Sentinel by NOVARTIS

__Two major mechanisms result in the signs of Heartworm disease seen in dogs. The first is due to the damage the worms cause to the arteries in the lungs (called the Pulmonary arteries). The second is the mechanical obstruction of blood flow that results from the inflammation and the number of worms present.

When a dog is first infested with Heartworm there are no visible or detectable signs. The infection cannot be detected even with a blood test. The changes in the victim start to occur when the final molt of the Heartworm larvae occurs and the immature L5 larvae arrive in the right ventricle and neighboring blood vessels. The arteries do not do well with worms living inside them. The artery lining is damaged within days, the body responds by inducing inflammation of the artery, called endarteritis, and other inflammation in the area to try to heal the damage. Unfortunately, the worms cause damage at a rate faster than the body can heal. The arteries over time develop certain characteristics that are typical of Heartworm disease, often these changes can be seen on x-rays. The vessels become tortuous and dilated. Blood clots and aneurysms are a common side effect, and complete blockage of small blood vessels can occur. The blood re-routes to non-worm burdened arteries. The result is complete and partial blockage of blood vessels and fluid begins to accumulate around these blood vessels in the lungs. This results in a loss of useful lung tissue and reduces the effective area of the lungs to oxygenate the blood for the body’s needs. As a result of the inflammation, blood vessel obstruction, and fluid accumulation, coughing results. The dog or cat displays exercise intolerance, nosebleeds, shortness of breath and a type of pneumonia may occur secondary to the increase in lung inflammation (called pulmonary eosinophilic granulomatosis).

As immature L5 worms continue to arrive and mature in the heart and lungs, the total number of worms at various stages of maturity increase and as they grow in size and number the above conditions take their toll. The host’s reactions become more significant and the signs worsen. More and more blood vessels and the surrounding lung tissue are damaged and not useful to the dog and this results in an increased resistance to blood flow through the lungs. This “backup” increases the blood pressure (hypertension) in the right side of the heart and Vena Cava due to the obstruction of blood flow. With accumulation of even more fluid in the lungs, the end result is the signs of actual heart failure. The severity depends on the number of worms present and the dog’s reaction to the worms. The failing, weakened, stretched heart muscle results in rhythm abnormalities, fluid accumulation in the lungs (called pulmonary edema) and exercise intolerance.

Over time, the immune system becomes “turned on” at a rate higher than normal. This puts extra proteins (in the form of antibodies) into circulation and they can settle out in the various organs of the body that are delicate in nature such as the eye, kidney, blood vessels, and joints. This causes inflammation, more tissue damage, and pain.

One of the most severe signs of heartworm is called Caval Syndrome or Vena Cava Syndrome. This is seen when there are large numbers of adult worms (usually around 100 or more) in the heart. There is almost complete blockage of all blood flow. Many times there will be no signs of heart disease prior to the animal’s collapse. When fainting and collapse does occurred it is accompanied by severe shock, red blood cell destruction, and often death within 1-2 days. Sometimes the only chance for survival in these cases is for the veterinarian to surgically remove the worms from the heart through the jugular vein. If enough worms can be removed to re-establish sufficient blood flow, there is some chance of survival.