Canine Heartworm Disease

Heartworm Disease – What Is It and What Causes It?

Heartworm disease is a serious disease that results in severe lung disease, heart failure, other organ damage, and death in pets, mainly dogs, cats, and ferrets. It is caused by a parasitic worm called Dirofilaria immitis. The worms are spread through the bite of a mosquito.  The dog is the definitive host, meaning that the worms mature into adults, mate, and produce offspring while living inside a dog.  The mosquito is the intermediate host, meaning that the worms live inside a mosquito for a short transition period in order to become infective (able to cause heartworm disease).  The worms are called “heartworms” because the adults live in the heart, lungs, and associated blood vessels of an infected animal.  

 

In the United States, heartworm disease is most common along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states.

The Heartworm Lifecycle in Dogs

In an infected dog, adult female heartworms release their offspring, called microfilariae, into the dog’s bloodstream.  When a mosquito bites the infected dog, the mosquito becomes infected with the microfilariae.  Over the next 10 to 14 days and under the right environmental conditions, the microfilariae become infective larvae while living inside the mosquito.  Microfilariae cannot become infective larvae without first passing through a mosquito.  When the infected mosquito bites another dog, the mosquito spreads the infective larvae to the dog through the bite wound.  In the newly infected dog, it takes about 5 to 7 months for the infective larvae to mature into adult heartworms.  The adult heartworms mate and the females release their offspring into the dog’s bloodstream, completing the lifecycle.

Heartworm disease is not contagious, meaning that a dog cannot catch the disease from being near an infected dog.  Heartworm disease is only spread through the bite of a mosquito.

Inside a dog, a heartworm’s lifespan is 5 to 7 years.  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 350 worms.

How is a Dog Tested for Heartworms?

A veterinarian uses blood tests to check a dog for heartworms. An antigen test detects specific heartworm proteins, called antigens, which are released by adult female heartworms into the dog’s bloodstream.  In most cases, antigen tests can accurately detect infections with one or more adult female 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.

Another test detects microfilariae in a dog’s bloodstream.  Microfilariae in the bloodstream indicate that the dog is infected with adult heartworms (because only adult heartworms can mate and produce microfilariae).  The earliest that microfilariae can be detected in a dog’s bloodstream is about 5 months after it is bitten by an infected mosquito (because it takes about that long for the heartworms to develop from infective larvae into adults that mate and produce microfilariae).

What are the Symptoms of Heartworm Disease in a Dog?

The severity of heartworm disease is related to how many worms are living inside the dog (the worm burden), how long the dog has been infected, and how the dog’s body is responding to the presence of the heartworms.  The dog’s activity level also plays a role in the severity of the disease and in when symptoms are first seen.  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:  General loss of body condition, 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 work, x-rays, hospitalization, and a series of injections.     

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.   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 best!  Talk to your dog’s veterinarian to decide which preventive is best for your dog.

A dog MUST be tested within 12 months to get heartworm preventive.

The Providence Veterinary Hospital Blog is a publication of Peter Herman, VMD, at the Providence Veterinary Hospital, 2400 Providence Ave. in Chester, PA. Contact Dr. Herman at 610-872-4000 or visit us at http://www.providencevet.com.