Heartworm Disease in Dogs, an Explanation|Healthy LifeStyle

Heartworm Disease in Dogs, an Explanation

By: PAUL SKELLENGER

I can remember being devastated as a second grader to find out that our family dog had heartworms and was dying and there was nothing we could do. That was 1966. Thankfully, much has changed since then. Today, treatment for heartworms is available and better yet, it can be prevented. Heartworms are an internal parasite of both dogs and cats. These worms live in the bloodstream and cause life threatening disease. Heartworms are a leading cause of lung and heart disease in dogs and dogs are dying of this disease every day. Heartworm disease is caused by the parasite, Dirofilaria immitis. It is transmitted by mosquitoes and has been found in every state in the US. It only takes one mosquito bite for your dog to get heartworms and, without prevention; there is a 100% chance your dog will get it.

The way your dog becomes infected is the stuff of nightmares. You see, dogs are the reservoir of infection. This means that there are many dogs out there that are infected and when a mosquito bites an infected dog, it ingests immature heartworms known as microfilaria. Microfilaria is the larval form of heartworms that change within the mosquito to reach the third stage. It is the third stage that can infect your dog. When the infected mosquito bites your dog, it deposits third stage microfilaria on your dog’s skin and the microfilaria enters the dog through mosquito’s bite wound. The microfilaria then travel or migrate through the tissues (meaning skin, muscle, connective tissue) and mature to become a juvenile adult or fifth stage larvae. This process takes about 50-70 days in the dog. Sometime between 70 and 110 days after the dog is infected, the larvae reach the blood stream and end up in the arteries of the lung (pulmonary arteries). These juveniles then grow up to produce microfilaria of their own in about 6.5 months after the dog is infected.

Heartworms cause the most harm just by being present in the major arteries of the lung, the pulmonary arteries (pa). They wreak havoc in these arteries and the entire dog suffers. It happens on many levels and here is a list of the damages.

1. Injury to the lining of the pa (pulmonary arteries).

2. Increased pressure within the pa, known as pulmonary hypertension, due to the presence of the heartworms, the thickening of pa walls, scarring and blockage of pa, and pieces of heartworm that lodge in pa or clots that form and lodge in pa (known as heartworm emboli). Pulmonary hypertension is then responsible for right sided heart disease, even right sided congestive heart failure. The severity of pulmonary hypertension is dependent on the number of adult heartworms, thus, the larger the number of heartworms, the more severe the disease.

3. Changes within the lung itself. These changes include the infiltration of lung by eosinophils, a white blood cell that takes part in the immune response to allergy and parasitism. Eosinophils are capable of causing and perpetuating a severe inflammatory response which damages the lung and leads to scarring of the lung. Other white blood cells and inflammatory cells may invade the lung causing damage. The heartworms can actually block an artery (thrombosis) and, in areas where thrombosis has occurred, nodules made up of inflammatory cells known as granulomas may form.

4. Vena Cava Syndrome. This syndrome strikes fear in the heart of every veterinarian. It can happen when there are excessive numbers of adult heartworms in the pulmonary arteries and the heart to the extent that they block blood returning from the liver and the back of the body. This results in severe pulmonary hypertension, the death of liver cells, and the destruction of red blood cells. Large numbers of adult heartworms can interfere with the closing of the valve on the right side of the heart (tricuspid valve) and cause increased speed of flow through this valve exacerbating these problems.

5. Serious injury to other organs, especially the kidneys, due to the deposition of damaging immune complexes formed between heartworm proteins and antibodies the dog produces against them.
If you are one of the lucky ones, your dog can be diagnosed before any symptoms occur. Just in case you were wondering what the symptoms were, here is a list:

1. None - dogs with only a few adult heartworms are not likely to develop significant symptoms. When symptoms do occur, they are usually delayed at least 1 year post infection and in most cases may not occur for many years.

2. Cough.

3. Difficulty breathing.

4. Intolerance to exercise, tiring easily. Note: 2-4 depend upon the severity of damage to the lung and/or the presence of right sided heart disease.

5. Weight loss - usually assoc. with severe heart &/or lung disease.

6. Coughing up blood - usually assoc. with pulmonary embolism.

7. Bloated abdomen due to fluid collection (ascites), a classic sign of right sided heart failure.

8. Bloody looking urine from pigments released during red blood cell destruction, pale mucous membranes (anemia), jaundice (icterus), weakness and lethargy, and signs of right sided heart failure (all due to Vena Cava Syndrome).