 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/cat--although they do cause problems.
      As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog/cat and ingests the micro filariae during a blood meal. The micro filariae develop further for 10 to 30 days in the mosquito 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/cat. The mosquito bites the dog/cat where the hair coat is thinnest. However, having long hair does not prevent a dog/cat from getting heartworms.
      When fully developed, the infective larvae enter the bloodstream and move to the heart and adjacent vessels, where they grow to maturity in 2 to 3 months and start reproducing, thereby completing the full 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, particularly where mosquitoes are prevalent.

Contagion
      The disease is not spread directly from pet to pet. An intermediate host, the mosquito, is required for transmission. Spread of the disease therefore coincides with the mosquito season. The number of dogs/cats 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 dogs/cats 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/cat under 1 year of age because the young worms (larvae) take up to 7 months to mature following establishment of infection in a dog/cat.

Effects on the Dog/Cat

      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/cats 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 011 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/cats 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/cats 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.

Diagnosis
      There is currently no approved treatment for heartworms in cats. 
      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 micro filariae are seen, the test is positive. The number of micro filariae seen gives us a general indication of the severity of the infection. However, the micro filariae are seen in greater numbers in the summer months and in the evening, so these variations must be considered. Approximately 20% of dogs do not test positive even though they have heartworms because of an acquired immunity to this stage of the heartworm. 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 micro filariae.

      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 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.

Treatment
      There is currently no approved treatment for heartworms in cats so prevention is of utmost importance. 

     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.

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

      Complete rest essential after treatment: 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 I 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. 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 microfilariae.. Two weeks later a test is performed to determine if microfilariae are present. If they have been all killed, the treatment is complete. If there are still some present in the blood, treatment for micro filariae is repeated.

      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.

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 begin a heartworm prevention program. There are medicines that can be used to prevent heartworm infection .. We have a chewable tablet that is given only once monthly. Revolution" is a topical product that is applied once monthly. Proheart is an injection that protects for six months and is recommended because owner compliance is easy and we do not have to worry about missing or getting off schedule on the monthly products. Their costs are all similar. One of these should be started immediately after the treatment is completed.