What are heartworms?
Heartworms are circulatory system parasites transmitted by mosquitoes. All dogs and cats are at risk for heartworm disease regardless of if they are strictly indoors or not! They are not directly contagious from one dog to another, and they are unrelated to intestinal worms. Heartworm disease is quickly becoming epidemic in our area, infecting more and more dogs each year. One in five dogs in Franklin County not on heartworm prevention is heartworm positive. We recommend keeping all pets on heartworm prevention monthly year-round!
How does a pet get infected?
When a dog is bitten by a mosquito carrying heartworm larvae (microfilaria), the larvae are transferred to the dog’s skin, where it travels through the bloodstream into the heart, and develops into an adult heartworm. Adult heartworms can exceed twelve inches in length! The adult heartworms in the heart interfere with its function, causing damage to the heart, heart failure, and possibly death if left untreated. As they travel, heartworms can also cause serious lung disease, especially in cats.
Damage to the Pulmonary Arteries
Arteries do not do well with worms living inside them. The lining of the artery becomes damaged within days of the worm’s arrival. Cells of the immune system are called into the area, but the worm is far too big for these tiny cells to destroy. The resulting inflammation; however, continues to damage the artery. The arteries dilate and become tortuous, which may be visible on a radiograph. Aneurysms and abnormal blood clotting called embolism results. Blood is shunted to other arteries that are not plugged up by worms, and fluid begins to accumulate in the lung around the worm-filled arteries. Blood being sent to the lung is not efficiently oxygenated and areas of lung become consolidated and unable to participate in providing oxygen to the blood.
Coughing and exercise intolerance result as areas of the lung are affected.
Nose bleeds may occur due to abnormal blood clotting in the lung.
A form of non-infectious pneumonia (pulmonary eosinophilic granulomatosis) can result from excessive infiltration of inflammatory cells into the lung in response to the parasite
Blood normally is pumped with ease through the arteries of the lung. With the arteries plugged with worms, the heart must pump harder against the pressure of the plugged arteries. This condition is called pulmonary hypertension and the right side of the heart must drastically increase its ability for the heart to work. It may be strong enough and it may not be.
If worms begin backing up into the heart, there will be less space in the pumping chamber for blood to be pumped. The heart must pump through the high pressure system of the plugged arteries using less blood then normal. In order to meet the body’s oxygen demand, the heart must pump faster and stronger still. There may come a point when the heart simply is not strong enough.
When the heart muscle begins to thicken (as any over-worked muscle will), it may not conduct electrical impulses normally. This means that the pumping/filling rhythm can be disrupted and an arrhythmia may result.
In any heart disease, arrhythmia is a possibility;
when arrhythmia is a possibility, so is sudden death
If the right side of the heart becomes too weak to keep up, fluid may accumulate in the chest and abdominal cavities, leading to a pot-bellied appearance and/or difficulty breathing.
Chronic Immune Stimulation
When a dog goes without treatment for heartworm disease, its immune system becomes chronically stimulated. Antibodies, which are not only important tools of the immune system but are also inflammatory proteins, are produced in high amounts all the time. These antibodies can cause a lot of trouble by settling in the delicate membranes of the eye, kidney, blood vessels, and joints. Antibodies that are stuck in these areas call in inflammatory cells and damage these delicate membranes, thus setting up tremendous tissue damage and pain.
Caval syndrome represents an especially disastrous form of heartworm disease. Here, there are so many worms at one time (around 100) that the entire right side of the heart is filled with worms and they are backing out into the large veins that feed the right side of the heart. Usually there have been no signs of heart disease prior to the collapse, shock, and red blood cell destruction associated with this syndrome. Death usually occurs within 1 to 2 days and the only effective treatment is to open the dog’s jugular vein and physically remove the worms with a special clamp. If enough worms can be removed to re-establish blood flow, the dog may survive.
Heartworm disease is a highly significant problem and must be managed both by dealing with the worms and by dealing with the heart disease.
Heartworms in Cats
Heartworm disease in cats is quite a bit different from dogs. Cats are so small that only one adult worm can be enough to cause heart failure, plus in cats there is much more inflammation involved with the immature worms. See details on infection, disease, treatment, and prevention of heartworms in cats.
How Do We Prevent Heartworm Disease?
We begin puppies and kittens on heartworm preventive medication, without testing, when they are less than six months of age. Adult dogs and puppies older than six months should be tested before beginning preventative medication. The medication should be given once a month, on the same day of every month, all year round. We carry Revolution and Heartgard as monthly heartworm prevention.
When Do We Recommend Heartworm Testing?
We test all dogs over six months of age for heartworm disease before starting medication, and retest annually thereafter. If your dog has been off heartworm prevention we recommend retesting 6 months after restarting prevention. Though the heartworm medications we prescribe are very safe and effective, no medication is 100% effective. If your dog has contracted heartworm disease, we can treat to remove the worms, but nothing can cure the damage they do to the heart – this is why annual testing is vital.