Mosquitoes, Dogs and Heartworm Disease

Mosquitoes can carry the Heartworm parasite, a dangerous and common parasite that can affect your dog's heart and other organs. Get the facts on Heartworm prevalence, screening and prevention to keep your pup safe with the resources below, then learn more about other parasites that affect dogs at our parasite prevention and screening campaign.

Mosquitoes, Dogs and Heartworm Disease

Reviewed By Peter Kintzer DVM, DACVIM

Yes! Unprotected dogs are at risk of becoming infected with Heartworm, a potentially deadly parasite, when they're bitten by a mosquito. Read more>

Mosquitoes, Dogs and Heartworm Disease

We've compiled the Heartworm prevalence data from thousands of veterinary clinics to help you work with your veterinarian to protect dogs from Heartworm. Find out what the heartworm prevalence is in your own zip code: Check out our disease prevalence maps here>

Mosquitoes, Dogs and Heartworm Disease

Reviewed By Peter Kintzer DVM, DACVIM

This potentially-fatal parasite is easy to screen for and prevent, but much harder and expensive to treat. Find out what you can do to protect your dog and why your veterinarian may want to test for this common parasite. Read more>

Mosquitoes, Dogs and Heartworm Disease

By Dr. Ruth MacPete

Dr. Ruth covers the basics of this hard-to-treat parasite and how you can work with your veterinarian to keep your pooch safe. Read more>

Mosquitoes, Dogs and Heartworm Disease

Drug-Resistant "Super Heartworms"?

By Dr. Ernie Ward

Most of us are familiar with news stories describing how certain bacteria have become resistant to previously life-saving antibiotics. Now we have drug-resistant parasites, as in a newly discovered strain of super heartworms. Read more>

Reviewed on:

Wednesday, September 9, 2015


Your veterinarian may perform a blood test to determine whether your pet has the disease. A blood sample is tested for the antigens (proteins) produced by adult heartworms. The sample may also be examined under a microscope for the presence of the heartworm larvae. More laboratory tests may be required to make a diagnosis, especially in cats, as the disease can be harder to diagnose in felines. A negative test result for the larvae does not rule out feline heartworm infection, as the larvae often are found only temporarily in an infected cat. However, if circulating larvae are found, it becomes a confirmation that heartworm disease is present.

Heartworm in Dogs: What You Should Know About the Mosquito-Borne Parasite

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.

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:

  • Heartgard ®Plus for Dogs (chewable, ivermectin/pyrantel)
  • Tri-Heart®Plus for Dogs (chewable, ivermectin/pyrantel)
  • Iverhart Max® for Dogs (chewable, ivermectin/pyrantel permeate/prziquantel)
  • Sentinel® for Dogs (chewable, milbemycin oxime/lufenuron/praziquantel)
  • Revolution® for Dogs (topical, selamectin)
  • Advantage Multi™for Dogs (topical, imidacloprid + moxidectin)
  • Trifexis (Milbemycin and spinosad)
  • ProHeart® 6 (injectable, given only by a veterinarian) — Lasts 6 months. Not effective against intestinal parasites.

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.

What Is Heartworm Disease?

Heartworm disease is an invisible, but potentially fatal threat that occurs in dogs and other animals. Heartworms (Dirofilaria immitis) are parasitic worms that are transmitted to dogs by mosquitoes. These microscopic larvae develop under the skin, then migrate to the blood vessels of the heart and lungs of the infected animal where they rapidly grow, becoming adults that are 5-12 inches in length. 1

Heartworms are an invisible threat because they cause severe damage to the vessels of the dog’s lungs, typically long before any symptoms appear. While treatment is available, heartworm disease causes lasting damage to the heart, lungs, and arteries, and can affect the dog’s health and quality of life, long after the parasites are gone.

Watch the video: Save money- cheap Heartworm medication for your dogs! BEWARE read below!! (July 2021).