Heartworms damage the lining of the pulmonary arteries causing them to leak fluid into the lung tissue. Large numbers of worms also physically reduce the blood flow to the lungs and create turbulence which further reduces the hearts ability to pump blood to the lungs especially in response to exercise. When the number of worms exceeds 1 worm per kg of dog body weight they will start to move into the right side of the heart. This combined with the increase in resistance against which the heart is working leads to enlargement of the right heart which is one of the characteristic findings seen on Xray.

Clinical signs of heartworm relate to the number of worms present in the pulmonary arteries. Low numbers of worms will generally not produce any signs of illness. Early clinical signs are coughing and poor tolerance to exercise. More severe signs are typical of lung disease with persistent coughing occasionally containing blood and panting or difficulty breathing.

Heartworm can be diagnosed through testing of the dogs, blood. Antigen tests detect the presence of adult worms in the heart, while filter tests are used to detect microfilaria in the blood. The antigen tests are accurate and simple to perform being available in kits for use in the veterinary clinic. Filter tests are often used to confirm a positive antigen test. Where the infection consist mainly of male worms (which don't produce microfilaria), or only a small number of worms, microfilaria may not be detected in the blood. This is referred to as an occult infection. Xrays of the chest may reveal the degree of damage to the heart and lungs of more severely effected animals.

Treating dogs diagnosed with adult heartworm infection involves the use of adulticides (drugs which kill adult worms). Originally arsenic based compounds were used however these had frequent side effects and a low safety margin. Newer families of adulticides have proven to be more effective and much safer to use. The drugs are normally given by injection over a 48 hour period during which time the animals are monitored for any complications. Treating the clinical signs with drugs directed at the symptoms of heart and lung disease is usually started before killing the adult worms. After the adulticide has been administered it is essential to rest the dog for around 4 weeks. This is to reduce the risk of complications arising from the bodies reaction to dead worms passing out of the pulmonary arteries. Corticosteroids may be prescribed for part of this recovery period to reduce any inflammatory reaction to these dead worms. Aspirin is also recommended in some cases to help prevent and dissolve blood clots and reduce damage to the red blood cells following treatment. Some veterinarians may recommend against treating older dogs with few clinical signs however it is usual to treat all other infected dogs.

Drugs which kill microfilaria (microfilaricides) are given four weeks after adulticide therapy if the dog is still has a positive filter test. This may be repeated a further four weeks later if they are still present in the blood. Where microfilaria persist after this period it is likely adult worms are still present.

Prevention is the best policy and is achieved by maintaining a constant level of drug which kills the infective larva in the dog. Diethylcarbamazine (DEC) given daily has been the traditional therapy however this is being replaced by a number of newer drugs including Ivermectin and Milbemycin which can be given monthly. Puppies should be started on preventative drugs from as early as 8 weeks of age however dogs older than 6 months should be tested for the presence of adult heartworm before starting on these drugs.