1. Home

Heartworm

Heartworm Disease, the Treatment and Prevention

By

Heartworm

A puppy like this Viszla is at higher risk if outside hunting all the time.

Image © Ryan McVay/Getty Images

I hate mosquitos not only because they’re itchy aggravation, but these nasty vampires spread deadly heartworm disease. That can make your puppy sick or worse—it could kill her.

Dogs are the natural host, and heartworms have been a problem at least since 1922 when they were first discovered. Today heartworms are found all over the world.

What Are Heartworms?

The heartworm Dirofilaria immitis belongs to a group of parasites called filarids, and is a type of roundworm. They live in the right heart chambers and pulmonary arteries—the lungs—of infected dogs. As you can imagine, lungs and heart filled with worms can damage and interfere with normal organ function.

You won't be able to tell if your puppy has heartworms. You can't see them the way you can fleas or ticks. And your puppy won't even act sick until she's been infected for quite a while.

Heartworm Lifecycle

An intermediate host, the mosquito, is necessary to transmit the disease. Mosquitoes ingest baby heartworms called microfilariae when taking a blood meal from an already infected dog.

The immature parasites spend about three weeks developing inside the mosquito and migrate to the mouthparts of the insect. When the mosquito again takes a blood meal, larvae are deposited upon the skin and gain entrance to the new host's body through the bite wound left by the mosquito.

Once inside the dog’s body, the immature heartworm undergoes many more molts and development stages. Finally it migrates to the heart and pulmonary arteries where it matures.

Adult worms can reach four to twelve inches in length. It's not uncommon for infected dogs to carry dozens of worms; more than 250 have been found in a single dog. Adult worms mate and females shed as many as 5000 microfilariae each day into the dog's bloodstream. These microfilariae must be ingested by a mosquito to continue their development, but can remain alive and infective in the dog's bloodstream for up to three years.

The life cycle takes about six to seven months. Puppies can be infected with microfilariae and not show signs of disease even when tested for many months. They might not show symptoms for years, but damage continues as long as they're infected.

All dogs can get the disease, but those exposed more often to mosquitoes. That means outdoor puppies that live near prime mosquito breeding grounds like swamps or standing water are at highest risk.

Heartworm Disease Symptoms

Heartworms can live in the dog for up to five years. Initially, the dog may not show any ill effects, but symptoms develop and grow worse over time. Common signs are coughing, shortness of breath, and reluctance to exercise. Infected pups may faint after exuberant play or games.

Eventually the dog becomes weak, listless, loses weight, and may cough up blood. Severe signs of late-stage disease are congestive heart failure that may result in sudden collapse and death.

Diagnosis

Traditional tests look for microfilariae in the bloodstream. The veterinarian draws a sample of the dog’s blood, and looks at it under the microscope to find the baby worms. But modern diagnosis is based on a combination of factors. Rather than visually searching for the microfilaria, blood screening antigen tests can detect the presence of adult female worms even before they’ve had babies. X-rays and echocardiography examine heart and lung changes, and urinalysis looks for telltale signs of protein .

Dogs fall into four categories once diagnosed. The lowest risk category, Class 1, tend to be young dogs or those with early infections and few symptoms, with no heart damage visible. Moderately affected Class II dogs have mild or intermittent symptoms though still relatively health, but have evidence of heart damage. Class III dogs are severely affected. Class IV Caval Syndrome dogs collapse and will die from their worm-load unless the worms are surgically removed.

Treatment

Heartworm treatment addresses the different life stages of the parasite. The newborn microfilariae swimming in the bloodstream, and the “adolescent” stages migrating through the dog’s skin, must be eliminated first. That serves two purposes.

If only the adult worms in the heart are treated these immature parasites would replace them as they mature. Killing these immature parasites first also reduces the numbers of adults that later need to be treated. Some of the monthly preventive medications can be given by the owner at home for two or three months to safely eliminate these immature heartworms before treating the adults.

Once the immature parasites have been treated, the adult worms are killed with a series of two or three treatments of a worm-killing poison called melarsomine dihydrochloride. This substance is related to arsenic and injected in the muscles of the dog’s back. This treatment can be hard on even a healthy dog’s body. The injection hurts and may require pain medication and follow up care to prevent potential abscess.

As of August 2011 the only FDA-approved heartworm treatment, Immiticide (made by Merial) became temporarily unavailable. Your veterinarian can refer to recommendations for alternative heartworm management at the American Heartworm Society.

The treated dog can go home, but must be confined for at least a month. That allows the dead worms to be absorbed by the body. Exercise could cause dead worm debris to move into the blood stream and cause a blockage—embolism—that damages the lungs or prompt heart failure.

Ivermectin Treatment Option

The injection treatment can be painful and quite expensive. In the past, low risk dogs diagnosed as Class I could instead be treated long-term with ivermectin-based heartworm preventive products usually in a pill form. It won’t kill the adults but will sterilize them so they can’t reproduce, and shortens their lifespan. And it kills the immature stages of the parasite. These dogs remain heartworm positive over a two-year period, and symptoms can get worse until the adult worms eventually die of old age. Your veterinarian can best advise you about your dog’s treatment.

UPDATE!As of August 2013, the experts recommend AGAINST using heartworm preventative medications like Ivermectin in a “slow-kill” method because it may encourage drug-resistance in the heartworm, and so make it more difficult to successfully treat or protect dogs. That's due in part because ss many as 20 percent of dogs infected with heartworms continue to have circulating microfilariae for at least a year or longer when receiving monthly treatment. When they mature and reproduce, they can potentially spread more drug-resistant parasites. Therefore, the current preferred method remains adulticide treatment--drugs to kill off the adult heartworm.

However, the current recommendation stands regarding pre-treating with Ivermectin for two months to kill the microfilariae before treating with melarsamine to kill adult worms. This, along with Doxycycline (an antibiotic) and maybe a steroid-type medication, helps reduce the risk of lung damage complications that can be associated with dying worms. Your veterinarian will have the latest information for your pet.

Heartworm Prevention

The changes in treatment won't make any difference to prevention protocols. It is much easier and less expensive to prevent heartworm disease in your puppy. According to Dr. Wallace Graham, president of the American Heartworm Society, puppies should begin preventatives at six to eight weeks of age. For pups six months old or older that haven't been on preventative, a heartworm test should be given before starting medication and the dog tested six months later to be sure there are no parasites. Annual tests thereafter ensure your puppy stays healthy. Although some geographic regions like the southern states and Mississippi Delta region have a higher incidence of heartworms, the disease has been found in all fifty states.

There are several heartworm preventive medications available, some in chewable tablets and others combined with flea or other parasite preventive products as a spot-on treatment. Ask your veterinarian to recommend the best option for your puppy.

©2014 About.com. All rights reserved.