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Heartworm in Dogs – Causes, Signs, Treatment & Prevention

Written by: Jackie Brown

Last Updated on March 24, 2024 by Dogster Team

A sick senior dog.

Heartworm in Dogs – Causes, Signs, Treatment & Prevention

Heartworm in dogs is a serious condition caused by an internal parasite that’s transmitted by mosquitoes. After a mosquito bites your dog, heartworm microfilaria (baby worms) pass into the bloodstream, where they eventually mature into larvae, which will grow into adult heartworms. Infected dogs can have hundreds of these worms in their heart, lungs and arteries.

“These parasites cause irreversible permanent damage to the blood vessels in the heart and lungs,” explains Ernie Ward, DVM, CVFT. “These little larvae are circulating throughout the entire body, congregating in the kidney and the liver vessels, and then they grow up into these 14-inch long monster worms that reside in the heart and lungs.”

What dogs are at risk for heartworm disease?

A sick white dog wrapped in a green blanket.
Unfortunately, heartworm can affect any and all dogs around the United States. Photography by Anna Hoychuk / Shutterstock.

Heartworms are found anywhere mosquitoes are found. In the U.S., the parasite has been identified in all 50 states and Puerto Rico. For this reason, all dogs, regardless of where they live, should be on heartworm prevention.

How is heartworm in dogs diagnosed?

“In addition to year-round prevention, the American Heartworm Society recommends an annual heartworm test for all dogs, regardless of where they live or whether they are on preventive,” says Andrea Sanchez, Senior Manager of Veterinary Policies and Standards for Banfield Pet Hospital. “Annual testing is important because the time from infection to adult worms is roughly six months, meaning some pets may have been infected with juvenile worms that did not originally result in a positive test but have since matured into adult worms that are doing irreversible damage.”

Heartworms are diagnosed with a simple blood test.

How is heartworm in dogs treated?

“If a dog contracts heartworm disease, it’s expensive to treat and it’s challenging to treat,” Dr. Ward explains. “It’s a series of injections of a drug to kill the adult heartworms. We have to do that very carefully because when these worms die they deteriorate and they break off into little pieces. Those little fragments of the heartworms can cause blood clots throughout the body, which can even be fatal, especially if they’re in the lungs, [which is] a condition known as pulmonary thromboembolism.”

One important component of heartworm treatment is exercise restriction. You also want to prevent your dog from becoming overly excited or upset. “We’re trying to keep the dog calm and as immobile as possible,” Dr. Ward says. “We really want to maintain a low heart rate because if you have rapid pumping of the heart, you’re more likely to dislodge a heartworm fragment. House rest, and no running or playing during the treatment period.”

Heartworm treatment lasts for months, so this part of the home care may prove challenging for dog owners. However, it’s a critical part of treatment, and can even be a matter of life or death. “If the dog somehow escapes the house or barks at a neighbor or somehow they get very excited, I have actually had patients that literally dropped dead,” Dr. Ward says. “They develop one of those blood clots and within just a matter of minutes, they die. It’s a heartbreaker and just another reminder of how serious this condition really is.”

How do you prevent heartworm in dogs?

Preventing heartworm is easy with just a monthly chewable pill, spot-on topical medication or a six-month injection (given by your vet), but despite the plethora of preventive options, many dogs still aren’t protected.

“It’s as simple as giving a preventive once a month and yet people often fail to do that,” Dr. Ward says. “We get busy, have a family to take care of and a job to maintain, and so we forget the dog’s monthly heartworm preventive. I think cost is a barrier — even though it’s relatively inexpensive, it’s still an additional cost that you have to incur.” The good news is, many affordable and effective generic heartworm preventives are available.

Are heartworm preventatives safe for dogs?

Some pet owners are concerned about the safety of heartworm preventives, but veterinarians feel confident using them. “Heartworm preventives, and many other veterinary medications, undergo extensive testing for safety and efficacy,” Dr. Sanchez explains. “Side effects, while always possible, are rare. Any concerns about a medication reaction should be brought immediately to your veterinarian’s attention.”

What about ivermectin and heartworm?

Some herding breeds, such as Collies, and some sighthounds, have a known genetic sensitivity to one type of heartworm preventive — ivermectin. Not every dog of an affected breed will have the gene that causes the sensitivity. “Genetic tests are available that can help identify dogs that may be at risk,” Dr. Sanchez explains. “In certain situations, there is no increase in risk for side effects with medications. Other times, reduced dosages may be recommended, or a different medication may be prescribed by your veterinarian. As with any medication for your pet, it is always best to partner with your veterinarian on the most appropriate option for your individual pet’s needs.”

Tell us: Has your dog ever experienced heartworm?

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