Understanding Heartworm Disease in Dogs: Causes, Symptoms, Treatment, and Prevention

Understanding Heartworm Disease in Dogs: Causes, Symptoms, Treatment, and Prevention

When I (Steph) was a child, I used to accompany my mom and our Shetland sheepdog, Brandy, to the veterinarian for Brandy’s annual wellness exams. I can clearly remember the large mason jar full of cloudy, yellowish liquid that sat on the corner of the vet receptionist’s desk. You had to look at the jar every time you walked up to the front desk to check in. Inside the jar was a preserved heart surrounded by what looked like fifty strands of cooked spaghetti. I’ll never forget when my mom told me these spaghetti strands were, in fact, preserved heartworms.

Heartworm Disease in Dogs: One of the most serious and potentially lethal canine diseases is heartworm disease. It is prevalent throughout the United States and found all over the world. If left untreated, heartworms are silent killers that will damage your dog’s heart, lungs, liver, and kidneys, ultimately leading to heart failure, lung damage, organ failure, and death.

If you are a dog owner, you are likely well aware that it is important to protect your dog against heartworm disease. You probably give your dog regular heartworm prevention medicine to ensure your four-legged friend’s health and wellbeing. But have you ever stopped to wonder what exactly heartworm disease in dogs is? What causes heartworm disease, and how do dogs contract heartworms? What are the symptoms of canine heartworm disease? Can dogs with heartworms be successfully treated and experience a full recovery?

Here is everything you need to know about the causes, symptoms, treatment, and prevention of heartworm disease in dogs.

What Is Heartworm Disease?
Heartworm disease is a serious and potentially deadly disease caused by parasitic roundworms called Dirofilaria immitis. The disease is spread from host to host by mosquitos that are carriers of adult heartworm offspring, or microfilariae.

Microfilariae become infective heartworm larvae within a mosquito. When a mosquito carrying infective larvae bites a dog, the larvae enter the dog’s bloodstream. Over the course of 2 to 3 months, heartworm larvae travel through the dog’s bloodstream, ultimately settling in the heart, pulmonary artery, lungs, and surrounding blood vessels. After 6½ to 7 months, the heartworm larvae mature into adult heartworms and begin producing their own larvae.

Adult female heartworms grow 10-12 inches in length, while male heartworms can grow to be 4-6 inches long. Heartworms can live in your dog’s body for 5 to 7 years. Left untreated, heartworms will reproduce within a dog and begin blocking blood flow to and from the heart, lungs, and surrounding organs. This blockage can eventually damage the heart, lungs, liver, and kidneys and lead to heart failure, irreversible lung damage, and organ failure.

Where Is Heartworm Disease Found?
Canine heartworms are carried by 70 different species of mosquitoes and can be found all over the world and in the United States. They were discovered in the United States 165 years ago.

Because heartworm disease is spread via mosquitos, the disease is most widespread in mosquito-friendly climates. These are typically warm, wet environments with temperatures ranging from 50 to 95 degrees Fahrenheit. Mosquitoes cannot function at temperatures less than 50 degrees Fahrenheit.

Heartworms can be found throughout the United States but are most prevalent in the Southeast, Gulf Coast, and Lower Mississippi Valley. Unsurprisingly, heartworm disease spreads most rapidly during the warmer months of the year when mosquitoes breed and are most active. “In 2019, in the worst-hit states, almost 10 percent of dogs tested positive for heartworms,” says Dr. Jamie Richardson, medical chief of staff at Small Door Veterinary.

Dogs that are shipped from state to state may also bring heartworms into regions where they were previously not a problem. According to a study published in May 2019, 130 animal shelters and rescue organizations brought about 114,000 dogs to Colorado between 2014 and 2017. This ultimately caused heartworm disease to increase by 67.5 percent.

What Dogs Are Most at Risk of Heartworm Disease?
All dogs in the United States are at risk of heartworm disease if preventive measures are not taken to avoid contraction. Dogs that live in warm, wet climates with thriving mosquito populations are most at risk of exposure. Areas with strays or wildlife (i.e., coyotes, foxes, and wolves) also pose a greater risk to our four-legged family since strays and wildlife are often carriers of heartworms.

