Everything You Need to Know About Heartworm in Dogs

A word of warning: Don’t read this article on your lunch break unless you have a strong stomach. You see, heartworm lives up to its name—and then some. It’s a disease characterized by parasitic worms living inside a heart. It’s, in a word, gross. But it’s also easily preventable and very important to understand for your dog’s health. Forks down, people. Here we go.

Scientifically they’re known as Dirofilaria immitis. Basically? Worms. Specifically: Parasites that can reach up to 14 inches in length. According to VCA Animal Hospitals, these worms are roughly ⅛ inch (5 mm) wide (think: udon noodles). These worms can live up to seven years in dogs, constantly reproducing millions of baby worms (called microfilariae) in that time. The adults love cozying up inside an animal’s heart, lungs, arteries and large blood vessels, while the microfilariae swim through small blood vessels. There can be hundreds of worms present in your pup by the time she’s diagnosed.

Heartworms are blood-borne, which means the bloodstream is their mode of transportation and transmission (aka dogs can’t give it to each other, but an infected dog is bad news for the neighborhood). If a mosquito bites a host with heartworm it ingests microscopic microfilariae into its bloodstream. Two weeks later, the baby microfilariae are now teenage larvae. As the Heartworm Society puts it, thus begins the “infective stage.” If the mosquito bites a dog during this time, the larvae will flow into the dog’s bloodstream like kids down a waterslide. Within six months, adult worms will be alive and thriving in the dog’s heart, pulmonary arteries (which deliver blood to the lungs) and blood vessels.

Obviously, circulatory and pulmonary systems full of worms is bad. The adult worms block major blood vessels, preventing blood from getting to the kidneys, liver and lungs (which could lead to organ failure). They also clog the heart and prevent heart valves from doing their jobs (which could lead to cardiovascular failure). Again, it’s a serious, life-threatening disease that could be fatal if left untreated in dogs, cats, foxes, coyotes, ferrets and wolves.

Catching heartworm early is critical to making a full recovery. Unfortunately, according to Dr. Katja Lang, DVM at Heart of Chelsea Veterinary Group, “There are often no signs of illness in the early stages of heartworm disease.”

As the worms mature and multiply, dog owners might notice coughing, swollen bellies, labored breathing and exhaustion or disorientation after walks or exercise. The only way to be sure is with a vet’s diagnosis.

One condition in particular, called caval syndrome, does occur suddenly if heartworm has gone untreated for a long time. Signs include dark (sometimes bloody) urine, pale gums and an abrupt loss of breath. If this happens, emergency surgery is necessary to save the dog’s life.

The good news, Dr. Lang adds, is that “dogs can make a full recovery if [heartworm] is caught early and treated aggressively.” Plus, there are tons of preventative measures pet parents can take to avoid this fiasco entirely.

“The best treatment is often the ‘fast-kill’ method,” says Dr. Lang. This includes a series of drug injections designed to kill both adult and baby heartworms.

Dr. Lang also notes pet owners must prioritize rest and relaxation during and right after treatment. Due to the decomposing worms in the bloodstream (sorry), dogs are at a higher risk of pulmonary embolism, the blockage of an artery in the lungs, and other major organ complications if allowed to be active during this time. 

In some advanced heartworm cases, those stupid worms have done so much damage, dogs may need to be on diuretics, heart medications or low sodium diets for the rest of their lives. 

“It is important for pet owners to understand that the treatment for heartworm disease is neither simple nor risk-free,” says Dr. David Dilmore DVM of Banfield Pet Hospital. “By the time heartworms are detected, it is possible that significant damage to the heart, vessels and lungs may have occurred. Some of this damage may be permanent.”

In severe cases, vets decide treating organ damage is safer than destroying the worms. (Not gonna lie, this last one is not a good diagnosis, as it usually means your dog won’t live much longer. That’s why prevention and yearly screenings are so important.)

The good news is, unlike Lyme disease prevention methods, which are intense and not suitable for all dogs, heartworm prevention is super doable and versatile. There are chewable tablets your dog can take monthly, vaccinations and annual (or biannual) testing to scan for signs of heartworm. Taking preventative measures is not only less painful on your pup, it’s much less expensive than treatment options and can protect from other parasites.

“All dogs, regardless of their age, gender or habitat—including indoor and outdoor pets—are susceptible to heartworm infection,” adds Dr. Dilmore. (Remember, all it takes is one mosquito!) Talk to you vet ASAP about heartworm prevention and screening if you haven’t done so in the past year.