<p>Canine heartworm disease (CHWD) is a common condition in many regions of the world. It is caused by the filarial (threadlike) worm Dirofilaria immitis that lives in the pulmonary arteries (blood vessels leading from the heart to the lungs) in most infected dogs. The name "heartworm" is a bit misleading because only in very heavy infections do the worms actually reside in the heart itself. </p><p>Nonetheless, the presence of these worms causes strain to the heart and an intense reaction in the blood vessels, resulting in problems for the dog. D. immitis is injected into the dog by infected mosquitoes. For this reason, CHWD is more common in tropical regions of the world, where the conditions of warmth and moisture are ideal for mosquitoes to live. </p><p>Predispositions for CHWD include living outdoors. Male dogs may be more likely to be infected than female dogs.</p><p>The most important predisposing factor is failure to receive heartworm preventative medication. All dogs living in an area where heartworm disease exists are at risk, even if they live entirely indoors.</p><p>Transmission occurs when a mosquito bites an infected dog and ingests heartworm larvae (baby heartworms), which live in the bloodstream. When the insect bites another dog or cat, some of the larvae are injected under the skin. The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.</p><p>Impact on the pet is variable. Dogs can be without symptoms if the infection is light or has occurred recently. In some cases HWD can cause severe debilitation and eventually may be fatal.</p><p>Because heartworm disease is easily preventable, it has become a less common diagnosis in pets that receive regular veterinary care.</p><p>What To Watch For</p><p>Symptoms don't usually develop until damage has already occurred to the heart. Dogs can have a wide range of symptoms, with some dogs being completely asymptomatic (no symptoms at all). Symptoms usually occur because of heart failure. These include:</p><p> Coughing</p><p> Coughing up of blood (hemoptysis)</p><p> Heavy breathing</p><p> Unwillingness to exercise</p><p> Signs of right sided congestive heart failure, which include fluid distention of the belly, pulsation of the jugular veins in the neck when the dog is sitting or standing and heavy breathing.</p><p>Preventative Care</p><p>Prevention of heartworm disease is easy and should be undertaken in all dogs. These include monthly preventatives.</p>