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Heartworm Disease Facts - Why Treatment is so Important

Updated: Jan 12, 2021

Summer may be coming but heartworm season is year round in North Texas and our state ranks among the top 10 states where heartworm disease is most prevalent. Since heartworm can cause life-threatening health issues, prevention is something that should be on the mind of every pet owner. Prevention is simple, and we are here to answer your questions so your pet can live a long, happy, and healthy life!

Did you know?

When a pet comes into HSNT's care we test for Heartworm disease. If a dog is heartworm positive, we cover the cost of treatment in our clinic so that they have a chance for a long and happy life with their new forever family.

Heartworm Disease - What Is It and What Causes It?

Heartworm disease is a serious disease that causes severe lung disease, heart failure, organ damage, and finally death in pets, primarily dogs, cats, and ferrets. The disease is caused by a parasitic worm called Dirofilaria immitis. The worms are spread through the bite of a mosquito. Dogs are the most common hosts and the worms mature into adults, mate, and produce offspring while living inside a dog. The worms are called “heartworms” because the adults live in the heart, lungs, and associated blood vessels of an infected animal.

Quick Facts

1 . Heartworm disease is NOT contagious, meaning that a dog cannot catch the disease from being near an infected dog or other animal.

2. Heartworm disease is ONLY spread through the bite of a mosquito. Inside a dog, a heartworm’s lifespan is 5 to 7 years.

3. Adult heartworms look like strands of cooked spaghetti, with males reaching about 4 to 6 inches in length and females reaching about 10 to 12 inches in length. The number of worms living inside an infected dog is called the worm burden. The average worm burden in dogs is 15 worms, but that number can range from 1 to 250 worms.

Can a dog get it even if they live primarily indoors?

Yes. Every dog whether an outside or inside dog, is susceptible to a mosquito. It is important that every dog be given preventatives to avoid this life threatenting disease.

How is a Dog Tested for Heartworms?

A veterinarian uses blood tests to check a dog for heartworms. The earliest that the heartworm proteins can be detected in a dog’s bloodstream is about 5 months after it is bitten by an infected mosquito.

When Should a Dog Be Tested for Heartworms?

Dogs that are 7 months of age and older should be tested for heartworms before starting heartworm prevention. A dog may appear healthy on the outside, but on the inside, heartworms may be living and thriving. If a heartworm-positive dog is not tested before starting a preventive, the dog will remain infected with adult heartworms until it gets sick enough to show symptoms.

Annual testing of all dogs on heartworm prevention is recommended. Talk to your dog’s veterinarian about the best time for your dog’s annual heartworm test.

Does Heartworm Prevention stop Heartworm Disease if my pet has it?

No, heartworm preventives do not kill adult heartworms. Also, giving a heartworm preventive to a dog infected with adult heartworms may be harmful or deadly. If adult heartworms are already in the dog’s bloodstream, the preventive may cause the worms to suddenly die, triggering a shock-like reaction and possibly death.

Can People Get Heartworms from Their Pets?

No, people cannot get heartworms from their pets. Heartworms are only transmitted by the bite of an infected mosquito. In rare cases, people can get heartworms after being bitten by an infected mosquito. But because people are not a natural host for heartworms, the larvae usually migrate to the arteries of the heart and lungs and die before they become adult worms.

What are the Symptoms of Heartworm Disease in a Dog?

Symptoms of heartworm disease may not be obvious in dogs that have low worm burdens, have been recently infected, or are not very active. Dogs that have heavy worm burdens, have been infected for a long time, or are very active often show obvious symptoms of heartworm disease.

There are four classes, or stages, of heartworm disease. The higher the class, the worse the disease and the more obvious the symptoms.

· Class 1: No symptoms or mild symptoms such as an occasional cough.

· Class 2: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.

· Class 3: More severe symptoms such as a sickly appearance, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.

· Class 4: Also called caval syndrome. There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is risky, and even with surgery, most dogs with caval syndrome die.

Not all dogs with heartworm disease develop caval syndrome. However, if left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death.

Is There a Treatment for Heartworm Disease in Dogs?

Melarsomine dihydrochloride (available under the trade names Immiticide and Diroban) is an arsenic-containing drug that is FDA-approved to kill adult heartworms in dogs. It's given by deep injection into the back muscles to treat dogs with stabilized class 1, 2, and 3 heartworm disease. Another drug, Advantage Multi for Dogs (imidacloprid and moxidectin), is FDA-approved to get rid of microfilariae in the dog’s bloodstream. Advantage Multi for Dogs is a topical solution applied to the dog’s skin.

The treatment for heartworm disease is not easy on the dog or on the owner’s pocket book. Treatment can be potentially toxic to the dog’s body and can cause serious complications, such as life-threatening blood clots to the dog’s lungs. Treatment is expensive because it requires multiple visits to the veterinarian, blood tests, x-rays, hospitalization, and a series of injections.

The Good News? The Best Treatment is Prevention!

Many products are FDA-approved to prevent heartworms in dogs. All require a veterinarian’s prescription. Most products are given monthly, either as a topical liquid applied on the skin or as an oral tablet. Both chewable and non-chewable oral tablets are available. One product is injected under the skin every 6 or 12 months, and only a veterinarian can give the injection. Some heartworm preventives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas, ticks, and ear mites).

Year round prevention is recommended and life saving! Talk to your dog’s veterinarian to decide which preventive is best for your dog.

The American Heartworm Society advocates to “Think 12." Give dogs 12 months of heartworm prevention and get them tested for heartworms every 12 months.

Pets and People, Saving Each Other

HSNT’s mission is to act as an advocate on behalf of all animals and to ensure their legal, moral and ethical consideration and protection; to provide for the well-being of animals who are abandoned, injured, neglected, mistreated or otherwise in need; to promote an appreciation of animals; and to instill respect for all living things.

Information for this blog post was cited from studies originally published by the Federal Drug Administration ( ) and The American Heartworm Society ( )

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