When can a puppy have heartworm medicine

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Three dogs poking their heads out of a red doghouse.
Three Dogs Poking Their Heads Out Of A Red Doghouse.

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, the 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 it'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.

adult black Labrador retriever walking outdoors
adult black Labrador retriever walking outdoors

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 more molts and development stages. Finally, it migrates to the heart and pulmonary arteries where it matures.

Adult worms can reach 4 to 12 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 the damage continues as long as they're infected.

black and white Border Collie standing on ground
black and white Border Collie standing on ground

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


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. 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.

tri-color cocker spaniel on green grass
tri-color cocker spaniel on green grass

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 in relatively good 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.


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 into 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.

photo of brown and black German Shepherd
photo of brown and black German Shepherd

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 bloodstream 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. It kills the immature stages of the parasite. These dogs remain heartworm positive over two years, 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.

As of August 2013, the experts recommend against using heartworm preventative medications such as 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 as 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 currently 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 melarsomine 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 dead worms. Your veterinarian will have the latest information for your pet.

two gray dogs
two gray dogs
black and tan puppy on ground
black and tan puppy on ground
pug mix sitting on grass field
pug mix sitting on grass field

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, former 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.