Heartworm in Dogs: Diagnosis, Treatment and Prevention

Heartworm in dogs is 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. Dogs Infected with heartworms can have hundreds of these worms in their heart, lungs and arteries.

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

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Since heartworms are found anywhere mosquitoes are found, all dogs, irrespective of where they live, should be on heartworm prevention because they are all at risk of the infection.

Diagnosis for Heartworm in Dogs

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 essential 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 in dogs are diagnosed with a simple blood test.

How is Heartworm Treated in Dogs?

Heartworm treatment in dogs is expensive and 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 equally be deadly, especially if they’re in the lungs, which is a condition known as pulmonary thromboembolism.”

One vital piece of heartworm treatment is to restrict your dog from exercise or jumping, running, and playing around. You have to prevent your dog from becoming too 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.”

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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.”

Preventing Heartworm in Dogs

Heartworm can be prevented in dogs with a monthly chewable pill, spot-on topical medication or a six-month injection, but despite the lots of preventive options, many dogs still aren’t protected.

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“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 monthly heartworm preventive for the dog. I think cost is a barrier, even though it’s moderately low-cost, it’s still an additional cost that you have to incur.” The good news is, many cheap and active 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 are possible, but very 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 called ivermectin. Not every dog breed will have the gene that causes the sensitivity.

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“Genetic tests are available that can help identify dogs that may be at risk. In certain situations, there is no increase in risk for side effects with medications, explains Dr Sanchez. Other times, reduced dosages may be recommended, or a different medication may be prescribed by your veterinarian.


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