How Do Dogs Get Heartworms? The Ultimate Guide to Keeping Your Dog Safe

The deadly and invasive parasite kills hundreds of pets per year. Here’s how to protect your dog from heartworm.

By Stephen C. George
Jul 24, 2023 1:00 PM
Dog at vet
(Credit: all_about-people/Shutterstock)


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By itself, Dirofilaria immitis looks about as threatening as a strand of limp spaghetti. In fact, it looks a lot like a strand of limp spaghetti. But in their hundreds, when these unassuming little roundworms grow to adulthood, they can cause serious and often fatal problems for your pet. And the worst part is, you might not see the signs until it’s too late.

We’re talking, of course, about heartworms, a devastating parasite that infects about 1 in every 100 dogs. Here’s what you need to know about heartworms and the most effective ways to save your pet from infection.

How Do Dogs Get Infected?

(Credit: BlueRingMedia/Shutterstock)

Like too many other diseases and parasites, heartworms are transmitted by mosquito. When an infected mosquito bites your dog, they deposit larvae that make their way to the heart, lungs and adjacent blood vessels. There, they eventually grow to full size over several months.

Dogs are considered the definitive host for heartworms, which means that the parasite can grow to maturity and reproduce all within the host. And grow they do: Adult worms can reach as much as a foot in length, clogging blood vessels and damaging organs as they go through their lifecycle. While adult worms can live as long as 5 or more years in a host body, it usually doesn’t take that long — mere months — for an infected dog to become very ill. 

How to Check If Your Dog Has Heartworms

(Credit: ThamKC/Shutterstock)

Unfortunately, infected pets can be asymptomatic for months after infection, which is why many dogs may already have a pretty severe infestation by the time they’re diagnosed. That said, common heartworm symptoms to watch for include loss of appetite, weight loss, low energy and a persistent cough.

Unfortunately, these can be symptoms of a lot of other problems, making it hard to guess whether your dog is suffering from a minor malady or a potentially fatal disease. This is why the American Heartworm Society and others (including your vet) recommend getting your pet tested for heartworms annually starting from the age of six or seven months.

Once they have a serious case, heartworm treatment for dogs can be expensive and hard on your pet, especially if surgery is involved to remove the most dangerous worms from vital organs. Even medication to treat heartworm disease can be toxic or cause other side effects.

Read More: Do Parasites Rule the World?

Heartworm Treatment for Dogs

(Shutterstock/Javier Brosch)

The best way to deal with heartworms is never to let your dog get them in the first place. The FDA and others therefore recommend giving your dog year-round preventive medicine.

Many owners opt for a pill that they can give their dog every month. But if you’re concerned about accidentally skipping a dose, there are preventive medications that can be delivered by injection every six or 12 months. Talk to your vet about the best option for your dog. 

It’s worth noting that most preventive medications typically work by killing only heartworm larvae and do not affect adult heartworms — another reason why getting annual tests and sticking to a consistent preventive regimen is so important. 

Do Cats Get Heartworms?

(Credit: mvatrabu/Shutterstock)

While dogs may be natural hosts for heartworms, the parasites can infect other animals, too. Canine species such as wolves, coyotes and foxes are particularly susceptible to infection, as are some other mammals.

Cats are not common targets for the parasite — their risk of infection is a fraction of what it is for dogs — but it can happen. As with dogs, cats can only get heartworms through a mosquito bite (your pet can’t get it from contact with an infected dog or cat).

As a rule, heartworms don't thrive in a feline body — a cat with heartworm disease may only have a few adult worms (whereas a dog could have hundreds). But even those few adults are enough to harm or kill the host. To make matters worse, feline heartworm disease is currently untreatable: The medicines used on dogs would be fatal to cats. Once again, preventive measures are best and your vet can recommend some options if they feel there’s a risk your cat could get infected.

Can Humans Get Heartworms?

(Credit: Mix and Match Studio/Shutterstock)

Meanwhile, there is another mammal known to become infected with heartworms: humans. To be clear, the risk is very small — in the past 80 years, fewer than 100 documented cases in the U.S. have been reported — but it’s not unheard of.

Luckily for us, humans are even less hospitable to heartworms than cats. Most heartworm larvae in the human body would die before growing to any appreciable size, so you wouldn’t need invasive surgery or medications to remove them (thank you, immune system).

However, even in death those larvae can cause inflammation in the lungs, and tissue from dead heartworm larvae can accumulate in blood vessels. These conditions would require medical treatment, so by all means consult your doctor if you find yourself with an unusual cough (especially if that cough produces blood), chest pain, shortness of breath, or unexplained fever or chills. Again, the odds are vanishingly small that you’d have heartworms, but any of those symptoms warrant a medical evaluation anyway.

Read More: The Origins of Dogs

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