dog worried about heartworms

As a concerned pet parent, you want to protect your best buddy and keep them free of icky diseases. One of the illnesses you may have heard of and wonder if you should be worried about is heartworm disease

This canine-specific disease is relatively uncommon, but it can be devastating if your precious pup is unlucky enough to be infected. That’s why it’s important to be familiar with the signs of heartworms in dogs and know what to do if Fido catches it.

What is Heartworm Disease?

Since not all diseases are intuitively named (for example, ringworm is a fungus, not a worm) it’s reasonable to wonder: what are heartworms, anyway? They’re not literally worms in your dog’s heart…right?!

Unfortunately, that’s exactly what they are. The parasitic worm Dirofilaria immitis lives within the heart and blood vessels of an infected dog (or other canine). If they build up, the sheer mass of these worms restricts blood flow to or from the heart. This causes internal inflammation, severe organ damage and organ failure if untreated.

Horrifyingly, an individual adult heartworm can be up to 1/8” wide and over a foot long! If not killed by treatment, they naturally live for around 5-7 years. The average number of adult heartworms living in an infected dog is 15, but there have been cases where over 300 adult heartworms were found in a single animal!

How do dogs get heartworms?

If you’re wondering what causes heartworms in dogs or how this disease is transmitted, it’s thanks to another pesky parasite we all know and hate: mosquitos

The heartworm life cycle begins when new heartworms are born within the body of an infected dog. When a mosquito bites this unlucky pup, it picks up microscopic baby worms along with its meal. 

The mosquito is an intermediate host for heartworms, which means the baby heartworms live in the mosquito’s body for a week or two as they grow into heartworm larvae. Then, when the mosquito bites another dog, the larvae are transmitted to this new host.

Some animals like cats and ferrets can also carry heartworms, but dogs (and other canines such as wolves, coyotes and foxes) are the “natural host” for heartworms — which means that’s where the worms can comfortably grow to adulthood, mate and reproduce.

Once heartworm larvae enter a dog’s body, it takes another six months for them to fully grow to adulthood. The worms then mate and reproduce, producing microscopic baby heartworms to continue the cycle.

How significant is my dog’s risk of heartworm?

Heartworms are most prevalent where mosquito populations are high. Dogs in hot and wet climates, such as the southern United States, are at highest risk of heartworm disease.

However, mosquitos are widespread, and therefore heartworm disease has been detected across North America and even in Canada. Regardless of where you live, take these creepy crawlies seriously!

Signs and Symptoms of Heartworm Disease

Early stage symptoms

The early signs of heartworms in dogs can be difficult to pinpoint. Frighteningly, a dog infected with heartworms may show no symptoms at all for several months while the worms grow and mature. 

The point at which warning signs of heartworm disease do begin to show varies for each dog. How active the dog’s lifestyle is, their age and size, other health conditions they may have, and the “worm burden” (the number of heartworms they are infected with) all play a part.

When symptoms do begin, the most common early signs of heartworm disease include:

  • A persistent cough
  • Shortness of breath
  • Tiring easily after exercise, or low energy overall
  • Loss of appetite 
  • Loss of weight

How do you know if it’s heartworms or kennel cough? What does heartworm cough sound like?

Defining if your dog has a “persistent cough” can be tricky. If you’ve noticed your dog coughing, you may wonder if the cause is something unusual like heartworms, or a more common ailment like kennel cough (bordetella).

A “heart cough” in dogs — which can be from heartworms, asthma or a heart condition — is a soft, dry cough that persists for several seconds or even minutes at a time. It can happen anytime, but is particularly triggered by exercise or excitement.

By contrast, kennel cough tends to be a strong, harsh sound. Often a dog with kennel cough will cough loudly a few times, make a gagging sound, and then stop. These coughing episodes will be sporadic throughout the day and not triggered by anything in particular.

Late stage symptoms

As heartworm disease progresses, the infected dog’s heart, lungs and other organs become increasingly impacted and blood flow is severely reduced. This then triggers a cascade of alarming symptoms, including:

  • Swollen abdomen or legs
  • Anemia
  • Signs of heart failure or heart murmur
  • Weak pulse
  • Abnormal sounds from the lungs
  • Fainting or collapsing after exercise
  • Liver cirrhosis
  • Jaundice

At the most extreme stage of heartworm disease, the amount of worms may even trigger the sudden onset of a lethal condition called caval syndrome. As the American Heartworm Society warns: 

“Dogs with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life-threatening form of cardiovascular collapse. This is called caval syndrome, and is marked by a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the heartworm blockage, few dogs survive.”

Testing For and Diagnosing Heartworm Disease

When should dogs be tested for heartworms?

If you are seeing the warning signs and symptoms of heartworm disease, it’s time to tell your vet and get Fido tested! A blood test called an antigen test can detect the presence of adult heartworms in your dog’s body. 

Healthy dogs should be tested for heartworms once a year. A puppy’s first heartworm test should be performed at 7 months. If it is negative, the puppy can then be started on a preventative. They should be tested again 6 months later, and once a year thereafter. 

What if my dog has no signs of heartworms?

