Learn About Heartworm Disease in Dogs

April 7, 2021
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Written by Pumpkin Care Pet Insurance,
reviewed by Stacy Choczynski Johnson, DVM


Heartworm disease is a serious condition that can have lifelong effects on the health of your pet – even when it’s successfully treated. The good news is, heartworm disease is preventable, so you can rest easy if you’re well informed and on top of your pup-parent game! Here’s everything you need to know about heartworm disease in dogs, including symptoms, treatments, prevention, and more.

First and foremost: What is heartworm disease?

Heartworm disease is an illness caused by – you guessed it – the parasitical monsters known as heartworms. Heartworms can be up to 12 inches in length and take up residence in your dog’s heart and the vessels that move the blood from the heart to the lungs. Once there, they can cause dangerous blockages and cut-off blood flow, leading to red blood cell lysis (destruction of red blood cells with release of their contents); heart failure; and stress on the kidneys and liver.

The parasite’s official scientific name is Dirofilaria immitis, but it’s commonly called “heartworm” because of the damage it can do to a pet’s cardiovascular health when it creates a home in the heart, lungs, and blood vessels.

What do heartworms look like?

Imagine the nastiest spaghetti squash you’ve ever seen. Take the stringy part out, bleach it, and stretch each “spaghetti” to about a foot in length. Not something you want to see in real life.

How do dogs get heartworms?

Heartworms are parasites, and unfortunately, dogs are natural hosts for them. This means immature heartworms can grow, mature, and procreate all in your dog’s body. When a mosquito bites a dog (or other animal) infected with heartworms, it can pick up microscopic baby heartworms from the dog’s blood.

Over the course of 10 to 14 days, these worms can mature into what the American Heartworm Society dubs the “infective stage.” If an infected mosquito bites another dog, it can transmit the infective larvae to that animal (as if mosquito bites weren’t dreadful enough on their own, right?).

How long does it take for a dog with heartworms to show symptoms?

Some dogs live with heartworms for a long time with little to no outside indicators of infection, especially if they don’t have many heartworms. That said, heartworms have a lifespan of five to seven years, so unfortunately they have plenty of time to wreak havoc on your dog’s system.

What are the first signs of heartworm infection in dogs?

According to veterinarian and author Dr. Margit Muller, many dogs don’t actually show any symptoms for the first few months of heartworm infection, though active dogs are most likely to suffer from symptoms the soonest. Dogs that do begin to exhibit signs may simply have a mild persistent cough, or show exercise intolerance and want to return home early from a long run or hike.

With severe heartworm disease, dogs may show the following symptoms:

• Difficulty breathing
• Lethargy after moderate activity
• Decreased appetite and/or weight loss
• Swelling of the belly due to fluid accumulation
• Syncope where a dog will “pass out”

In addition, your veterinarian may be able to feel liver enlargement, or hear abnormal lung sounds and a heart murmur.

While you should consult your veterinarian if your pet has any of these symptoms – if you notice your dog has trouble breathing, dark urine, and/or pale gums – seek medical attention for them immediately. They may have developed caval syndrome, a potentially deadly cardiovascular issue that requires immediate surgery for your fur baby’s survival.

What are long-term symptoms of heartworm in dogs?

The longer heartworm disease goes undiagnosed and untreated, the more serious the symptoms can become.

“Long-term symptoms occur when the adult worms cause inflammation of the blood vessels and can block blood flow,” Dr. Muller tells Pumpkin. “This leads to clots in the lungs and heart failure. Heartworm disease can also lead to liver or kidney failure.”

How is heartworm disease diagnosed?

Because heartworm disease shares symptoms with so many other health conditions dogs may face – and because your dog may not exhibit any symptoms at all for months – special tests are necessary for diagnosis.

Your veterinarian will simply have to take a small blood sample from your dog, the American Heartworm Society notes, in order to check for heartworm proteins. If your dog tests positive for heartworm proteins, your veterinarian may order more tests, including confirmatory blood tests, X-rays and a heart ultrasound called an echocardiogram, or opt to start treatment immediately.

