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Heartworms are a scary disease for dogs. Heartworm disease is caused by a parasite, a worm known as Dirofilaria immitis. This worm lives inside of the heart and the blood vessels.
Signs of heartworm disease are caused by damage to the heart and blood vessels as a result of the heartworms living in them. In addition, the body's attempt to rid itself of the worms results in chronic immune stimulation, which further complicates the disease.
Heartworm Infection Versus Heartworm Disease in Dogs
Heartworm infection starts when the dog is bitten by an infected mosquito, which injects heartworm larvae (known as microfilaria) into the dog's skin.
The larvae injected into the dog by the mosquito then proceed to go through their maturation cycle and eventually end up living as adults inside of the heart and lungs. Dogs may be infected with heartworms but not yet have adult heartworms living inside of the heart and blood vessels.
Dogs that are infected with heartworms may not be sick. Because they do not yet have adult heartworms living in the heart and blood vessels, the damage to these tissues has not yet occurred and the signs of heartworm disease will not be evident.
In contrast, dogs with heartworm disease are sick.
Heart and Lung Disease Develops in Canine Heartworm Disease
Once the adult worms have moved into the blood vessels and the heart, the damage begins to occur. The worms living within the pulmonary arteries cause damage to the arteries themselves. The arteries become dilated and also take on a tortuous appearance.
At the same time, the worms in the pulmonary arteries make it more difficult for the heart to pump blood through the arteries to the lungs. As a result, the heart muscle, especially that on the right side of the heart, starts to thicken from the increased workload.
As heartworm disease progresses, blood clots are formed because of the damaged arteries. These clots flow through the arteries and eventually plug up some of the smaller blood vessels. When this happens in the lungs, fluid will begin to build up in the lung tissue surrounding the blocked blood vessel and the lung is not able to oxygenate the blood normally.
Signs of Heartworm Disease in Dogs
Coughing may be one of the first signs noted in dogs with heartworm disease. Exeercise intolerance is also common. These signs occur because of the damage to the heart and lungs done by the heartworms.
Pneumonia can also occur. When the body detects the heartworms in the pulmonary arteries and smaller vessels entering the lungs, it sends inflammatory cells into the lungs to try to fight the parasite. These inflammatory cells cause a non-infectious form of pneumonia as they attempt to kill the organisms.
Dogs with heartworm disease may also have abnormal clotting, especially in the lungs. Nosebleeds can happen as a result.
Cardiac arrhythmias (abnormal heart beats) can occur also because of the thickening within the heart muscle. An arrhythmia can cause fainting, weakness or even sudden death.
Caval syndrome is an especially serious form of heartworm disease. In caval syndrome, there are so many adult heartworms present that the entire right side of the heart is filled and the worms start to overflow into the vessels that lead into the right side of the heart. This leads to shock, red blood cell destruction and collapse. Death can occur relatively quickly in caval syndrome and, in some cases, this may be the first sign of heartworm disease.
Heartworm Treatment Options