How Do Dogs Get Heartworm Disease?

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The American Heartworm Society does not endorse any specific heartworm medication preventative and is not responsible for any cross-linked

What Is Heartworm Disease in Dogs?

Heartworm disease is a serious—sometimes fatal—disease caused by parasitic worms that grow in a dog’s heart, lungs, and arteries.

Heartworm disease can be easily prevented in dogs but it is unfortunately quite common, affecting at least a million dogs in the U.S. While heartworm disease in dogs is treatable, it can cause serious and lasting damage.

How Do Dogs Get Heartworm Disease?

Heartworms are spread by mosquitoes that carry infective heartworm larvae, which are tiny, immature heartworms. Not all mosquitoes carry heartworm larvae—only those insects that have previously fed on an animal with heartworm disease carry the larvae.

Within just five months of the dog being bitten by an infected mosquito, the heartworms that started out as tiny larvae grow to be up to 12 inches long.

Dogs and several wild canine species, including coyotes, foxes, and wolves, are considered natural hosts for heartworms and are highly susceptible to becoming infected if they are not on regular heartworm preventive medication. 

How Heartworm Disease Spreads in Dogs

Mosquitoes play an essential role in the life cycle of heartworms in dogs at two different stages of heartworm development.

Feeding on blood is a necessity for female mosquitoes because they need nutrients found in blood to lay their eggs. Female mosquitoes feed frequently, drinking roughly three times their weight in blood at any given feeding time.

A dog becomes infected with heartworms when a mosquito carrying heartworm larvae bites the dog and injects the larvae into the dog’s skin while feeding on the dog’s blood. These larvae quickly make their way into the dog’s bloodstream, then on to the heart, lungs, and arteries.

Within just five months of the dog being bitten by an infected mosquito, the heartworms that started out as tiny larvae grow to be up to 12 inches long. These spaghetti-like adult worms can live for up to seven years inside the dog.

Over time, as the worms die, the dead worms will begin to clog a dog’s heart, lungs, and arteries. Dogs also can continue to become reinfected if mosquitoes carrying larvae continue to feed on them.

Additionally, once heartworms reach adulthood, they can mate inside your dog, producing millions of microscopic babies called microfilariae that live in your dog’s bloodstream.

Here’s where mosquitoes play their second role in the heartworm life cycle. When a mosquito feeds on a dog carrying microfilariae, she ingests the baby worms along with her blood meal. The mosquito’s body then incubates these babies for 10–14 days, at which point they have become heartworm larvae and can be transmitted to dogs, cats, ferrets, and other susceptible animals.

Is Heartworm Disease Contagious to Other Dogs?

Your dog cannot catch heartworms directly from another animal infected with heartworms. A mosquito must bite an infected animal, then carry the heartworms to another animal to deposit. In other words, heartworm disease can only be carried and spread through mosquitoes. Any time there are mosquitoes and infected animals in your area, your pet will be at risk if they are not on heartworm prevention.

How Are Heartworms Treated in Dogs?

If your dog becomes infected with heartworms, they should be treated by your veterinarian as soon as possible to minimize the damage these parasites can cause. Damage begins to happen the minute heartworms reach the heart and lungs and continues to worsen if the worms remain there.

Because heartworms can cause lifelong damage, treatment should not be considered a substitute for prevention. The heart and lung damage caused by heartworms is never totally reversible. Heartworms cause thickening and hardening of arteries, inflammation of the lungs, and enlargement and stretching of the heart. If heartworm disease is untreated, dogs can develop heart failure and even die.

Treatment for heartworm is significantly more expensive than prevention, entailing multiple medications and veterinary visits over several months. Additionally, strict exercise and activity restriction are necessary before, during, and after treatment, depending on the severity of the heartworm disease case.

How Can I Prevent Heartworms in My Dog?

Anywhere there are mosquitoes, there is a risk of heartworms. But here’s the good news: Heartworm infection is easy to prevent.

As soon as your dog reaches 6–8 weeks of age, you can begin giving heartworm prevention. If your dog is older, they can start heartworm prevention at any time—it’s never too late to begin. Just be sure your dog is tested for heartworms first. If they were previously unprotected, it’s possible they could have heartworm disease—and if so, you will want to get them treated as soon as possible.

Heartworm preventives are safe, affordable, and highly effective—when given as directed, heartworm medication can be almost 100% effective in preventing heartworm infection in your dog.

Heartworm preventatives for dogs are available by prescription from your veterinarian and come in several formulations. Your veterinarian can discuss these options with you and help you determine which product and formulation makes sense for your dog and lifestyle.

It’s important to know that while some products protect against heartworms only, other products can protect your dog from other common pests like fleas, ticks, mites, and intestinal parasites.

Heartworm preventives are safe, affordable, and highly effective—when given as directed, heartworm medication can be almost 100% effective in preventing heartworm infection in your dog.

Whichever preventive medication you and your veterinarian decide is best for your dog, it’s essential that you give the preventive on time, every time. The American Heartworm Society recommends year-round heartworm prevention for all pets (this includes dogs, cats, and ferrets) to ensure they stay protected from deadly heartworms.

If you miss a dose or give heartworm preventives only during certain months of the year, your dog could become infected. Even if your dog spends most of their time indoors or you live in a cold-weather climate, they are still at risk. Mosquitoes easily come indoors and can also survive in outdoor protected areas for long lengths of time during the winter.

If you live in a community or locale where mosquitoes are present, the odds are good that infected dogs or wildlife are nearby as well. Keeping your dog on year-round heartworm prevention is a safe, cost-effective, and easy way to ensure your dog is protected from this deadly disease.

Jenni Rizzo, DVM, President of American Heartworm Society


Jenni Rizzo, DVM, President of American Heartworm Society


Jenni Rizzo, DVM is the President of the American Heartworm Society and President and Founder of IntroVet, a continuing education...

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