If your dog is on a heartworm preventative and not showing symptoms, you may wonder, “Are heartworm tests really necessary?” The answer is yes — absolutely. As the American Heartworm Society puts it:

“Heartworm medications are highly effective, but dogs can still become infected. If you miss just one dose of a monthly medication — or give it late — it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit a heartworm pill — or rub off a topical medication. Heartworm preventives are highly effective, but not 100 percent effective. If you don’t get your dog tested, you won’t know your dog needs treatment.”

Heartworms are much, much easier to treat in the early stage, before a dog even shows symptoms. By the time your poor pup is in late stage heartworm disease, they may have residual organ damage that impacts the rest of their life, even after the worms are killed.

Make sure your dog is on a heartworm preventative and gets tested every year to prevent this (literally) heartbreaking scenario!


Once you’ve received the disheartening news that your best furiend has tested positive for heartworms, you’ll probably have many questions about what you should do next. You may wonder if heartworm even can be cured, or if it’s something your dog will have to carry with them all their life. The good news is, heart disease can be completely cured — although it is not an easy process, for either you or your dog! 

What happens if my dog tests positive for heartworm disease?

The first step is evaluation. After an antigen test confirms the infection, your vet will order other tests such as x-rays, echocardiograms or a full blood panel to evaluate the extent of the disease and its impact on your pooch.

The second step is stabilization. If the heartworm disease has already caused extensive damage by the time it is detected, it’s important to treat your dog’s symptoms before addressing the root cause (the worms themselves).

Heartworm disease is stressful on your pet’s body, but the treatment is as well. That’s why it’s crucial to first make sure your dog is in stable condition and any negative effects of the heartworms are at least partially managed.

It’s also imperative to keep your dog on bed rest from the moment you find out they have heartworm disease. Excitement or activity aggravates the disease, so try to keep your pup still and calm. That means no more walks or games of fetch — brief potty breaks only! 

The third step is treatment. The medication that ultimately kills the heartworms is administered by a deep injection into your dog’s back muscles. It must be administered in a series of doses. Typically, vets allow a few weeks between the first and second dose, but only a day or so between the second and third doses. 

The first injection should kill the majority of the worms within a few days, but it will take weeks for your best buddy to recover from the strain. The Veterinary Centers of America advises:

“Complete rest is essential after treatment. The adult worms die in a few days and start to decompose. As they break up, they are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This resorption can take several weeks to months, and most post-treatment complications are caused by these fragments of dead heartworms. This can be a dangerous period so it is absolutely essential that the dog be kept as quiet as possible and is not allowed to exercise for one month following the final injection of heartworm treatment.”

At some point either before or after the adult heartworms are killed, a different drug will also be administered to your dog to kill any of the microscopic baby heartworms that might be present (and thus sparing another pet from the same fate!).

Because the entire treatment is so stressful on your pet’s body, antibiotics are also administered to prevent another infection from taking advantage of your buddy’s weakened state. 

Finally, once it is confirmed that all heartworms (adult or otherwise) are dead and gone, it is absolutely critical to start your dog on a heartworm preventative right away. Nobody will want Fido to have to endure another round!

Your vet will want to retest your dog for heartworms after 6 months and again at 12 months. After that, provided they stay on a preventative, your pooch should be in the clear — and feeling MUCH better without all those wriggly trespassers!

Heartworm Prevention

Thankfully, heartworm disease is relatively easy to prevent!

Heartworm prevention medication is available in a variety of forms: as a monthly pill, an injection to be given every six months, or as a topical cream. Some heartworm preventatives are also effective against other common canine parasites, such as fleas.

All heartworm preventatives work by killing heartworm larvae, but be aware that they cannot kill adult heartworms — which is why it is important not to miss a pill or injection, which may give heartworm larvae a chance to mature.

Finally, remember that unlike more common parasites such as bordetella or giardia, heartworms cannot be spread directly from pet to pet. Heartworms are dependent on mosquitos to carry their larvae to a new host.

So although your heartworm-positive dog should not be playing or roughhousing, they don’t necessarily need to be isolated from the other dogs or cats in the home — particularly if they have a favorite nap buddy!

That’s also why taking steps to protect your dog from mosquitoes (such as using mosquito sprays, repellents or traps) is another good way to reduce the risk of heartworms and other mosquito-borne diseases.

Can humans get heartworms?

If your dog is diagnosed with heartworms, you may immediately think of all the times your precious pup licked your face recently — and worry for your own health, too!

The good news is, there’s no reason to shun poor Fido in his time of need. People cannot catch heartworms directly from our pets. (Phew!)

In rare cases, humans can become infected with heartworm larvae after a bite from an infected mosquito. Thankfully, because humans aren’t the preferred host for heartworms, the larvae will die before they mature.


Heartworm disease is a serious and frightening diagnosis. If left untreated, it will almost certainly kill your pet.

If you test annually and watch for the key signs of heartworms in dogs — such as persistent cough, lethargy, and shortness of breath — you can catch the disease early and treat it before too much damage is done.

But as with many diseases, prevention is better than a cure. Staying on top of your dog’s heartworm prevention medication is the best way to protect your pooch from these nightmare-worthy worms!

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