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Heartworm is a potentially deadly parasite that is transmitted only by mosquitos, which pick up larval heartworms, called microfilaria, circulating in the bloodstream of infected animals.

Dogs and other canids, such as fox, wolves, and coyote, are considered the primary heartworm hosts, but these parasites can also affect other mammals, including cats and ferrets.

There are treatments available for dogs, but prevention is the wisest approach to dealing with heartworms.

What Is Heartworm Disease?

The heartworm in dogs that’s prevalent in the U.S. goes by the scientific name Dirofilaria immitis or D. immitis. It does not spread from dog to dog, but requires an intermediary, the mosquito, to infect new hosts.

The worms enter their host through a mosquito bite when it is taking a blood meal. Residue on the mosquito’s mouthpiece carries immature worms called microfilaria (which are only about 1/100th of an inch long) from an infected animal to an uninfected one. The immature worms travel through the bloodstream and, after about two months, settle in the right side of the heart, where they begin to grow.

They mature after six months and can live in the dog’s body for seven years, each reaching a length of up to one foot, and constantly producing offspring. After about a year, a dog may harbor hundreds of these worms, although 15 is the average burden. The worms cause inflammation and damage the heart, arteries, and lungs.

How Widespread Are Heartworms?

The first canine heartworms in the U.S. were discovered in 1856, in the southeast, according to Stanford University. While it was once more common in the Atlantic and Gulf coast states, the American Heartworm Society, in its first update since 2014, reports that heartworm is increasing throughout the country and worldwide. In the report, Cassan Pulaski, DVM, MPH states that, “…While southern regions of the country have historically been associated with heartworm, we now know pets all over the country are potentially at risk for heartworm disease throughout the year.” According to U.S. Food & Drug Administration, incidents of the disease have been reported in all fifty states. This is especially important to keep in mind if your dog travels.

Areas with large populations of wild or stray animals also pose an enhanced risk, but even dogs that stay inside most of the time are not totally safe from a mosquito’s bite. Also, pets being shipped from state to state may introduce heartworm into regions where it was historically not a problem. This is especially notable due to the bringing in of shelter dogs from heartworm endemic states and from disaster relief efforts. After Hurricane Katrina in 2005, for example, about a quarter of a million pets traveled from the New Orleans area to new homes around the country. Some brought heartworm with them. AHS estimates that more than a million pets in the U.S. are infected. The AHS notes that climate and environmental change also contribute to the spread of the disease.

Should Your Dog Be Tested for Heartworm?

  • All dogs should be tested annually, even those on heartworm prevention medication year round.
  • Puppies under seven months old may be started on heartworm prevention even though it can take six months or more for blood work to test positive. After that, they should be tested in six months and again, six months later.
  • Adult dogs over seven years old that have not previously been on a heartworm prevention regimen should be tested.
  • Dogs that have missed a treatment or for whom treatment has lapse need to be tested. If you’ve missed even one dose, or been late to administer, your dog should be tested.

What Are the Symptoms of Heartworm Infection?

In the early stages, there may be no signs of disease. As the worms grow and multiply, symptoms will become evident, increasing in severity as the disease progresses. There are four classes of infection:

  • Class one has no symptoms or just mild cough.
  • Class two is marked by mild exercise intolerance and persistent cough.
  • Class three will result in greater exercise intolerance, abnormal lung sounds, weak pulse, syncope (fainting caused by impaired blood flow to the brain), decreased appetite, weight loss, and ascites (swollen belly due to heart failure).
  • Class four is known as caval syndrome, a life-threatening cardiovascular collapse, which is marked by labored breathing, pale gums, and dark coffee-colored urine, leading to complete organ failure and death.

How Is Heartworm Diagnosed?

Early diagnosis important; the earlier heartworm is detected, the better the chances for recovery. In many cases, a blood test can determine the presence of the parasite. The blood is tested for the presence of antigens (proteins) even if there is no evidence of microfilariae. If your dog is diagnosed with heartworm through a blood test, your veterinarian will use additional tests for confirmation and to make sure your dog can safely undergo treatment. These include:

  • Radiographs to pinpoint abnormalities in the right side of the heart and pulmonary arteries.
  • Ultrasounds to show abnormal organ shape as well as wriggling worms.
  • Echocardiography to see inside heart chambers and visualize worms.

Beagle at the vet

How To Treat Heartworms in Dogs

Confirm diagnosis: Treatment is expensive and complex, so your vet may want to perform additional tests to confirm the diagnosis.

Restrict activity: Although it may be difficult to restrict normal activity, especially in active dogs, this is crucial. Exertion will increase the rate of damage to the heart and lungs. Crate confinement may be necessary.

Stabilize the disease: If your dog has advanced symptoms or other medical conditions, preliminary therapies may be required.

Kill worms: The drug that is FDA-approved for treatment in the U.S., is melarsomine dihydrochloride, which is sold under the brand names Immiticide and Diroban.

It is administered by deep intramuscular injection into the dog’s lumbar region of the lower back and is recommended for disease that has not progressed past class three. Another FDA-approved drug, Advantage Multi for Dogs (imidacloprid and moxidectin), rids the bloodstream of microfilariae. In addition, other drugs such as heartworm preventives, antibiotics, and steroids may be part of your veterinarian’s protocol.

Surgery: In extreme cases, veterinarians will resort to surgery, physically pulling the worms out.

Follow-up testing: The first test should take place six months after successful treatment.

Preventing Heartworm

Treatment for heartworm is hard and sometimes dangerous for the dog and very expensive, which is why veterinarians are so adamant about yearly testing and administration of preventative compounds — what scientists call chemoprophylaxis. The current AHS guidelines include the following:

  • Year-round administration of a heartworm prevention medication.
  • Strict compliance to the schedule of dosing recommended by your vet.
  • Application of FDA-approved mosquito repellants.
  • Environmental control to reduce or eliminate mosquito breeding grounds, such as pools of standing water; the use of mosquito traps; and avoiding your dog’s exposure by limiting outdoor activities at dawn and dusk, mosquitos’ prime feeding time.

These medications, used for prevention, are also effective against other parasites, such as roundworms, hookworms, fleas, and tapeworms. Make sure to use a formulation specifically intended for dogs.

Following is a list of some preventatives. Discuss with your veterinarian about which one best fits your dog’s needs:

Sources: American Heartworm Society; Merck Veterinary Manual; Centers for Disease Control and Prevention

With heartworm on the rise across the country and abroad, you and your vet can follow sensible steps to protect and, if necessary, treat your dog:

  1. Year-round prevention.
  2. Early testing if you suspect your dog has heartworm.
  3. Adherence to all the components of treatment.
  4. Compliance with whatever follow-up your vet recommends.

This article is intended solely as general guidance, and does not constitute health or other professional advice. Individual situations and applicable laws vary by jurisdiction, and you are encouraged to obtain appropriate advice from qualified professionals in the applicable jurisdictions. We make no representations or warranties concerning any course of action taken by any person following or otherwise using the information offered or provided in this article, including any such information associated with and provided in connection with third-party products, and we will not be liable for any direct, indirect, consequential, special, exemplary or other damages that may result, including but not limited to economic loss, injury, illness or death.