Related Articles * Heartworm Information Center Diagnosis of Heartworm Disease Feline Heartworm Infection Heartworm Prevention Heartworm Preventive Comparison Heartworm Treatment Heartworm: The Parasite Heartworm: The Parasite The Heartworm Situation in Los Angeles What Happens in Heartworm Disease

(chemoprophylaxis)

Ivermectin (Heartgard Plus by Merial)

The development of this medication in the late 1980s represented a huge breakthrough in heartworm prevention. Preventive medication for the first time could be given once a month instead of daily.

If ivermectin is inadvertently given to a heartworm-infected dog with circulating microfilariae, adverse reactions would not be expected; in fact, this product is commonly used in the treatment of active infection

Ivermectin at the heartworm preventive dose is not strong enough to kill common intestinal parasites but it is available with pyrantel pamoate included (Heartgard Plus) to control hookwormsand roundworms.

There are breed-related sensitivities with ivermectin (i.e., collie-related breeds have some difficulties) though at the very low doses used in the prevention of heartworm disease are not a problem for any breed. (Note that at one time Merial had even contracted Lassie as
the "poster dog" for their heartworm prevention products).

See more information on ivermectin.

Selamectin (Revolution by Pfizer)

For more information on Revolution (from the manufacturer), visit www.revolutionpet.com

Milbemycin Oxime (Interceptor & Sentinel by Novartis)

Milbemycin, however, is able to control roundworms, hookworms, and whipwormswithout the addition of a second parasiticide. It is also available combined with Lufenuronfor the control of fleas.  There are no breed-related sensitivities for milbemycin.

Milbemycin oxime is also used effectively in the treatment of demodectic mange.

See Novartis Animal Health's site for more information on Sentinel.

Moxidectin (ProHeart by Fort Dodge)

Moxidectin is another relative of ivermectin.

On September 3, 2004, at the FDA's request, Fort Dodge  agreed to stop production and recall its heartworm medication ProHeart6 from the market until the FDA's concerns about adverse reaction reports associated with the product are resolved.

For more information visit www.Proheart6.com.

When to Start Prevention Each Year

Obviously the answer to this question is regional. Indeed it may be simplest to just use preventive medications all year round. There is more to transmission than just the presence of mosquitoes; it must also be warm enough to allow the development of microfilariae to infective L3s. A simple formula involves counting the degrees above 57 degrees F reached each day. Each degree is called a “heartworm development unit” and when 234 heartworm development units have accumulated within a 30-day period, conditions have been reached to allow the transmission of L3 heartworm to new hosts. A monthly heartworm pill or chewable must be given at the end a month in which 234-heartworm development units has accumulated.

When 30 days pass and 234 heartworm development have not accumulated, mosquitoes will die having reached the end of their natural lives before any microfilariae they carry can develop to L3s. Monthly heartworm preventive need not be given after a month under these conditions.

If all this sounds complicated, it is. In addition, most of us have better things to do besides monitoring average weather temperatures. It may be simpler to use the product all year round or just go by the recommendations of a practicing veterinarian in the region in question.

At March Animal Hospital we recommend year round heartworm preventative for two reasons.  (1)  The weather is very variable from year to year and becomes difficult to predict when exactly it is safe to stop preventative (usually after second deep frost) (2)  The medication will protect pets from intestinal parasites which they can be exposed to year round through contact with other animals stool.  It is also recommended that testing be done yearly because heartworm preventative is not 100% effective, and in rare instances animals on the preventative will develop the disease.  By testing yearly we will catch heartworm disease long before your pet becomes clinical and therefore have a better outcome if treatment is necessary.  Often if animals are on medication and tested regularly the companies will cover the cost of treatment if they come back heartworm positive.

For a side-by-side detailed comparison of how commercially available heartworm preventive products (Heartgard30, Revolution, and Sentinel) work, take a look at our heartworm preventive comparison chart.

What Happens in Heartworm Disease
By Wendy C. Brooks DVM, DABVP

Related Articles * Heartworm Information Center Diagnosis of Heartworm Disease Feline Heartworm Infection Heartworm Prevention Heartworm Preventive Comparison Heartworm Treatment Heartworm: The Parasite The Heartworm Situation in Los Angeles What Happens in Heartworm Disease

HEARTWORM DISEASE VS. HEARTWORM INFECTION
Before reviewing the clinical signs seen in heartworm disease, an important distinction must be made between heartworm disease and heartworm infection. Heartworm infection by definition means the host animal (generally a dog) is parasitized by at least one life stage of the heartworm (Dirofilaria immitis). Dogs with heartworms in their bodies do not necessarily have adult worms in their hearts; they may have larval heartworms in their skin only. Dogs with heartworms in their bodies are not necessarily sick, either. Dogs with only larvae of one stage or another are not sick and it is controversial how dangerous it is for a dog to have only one or two adult heartworms. These dogs are certainly infected but they do not have heartworm disease.

On the other hand, dogs with heartworm disease are sick. They not only have the infection but they have any of the problems listed below because of it. Fortunately, heartworm disease is both treatable and preventable. Further sections of this web area explain both treatment and prevention; we will now discuss the damage heartworms can do to a dog's body.

DAMAGE TO THE PULMONARY ARTERIES
Arteries do not do well having 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 (embolism) results. Blood is shunted to other arteries which 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 lost to the blood oxygenation process.
   
• 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.

HEART FAILURE
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 to work. It may be strong enough and it may not.

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 AND 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 cavity and abdominal cavity, 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 inflammatory proteins, are produced in high amounts all the time. These antibodies can cause a lot of trouble by precipitating in the delicate membranes of the eye, kidney, blood vessels, and joints. Antibodies stuck in these areas, call in inflammatory cells and damage these delicate membranes thus setting up tremendous tissue damage and pain.

CAVAL SYNDROME
Caval syndrome represents an especially disastrous form of heartworm disease. Here, there are so many worms present (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.

Learn more about heartworm treatment.

Learn more about heartworm in cats.

 NOTE- THIS FULL ARTICLE AND THE ACTUAL LINKS CAN BE OBTAINED BY GOING INTO THE PET LIBRARY SEARCH ENGINE ON THIS SITE.