While some medical conditions in dogs may run their course and resolve on their own, others can be devastating when left untreated. In dogs, a tiny bite from an infected mosquito leads to a cascading sequence of events with life-threatening consequences if treatment isn't initiated. Because treatment can be very expensive and takes a toll on the dog, prevention remains the best treatment.
Effects on Immune System
When heartworm disease is present, the dog's immune system is activated. Antibodies will be produced in an attempt to fight the disease. Left untreated, the dog's immune system will be chronically stimulated and antibodies will be released all the time. Over time, this chronic, long-term production of antibodies causes inflammation, pain and significant damage as the antibodies settle within the membranes of the eye, kidney, blood vessels, and joints.
Effects on Arteries
The arteries of dogs weren't meant to house worms inside them. When the worms establish in the arteries, immune system cells are sent to fight the worms, creating inflammation. These cells though aren't capable of destroying the worms, and the inflammation ultimately damages the lining of the arteries, causing them to become tortuous, thickened and dilated. Because the arteries end up being plugged with worms, left untreated, heartworm disease can potentially lead to blood clots and aneurysms.
Effects on Heart
When a dog's arteries fill up with worms, mechanical blockage to the blood flow causes the heart to beat faster and stronger to compensate for the pressure build-up. This greater workload causes pulmonary hypertension, which left untreated may lead to congestive heart failure. Other risks involve the thickening of the heart's muscle, which may lead to arrhythmia, and in heavy burdens, the development of caval syndrome causing shock, red blood cells destruction and even death within one or two days.
Effects on Lungs
When several important arteries are clogged with worms, blood will be re-routed to arteries without worms. Because fluid accumulates in the lungs due to plasma leaking out from small vessels and capillaries, the lungs will no longer be able to oxygenate the blood as needed. Also, the damaged lungs develop chronic lesions and scarring. Left untreated, clinical signs such as coughing, exercise intolerance, shortness of breath, and pneumonia due to lung inflammation may occur.
Effects on Liver and Kidneys
Because heartworms clog important blood vessels, the blood supply to important organs such as the liver and kidneys is reduced causing them to malfunction. When the liver is affected, dogs develop jaundice, anemia and associated weakness. When the kidneys are affected, toxins are allowed to accumulate into the dog's body causing many devastating effects.
Effects of Non-Treatment
In some cases, where no treatment is instituted, the dog may rid himself completely of the heartworms after several years, according to Scenic Hill Veterinary Hospital. However, there are also high chances that without treatment the affected dog will continue to get infested with more heartworms, and in the meantime, inevitably, irreversible damage to the heart and blood vessels will occur. Mild treatment initiated with low levels of ivermectin along with other drugs remains the treatment of choice for most dogs with heartworm.