Although less likely, even dogs that stay inside most of the time can be infected with heartworm by indoor mosquitoes. A female mosquito can live in a house for up to 3 weeks.

Regardless of a dog’s location, environment, or situation, it is always advisable to take preventative steps to protect your dog against heartworm disease.

Symptoms of Heartworm Disease in Dogs
Canine heartworm disease is a silent killer. Clinical signs and symptoms usually do not appear until the heartworms are at least 6 months old. As heartworm disease progresses (from stage I to stage 4), symptoms become more severe and include:

Stage 1 and Stage 2: Mild to Moderate Symptoms

  • Soft and dry cough
  • Depression
  • Low energy
  • Poor appetite
  • Weight loss
  • Dull coat
  • Excessive sleeping
  • High blood pressure
  • Stunted growth

Stage 3: Severe Symptoms

  • Allergic pneumonitis
  • Nosebleeds
  • Frothy blood-streaked mucus
  • Right-sided congestive heart failure
  • Bluish tongue, gums, lips, or skin (Cyanosis)
  • Difficulty breathing
  • Fainting after exercise
  • Bulging chest
  • Seizures
  • Blindness
  • Lameness
  • Dark bloody or coffee-colored urine

Stage 4: Caval Syndrome
Caval syndrome occurs when adult heartworms invade the vena cava (the vein bringing blood to the heart), the right atrium, and the right ventricle. It can cause high blood pressure in a dog’s lungs and congestive heart failure. According to the Journal of Veterinary Internal Medicine, 20 percent of heartworm‐positive dogs develop caval syndrome.

Heartworm removal surgery is the only treatment option. It has a 42 percent mortality rate. Without treatment, death generally occurs within 12 to 72 hours because of liver or kidney failure.

How Is Canine Heartworm Disease Diagnosed?
Heartworms are usually diagnosed with a Snap 4Dx test. This test requires three drops of blood and provides results in a matter of minutes. Snap 4Dx tests check for four different diseases that are transmitted by mosquitoes and ticks:

  • Heartworm disease
  • Lyme disease
  • Dog tick fever
  • Ehrlichiosis

The Snap 4Dx test detects the proteins of adult female heartworms. A blood test is followed up with a modified Knott’s or filter test. The modified Knott’s test uses a machine that spins your dog’s blood sample very quickly in a small circle to concentrate heartworm larvae. The filter test passes his blood through a very fine filter that traps the parasites. In both tests, your dog’s blood is filtered, stained, and then examined under a microscope on 10x power.

One in 5 dogs with heartworm disease will have a negative modified Knott’s or filter test. This is called an occult infection. It may mean that only one sex of the worm is present. Heartworm prevention medications can also make worms sterile.

Your veterinarian may use these additional tests to detect heartworms:

  • X-rays. Your dog’s X-ray will usually show an enlarged heart and swelling of the pulmonary artery.
  • Ultrasounds. An ultrasound can see inside your dog’s heart chambers. It can determine how many worms are living inside of him. The average worm burden is 15 worms but that number can range from 1 to 300 worms.
  • Electrocardiogram (EKG or ECG). An EKG traces the electric currents created by your dog’s heart. Deep S waves or tall P waves mean that the right side of his heart is enlarged.

Treatment of Heartworm Disease in Dogs
There are two types of heartworm treatment for dogs:

  • Melarsomine dihydrochloride (also known as Immiticide or Diroban) is an arsenic-containing drug that kills adult heartworms and immature heartworm larvae. Melarsomine is injected into the dog’s back muscles and generally costs between $200 and $400 depending on the size of the dog. Most dogs will receive an initial Melarsomine injection, followed by a strict month-long period of crate rest to prevent pulmonary thromboembolism, and then 2 additional injections. As Dr. Jamie Richardson from Small Door Veterinary explained to us: “The injections can be painful for your dog. On the days they are given, he must stay in the hospital for observation to monitor for allergic reactions.”
  • Advantage Multi for Dogs (also known as imidacloprid and moxidectin) is a topical solution that is applied to a dog’s skin. It removes heartworm larvae from the dog’s bloodstream. Side effects can include agitation, poor appetite, vomiting, bloody diarrhea, and muscle tremors due to pulmonary thromboembolism. A 6-month supply averages $133 for a 9 to 20 lbs dog.

