Heartworm is caused by a parasite (Dirofilaria immitis) which is transmitted by mosquitoes and has the potential to cause heart failure and death. Your pets do not have to come into contact with other animals to develop this disease.
 

It may only take a single bite from a carrier mosquito to infect your dog. The parasite larvae move through the tissue beneath the skin. These larvae then migrate to the heart and adjacent blood vessels of the lungs where they grow into adult worms.
In the early stages of heartworm infection there may be no outward signs of disease.

Life long heart worm prevention is recommended for all dogs and can be in the form of tablets given daily or monthly, monthly spot on medication or convenient annual injections. A simple blood test may be recommended before starting heartworm prevention for the first time or if some doses have been missed.