Thankfully, some pet insurances like Pumpkin will cover these costly heartworm disease diagnostics and treatments and reimburse 90% of eligible costs.

How much does it cost to treat heartworm disease in dogs?

Like almost anything else, there are a number of variables when it comes to the price of heartworm treatment for dogs, including the severity of the case, location, and the specific course of treatment given. That said, Dr. Muller advises, “The pure treatment might be around $500, but a complete examination and treatment including X-rays, blood tests, and proper follow-up testing might add up to $1,000 or more.”

Can heartworm disease in dogs be cured?

Yes, there is a cure for heartworm disease in dogs, but know that it’s a bit of a lengthy, complex, and expensive process. “There are injections available to treat heartworm,” Dr. Muller says, “but they have to be given repeatedly, usually two or three times.” Of course, all dogs vary and it may depend on the severity of your pet’s particular case.

It’s also important to note that dogs who have a lot of heartworms may not exhibit any more symptoms than dogs with fewer heartworms. However, the less severe the symptoms, the more likely they’ll respond positively to heartworm treatment with minimal complications.

According to the American Heartworm Society, heartworm treatment in dogs can take several months and methods:

• You’ll have to restrict and minimize your dog’s activity. Active dogs are more likely to have heart and lung damage from heartworms. You’ll want to keep your pet in a crate or restricted to one room, and keep walks minimal and limited to just relieving themselves. If you have a particularly energetic dog, your veterinarian may prescribe a sedative to keep them calm and restful.
• Your dog may require therapy to stabilize their condition before treatment, especially if they have severe heartworms or other serious medical issues that may pose complications.
• Your veterinarian will likely give your dog an injection of melarsomine dihydrochloride (brand names Immiticide and Diroban) to kill the worms. Some protocols include topical imidacloprid and moxidectin to rid your dog’s bloodstream of microscopic heartworm larvae.
• Your veterinarian may also prescribe antibiotics or steroids.
• In severe cases, your veterinarian may perform surgery to physically remove heartworms from your dog’s body.
• Your veterinarian will continue testing for heartworm disease after treatment, often starting at about six months after treatment concludes.

What happens if heartworm disease goes untreated?

“If untreated, the heartworm numbers can increase up to several hundred worms in the dog’s body,” Dr. Muller tells Pumpkin. “Heartworm disease causes lasting damage to the heart, lungs, and arteries, and can negatively affect the dog’s health and quality of life.” If the organ damage from heartworms is severe enough, it may be fatal.

Do I have to limit my dog’s activity and exercise if they’re heartworm positive?

No one wants to see their beloved pet bored and sad in a kennel or confined to a small space, but it’s imperative that you keep your dog at rest and as inactive as possible until they’ve completed their heartworm treatment, per the American Heartworm Society. 

When the treatment kills the worms in your dog’s body, the decomposing worms can cause blockages in blood vessels near the lungs. If your dog’s heart rate increases due to exercise or excitement, they’ll be at risk for a pulmonary embolism, which can be fatal at worst, and at best, cause an increased chance of serious complications.

How can I keep my dog entertained while limiting their activity during heartworm treatment?

Instead of playing fetch, roughhousing, or running with your dog, which can be harmful to their recovery after heartworm treatment, try exercising their brain instead! The American Heartworm Society recommends investing in chewable toys (think Nylabones and squeakers) to keep your pup occupied, and to have them try their paws at games like puzzles and “casinos,” where they have to work to get a small treat. This will keep your pup mentally stimulated, even if they can’t move around much. 

You can also fill Kong toys with their food to extend mealtimes, and keep them close to you while you work from home, read, watch TV, or do anything else around the house, so they don’t feel lonely.

Are some breeds more or less prone to heartworm infections?