Dogs cannot exercise during the heartworm treatment period and for 4 weeks afterward. Heightened activity increases the risk of pulmonary thromboembolism and other complications. Pulmonary thromboembolism is a life-threatening condition that can occur if a fragment of a dead adult heartworm travels and causes blockage in one of the pulmonary arteries of the lungs. The blockage prevents blood flow to a portion of the lung which then prevents the lung from delivering oxygen to the rest of the body.

Although there are serious risks associated with heartworm treatment, most dogs that do not have advanced heartworm disease see a successful recovery. Unfortunately, dogs with advanced heartworm disease may have already suffered lasting damage to their heart, lungs, blood vessels, kidneys, and/or liver.

Heartworm Prevention for Dogs
Heartworm disease in dogs can be easily prevented with heartworm prevention medications. A variety of these medications are available and work to kill heartworm larvae that have infected your dog within the last one to two months. The medications may also protect your dog from intestinal worms, fleas, ticks, and mites.

There are 3 different types of heartworm prevention medications. Oral tablets are the most popular household heartworm prevention medication and include popular options like Heartgard Plus, Advantage Multi, Tri-Heart Plus, and Trifexis. Mike and I give our dachshund Django Heartgard Plus each month.

Oral Tablets

  • Must be administered every 4 weeks
  • Dosage determined by weight
  • Can be hidden in food or a pill pocket
  • Make sure your dog does not spit or vomit out the pill
  • Mark your calendar to make sure that you give the heartworm medication on the correct day
  • Should be stored between 68°F and 77°F


  • Must be applied every month
  • Make sure your dog does not lick the product
  • Should not be applied to irritated/broken skin
  • Do not bathe your dog for at least 4 days after use
  • Should be stored between 39°F and 77°F


  • Can only be administered by a certified veterinarian
  • Must be given every 6 months
  • Has a 2-week window
  • Prevents hookworms
  • Safe for pregnant and lactating females
  • Cannot be given to dogs over 7 years of age

Popular Heartworm Preventatives and Their Active Ingredients:

  • Advantage Multi for Dogs (topical/imidacloprid and moxidectin)
  • Heartgard Plus for Dogs (chewable/ivermectin and pyrantel)
  • Iverhart Max for Dogs (chewable/ivermectin, permeate, praziquantel, and pyrantel)
  • ProHeart 6 (injectable, only available to veterinarians)
  • Sentinel Spectrum for Dogs (chewable/milbemycin oxime, lufenuron, and praziquantel)
  • Simparica Trio (chewable/moxidectin and pyrantel)
  • Revolution for Dogs (topical, selamectin)
  • Trifexis (chewable/milbemycin and spinosad)
  • Tri-Heart Plus for Dogs (chewable/ivermectin and pyrantel)

If you are not sure what heartworm prevention medication to give you dog, or if you are considering switching to a new medication, please consult your veterinarian. Your vet knows your dog best and can guide you towards the safest and most appropriate prevention medication based on your dog’s weight, health history, medication regimen, etc.

How Often Should My Dog Be Tested for Heartworm?
All dogs should be tested annually for heartworms, even if they are on heartworm prevention meds year-round. “Heartworm preventatives are highly effective, but they are not 100 percent foolproof,” Dr. Richardson says. “Sometimes heartworm pills are not swallowed, or they might be thrown up. Topical creams can be washed or licked off if they are not applied correctly.”

Django gets checked for heartworm each year at his wellness visit and before our vet gives us a new heartworm prevention medication prescription. The test is a simple blood draw and the results come back within minutes.

When else should you test your dog? If you miss a dose of your dog’s monthly heartworm medication or give it to him too late, your dog should be immediately tested for heartworm. Retest your dog six months later and then annually after that.

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