Symptoms and Treatment of Heart Worm in Dogs

"Heart worm in dogs is caused by bites from infected mosquitoes that transmit the heartworm (Dirofilaria immitis) into the dog. The worms lives in the pulmonary arteries and the heart, causing symptoms such as coughing, being short of breath and weight loss. The severity of symptoms depends on the number of worms and how long the worms have been causing a problem. The mean age for infected dogs is age 3 to 8. Treatment options include medications and possibly surgery. As the worms multiple and grow, it can lead to canine heart failure and death. In the United States it is more common in the West and Midwest, but has been diagnosed in all 50 states. Cases that can be treated by injection of a heartworm medication result in a cure 96% of the time. Test your dog for heartworms before buying a heartworm preventative in order to avoid complications. Only purchase name brands recommended by a veterinarian such as those at the bottom of this page. These products are almost 100% effective in preventing the disease."

There are two types of heart worm in dogs. In pulmonary heartworm disease, dogs have live heartworms in the pulmonary arteries. In caval syndrome, there are worms in the in the heart itself. The worms themselves may be alive or dead. Heartworm in dogs tend to be between 9 and 16 inches in length.

The disease can be lethal and infect the heart and lungs. As the worms grow they can interrupt blood flow. An infestation can include 100's of worms and grow for several years inside a dog.

Many people ask if heartworm medicine is necessary for a dog. The answer depends on where a dog lives. The answer is yes in high risk areas such as warmer climates. This includes the southern half of the U.S., with a focus on the southeast. Another indicator is the mosquito problem in your area. If there is a big mosquito problem, then dog heartworm treatment is probably necessary. Dogs should also get a preventative if they travel in these areas. The disease is somewhat rare in the most northern states such as Washington. It never hurts to consult with your veterinarian.

In areas where there are long lasting mosquito populations, and with over 70 kinds of mosquitoes carrying the disease, it is highly likely that one will bite and infect your dog. Prevention is the best approach to heart worm in dogs.

Picture of Hearworm In Dog

Heart worm in dogs adult. Picture shows canine heartworm in right
section of dog's heart. Heart worm dogs can grow up to 14 inches in length

Getting Heartworm Disease In Dogs

Dogs are highly likely to get heartworm disease if they are bitten by an infected mosquito. The mosquito spreads heartworm disease by biting an infected dog, drinking in the heartworm and then passing on the worm by biting another dog.

There are four factors that will affect how your dog reacts to heartworms:

  • Number of worms
  • Health of your dog's immune system
  • Length of time your dog has the worms
  • How active your dog is

Dogs that are active tend to get a more severe case of heartworm disease than inactive dogs. Also, large dogs tend to tolerate the disease better than small dogs since small dogs have smaller artery openings in the heart.

The risk of infection is higher in warmer climates of if the temperature remains above 64F for one month or more.

Symptoms of Dog HeartWorms

Dogs that have not had any treatments to prevent heartworm are checked for symptoms such as respiratory problems such as coughing (particularly during exercise), muscle weakness or lethargy, weight loss, shortness of breath or labored breathing, loss of appetite and vision problems. Other symptoms include high blood pressure (hypertension).

heart worm in dogs

Heart worm in dogs can cause an enlarged canine heart(Dirofilaria immitis)
Source: From the collection of Dr. Richard Wescott viaWashington State University


Heart worm in Dogs is diagnosed using a blood test that only requires a few drops of blood and sometimes x-rays. Blood tests usually do not detect the disease until the worms are at least 6 months old. Other signs such as respiratory distress, hypertension (high blood pressure) and kidney disease as also associated with heartworm.

Mild cases of heartworm in dogs can be detected while the disease is still treatable.

Treatment For Canine Heartworms

There are three treatments for heart worm in dogs. These include surgery, treatments using drugs that kill the heartworms (melarsomine dihydrochloride) or no treatment if your dog has other problems such as liver problems. Most dogs will need to be hospitalized.

Dog heartworm treatment can be expensive and traumatic. This is compounded by shortages of the required dog heartworm medication. Heartworm treatment starts with killing any larvae in the bloodstream and worms that are in the heart itself. After killing off all of the larvae, a medication called melarsomine (immiticide®) is used to kill adult worms. Dogs being treated with melasomine need to be confined to a crate and up to 8 weeks after receiving injections. The reason is to keep dogs from being too active to reduce the risk of a dog getting a pulmonary embolism or death. The only time they can leave the crate is to go to the bathroom. The most common side effect of this treatment is some swelling where the injection was given. With 2 doses, 96% of dogs are cured with this treatment.

If the heart worms dogs caused any damage to the heart, this will be permanent.

In severe cases, surgery is used to remove heartworm if your dog is at high risk.

After treatment, cage rest is often recommended. Most Veterinarians prefer to do this in their office, however, you can ask your Vet for a tranquilizer or keep our dog in a caged area.

Heartworm Flea Tick Dog Medicine

Heartworm preventatives should be used starting between ages 6 to 8 weeks. For dogs that have not been protected by one of the following treatments, a simple test in the Veterinarians office (antigen test) will determine if your dog has heartworms. Do not give a treatment until your dog has been tested first!

There are several treatments that are effective in preventing heart worm in dogs. These approaches stop worm larva from developing during the 1st 2 months after infection. These drugs are used every month or in larger intervals. Even if you forget to treat your dog for 2 to 3 months, as long as you continue for the next 12, the treatments should be effective. Most products have an automatic 15 day leeway built in. Also, it can take up to 6 months for the tests to read positive for dog heartworm after a dog has been bitten by a worm carrying mosquito.

We recommend buying name brand from a reputable company. Examples include:

  • Heartgard Plus: (prescription required) Convenient chewable that is very popular with veterinarians. Almost 100% effective in recent studies. Also helps to prevent roundworms and hookworms.
  • Interceptor: (prescription required) Comes in a chewable. Prevents heartworm, controls adult hookworm, and removes and controls adult roundworm and whipworm. Almost 100% effective. 97% effective if used every 3 months.
  • Revolution: Topical applied to skin of dogs 6 weeks of age or older. Almost 100% effective. Kills fleas and flea eggs, ticks and ear mites.

Also, the addition of a natural homeopathic product such as Parasite Dr., helps to support a dog's natural immunity, a step that could be of benefit during recovery and to prevent future dog worm problems.

Adult Heartworm and Dogs

It is rare for a human to contract heartworm. If an adult does get heartworm it is from a mosquito bite, not from contact with a dog.


Have A Dog Heartworm Question For Our Veterinarian?

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We will do our best to get back to you quickly (depends on how many questions we receive each day). If you do require an immediate response we suggest using this online dog veterinary service that is available now.

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