All dog breed types are susceptible to heartworms and the dangerous disease they cause.

Does where I live impact the likelihood of my dog getting infected?

Make no mistake: Heartworms can infect dogs anywhere in the world, though they tend to be more concentrated in some areas than others, with geographical infection rates varying widely from year to year. As of 2020, the Southeast United States (North and South Carolina, Florida, Georgia), Louisiana, Mississippi, Alabama, Texas; parts of Illinois, Oklahoma, Missouri, and Minnesota; and California are more prone to heartworms and the mosquitoes that carry them. 

However, climate change, pets infected with heartworm relocating to other areas, migration of wild animals (like foxes, coyotes, and wolves) and stray dogs, and wind carrying mosquitoes long distances spreads the parasites all over. Long story short: Dogs everywhere are at risk for heartworms, so don’t take any chances – even if you live in a big city or colder climate!“

Urban areas with high density buildings can also serve as warm microenvironments for mosquito breeding and some of the hardy mosquitos can apparently hibernate for up to 5 months,” Dr. Stacy Choczynski Johnson tells Pumpkin. “This means that us cold weather pet owners need to be vigilant about heartworm too!”

How can I prevent my dog from getting heartworm disease?

Year-round preventative medication for heartworm is a must. You can also take steps within your environment to reduce the likelihood of mosquito bites by eliminating any standing water outdoors, using mosquito traps, and timing your dog’s outdoor activity to exclude dusk and dawn, when mosquitoes are most active.

“I recommend reviewing your pet’s age, breed, and health history to determine the best heartworm prevention for your pet,” Dr. Choczynski Johson tells Pumpkin. “For dogs, we can suggest a long lasting injection or an oral medication that sometimes includes flea and tick prevention. For dogs and cats, there is also a topical option.”  

When should I get a heartworm test for my dog?

If your dog exhibits any of the symptoms mentioned above, visit your veterinarian immediately for evaluation and heartworm testing. Got a healthy pooch? Great! Your dog should be tested for heartworm disease annually, even if they’re on preventative medication and you have no reason to suspect infection. Your veterinarian will likely perform the test at your dog’s annual wellness exam. 

“The American Heartworm Society is now recommending that as veterinarians we screen for heartworm using two tests – the traditional antigen test and also a microfilarial test that will screen for larvae too,” Dr. Choczynski Johnson notes. “Vets are starting to crack down on heartworm now that we know it is in all 50 states.”

Many veterinarians will start puppies on heartworm prevention as early as their first vet visit – and annual heartworm testing in dogs usually begins around seven months old. This is because it can take up six months to detect heartworms! While protecting your pup from heartworm disease is an investment – the good news is some pet insurance providers like Pumpkin offer optional preventive care coverage for key parasite screening tests, including a yearly heartworm test for dogs.

Is canine heartworm disease contagious?

Here’s some more good news for pet parents: heartworms aren’t spread from dogs to other dogs, nor can you catch them from your pet (or vice versa). Dr. Muller explains, “No, heartworm isn’t contagious, as the heartworms are not transmitted from dog to dog. The bite of an infected mosquito is the only way heartworms are transmitted.”

What is the prognosis of heartworm disease in dogs?

According to Dr. Muller, the prognosis for heartworm in dogs depends on the stage and clinical manifestation of the disease. “Remember that the heartworm can stay alive up to six years in the dog,” she states. “In early stages without lung and heart failure, 98 percent of dogs will be cleared of heartworm with an intensive three-dose protocol. But the very best prognosis comes from regular prevention year-round.”

Heartworm disease is a complicated issue that requires immediate medical attention and TLC for your infected dog. Still, as the FDA notes – and both Dr. Muller and Dr. Choczynski Johnson agree – the absolute best treatment is simply adequate heartworm prevention. Your dog (and your wallet) will thank you for keeping them healthy, safe, and free of creepy-crawlies.

Download this helpful Heartworm Disease Guide for Dogs