Heartworm Disease In Dogs: All Your Questions Answered

Heartworm disease is a serious condition that can have lifelong effects on your pet’s health — even when it’s successfully treated. The good news is that heartworm disease in dogs is preventable, so you can rest easy if you’re well-informed and on top of your pup-parent game. 

Cats and other animals can also be infected with heartworms. However, because this infection is more common in dogs, we’ll be focusing on pooches in this guide.

Here’s everything you need to know about heartworm disease, including symptoms, treatments, and heartworm prevention tips from experienced veterinarians. Remember: the signs of heartworm in dogs can be subtle at first, and it’s your job to spot them.

What is heartworm disease?

Heartworm disease is an illness caused by the parasitical monsters known as heartworms, which are exactly as gross and dangerous as they sound. Heartworms can grow up to 12 inches long and take up residence in your dog’s heart and the vessels that move the blood from the heart to the lungs. 

Once these parasites take up residence inside your pet, they can create dangerous blockages that cut off blood flow, causing red blood cells to break down. This can lead to heart failure and put stress on a dog’s kidneys and liver, among other problems.

The parasite’s official scientific name is Dirofilaria immitis, and they’re called “heartworms” because of the damage they do to a pet’s cardiovascular health.

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 exactly something you’d want to find inside your dog, let alone inside their precious heart.

Heartworms gather in the heart and nearby blood vessels, and they look like a tangled bunch of thin, wiry strands. When pulled out during surgery or an exam, they appear as long, smooth, thread-like worms. If you have the stomach for it, you can find YouTube videos showing heartworm surgery and removal.

How do dogs get heartworms?

Heartworms are mosquito-borne 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 10-14 days, these worms 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?

The signs of heartworm in dogs can take months to appear, and all the while the infection is growing. According to veterinarian and author Dr. Margit Muller, most dogs won’t show symptoms for the first few months of heartworm infection. Active dogs are most likely to suffer from symptoms the earliest.

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 health.

What are the first signs of heartworm infection in dogs?

At first, heartworms are sneaky invaders: you might not even notice they’re there. But as they settle in and grow, your pup may start showing signs that something’s not right. Early symptoms can be easy to miss, but catching them can make all the difference.

Early heartworm symptoms include:

• A mild, persistent cough
• Fatigue after exercise
• Loss of appetite and weight loss

As the infection worsens, things can take a serious turn. Some advanced symptoms are:

• Breathing difficulties
• Lethargy
• Swollen belly from fluid buildup
• Fainting episodes (syncope)
• Abnormal lung sounds

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?

Heartworm disease shares symptoms with many other common dog diseases, making it difficult to recognize. To make things trickier, dogs may not show any signs for months, which is why your vet will do special tests for an accurate diagnosis.

Your veterinarian will take a small blood sample from your dog and test it for heartworm proteins. If your dog tests positive, they may need to start treatment immediately. Alternatively, they may order more tests like X-rays and a heart ultrasound called an echocardiogram.

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

Like almost anything else, there are several variables when it comes to the price of heartworm treatment for dogs. Factors include the severity of the case, location, and the specific course of treatment. 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.”

(Download this helpful Heartworm Disease Guide for Dogs)

Can heartworm disease in dogs be cured?

Yes, there is a cure for heartworm disease in dogs, but 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.

Dogs with a lot of heartworms might not show any more symptoms than dogs with just a few. But here’s the good news — if the symptoms are mild, there’s a better chance your dog will respond positively to heartworm treatment.

In some cases, surgery is necessary to physically remove the heartworms from your dog’s chest. In heartworm surgery, a special tool is inserted into the heart so the parasites can be removed.

According to the American Heartworm Society, heartworm treatment in dogs can take several months and involve multiple 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. 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.
• 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 clear out heartworm larvae from your dog’s bloodstream.
• Your veterinarian may also prescribe antibiotics or steroids.
• In severe cases, your veterinarian may perform heartworm surgery to physically remove the parasites from your dog’s body.
• Your veterinarian will continue testing for heartworm disease after treatment, often starting about 6 months after treatment ends.

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 if they’re heartworm-positive?

No one wants to see their beloved pet bored and sad in a kennel or crate. But it’s imperative that you keep your dog as inactive as possible until they’ve completed their heartworm treatment.

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. This 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?

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. Have them try their paws at games like puzzles, 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. 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 diseases they cause.

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

Heartworms can infect dogs anywhere in the world, though they tend to be more concentrated in warm areas, with geographical infection rates varying widely from year to year. 

According to the American Heartworm Society’s 2022 Heartworm Incidence Survey, the Southeastern and Gulf Coast states in the U.S. continue to report the highest percentages of positive cases. Mississippi has maintained the highest incidence for the past six years, followed by Louisiana, South Carolina, Arkansas, Alabama, Texas, Tennessee, Georgia, North Carolina, and Oklahoma. It’s no surprise that these are areas where mosquitoes are most active

Even so, remember that 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 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, but you can also take steps within your environment to reduce the likelihood of mosquito bites. Get rid of any standing water outdoors, use mosquito traps, and time 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.”  

How often should I get a heartworm test?

Got a healthy pooch? Great! Your dog should still 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.”

Is canine heartworm disease contagious?

We know that spotting the signs of heartworm in dogs can be terrifying. So, 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% of dogs will be cleared of heartworm with an intensive three-dose protocol. But the very best prognosis comes from regular prevention year-round.”

Prevention is the best cure

Heartworm disease is a complicated issue that requires immediate medical attention and TLC for your infected dog. Still, as the U.S. Food and Drug Administration notes — and both Dr. Muller and Dr. Choczynski Johnson agree — the best treatment is simply adequate heartworm prevention.

Keeping your pup safe with year-round heartworm prevention is the best way to avoid the stress, cost, and heartache of dealing with this nasty parasite. Want extra peace of mind? A pet insurance plan can help cover eligible vet bills in case of unexpected accidents or illness in the future. Learn more about Pumpkin Dog Insurance plans and keep your pup’s tail wagging.

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