The heart of the matter: Quick and easy heartworm prevention for your furry Valentine

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Dr. Anna Dunton-Gallagher

Valentine’s Day is the perfect time to talk about heartworm disease, because it is a day to celebrate hearts. Some of us have exciting dates planned, some of us have fun dates, and then some of us have dates planned with our always loving furry pets. We all love our pets and want to keep them safe, but did you know how easy and important it is to protect their hearts from parasites? A simple monthly pill can prevent heartworms, but treatment is involved and expensive.

What are heartworms?

The easy answer is that they are literal worms living in pet’s hearts. Moving, wiggling, worms living in the chambers of the heart. The microfilariae (which are baby worms) migrate through the bloodstream, then mature in the heart. Heartworms live for about two years. The pets do not clear the infection after two years, however, since the worms continue to reproduce, and more and more develop. Without treatment, some pets die because there are so many worms that they actually prevent blood from flowing at all. Heartworms can also be found in other places in the body if they migrate out of the heart. I have had colleagues even find heartworms during neuter procedures.

How do I prevent heartworm?

Heartworm infection is prevented by a monthly pill or topical treatment. There are several types of drugs depending on the brand, but they all work in the same manner. These drugs kill the microfilariae or “baby worms” within the bloodstream so that they don’t have a chance to migrate to the heart and mature. It is recommended by the American Heartworm Society year-round but is essential during the warmer months when mosquitoes are present. Because these medications cannot kill adult worms, it is important not to skip months when mosquitoes are present. If your dog is infected and several months go by, the medication will no longer be able to kill the microfilariae, and these will be progressing to adults.

How is heartworm spread?

Heartworm is spread by mosquitoes. It cannot be spread any other way. Mosquitoes carry the juvenile stage of worms from one infected animal to another. Dogs, foxes, coyotes, and wolves are all able to harbor and spread heartworms through mosquito vectors. The microfilariae become infective after spending time in the mosquito and are then deposited on the skin of pets when the mosquitoes bite. The microfilariae then migrate into the animal and into the bloodstream, where they eventually mature in the heart. Heartworm can be spread among wild animals and dogs, so even if your pet doesn’t live with other dogs they are at risk if you see mosquitoes.

What if my pet has heartworms?

Heartworm is detected on an annual screening test. Heartworm treatment consists of killing the adult worms, the microfilariae, and treating the side effects of the worm death. Your veterinarian will run a confirmation test and stage the heartworm disease based on looking at bloodwork and chest x-rays. Your pet will receive an injection of a medication that kills the adult worms, then one to two more injections over the next month. This injection must be given in the muscles along the back, and because it must be given in a sterile manner, your pet will have the hair shaved in those two areas.

They will also receive heartworm prevention pills beforehand and monthly for a year after treatment (at least.) This will ensure that any “baby worms” are killed before they have a chance to develop into adults. Dogs will receive two weeks to a month of antibiotic treatment that will help prevent a bacteria called Wolbachia that lives with the heartworms from developing.

There is another method that consists of treating the juvenile form monthly and waiting for the adults to die. This is less expensive, but not recommended. Worms can live at least two years, during which time they alter how the heart muscles work. These alterations will be permanent and often lead to irreversible heart damage.

How soon after we find heartworm is it treated?

Your pet should be treated as soon as possible after they are diagnosed with heartworm, ideally within two weeks. The danger of not treating heartworm is that the structure of the heart changes to accommodate the worms. As the blood flow changes, because it has to go around the worm, the heart and lung structure is affected. The longer the worms are living in the heart, the more damage is done which we cannot reverse.

What happens after treatment?

The number-one most important thing that I cannot emphasize enough is that dogs MUST be confined for four-six weeks after treatment. They can only go outside to go to the bathroom, and can never be off leash. Indoors they need to be crated or confined to a small space unless you are next to them. They cannot run, jump, play or have any sudden bursts of energy.

When we give the injection to kill the worms, they disintegrate into small pieces as they die. If dogs have a burst of energy or their heart is required to pump faster, there is a high chance that the pieces of worm will become lodged throughout the body. These act just like clots (by stopping blood flow) and can cause serious damage to the heart, lungs, organs or even lodge in the brain and cause sudden death. After six weeks the pieces will have dissolved to the point that they are no longer dangerous.

It can be very difficult for owners to keep a dog fully confined for that long, especially those that are used to daily exercise. Since they appear normal, the urge to let them loose can be very strong. This is the hardest part of treatment, and some dogs need mild sedation in order to relax. This period of time is a small price to pay, however.

Can cats get heartworm?

Cats CAN get heartworm. It is much more prevalent in the southern climates than Vermont, but is still a possibility. Cats are an aberrant host, which means that they cannot spread heartworms to mosquitoes. However, they can get the disease and suffer problems as well. There is also heartworm prevention for cats, so make sure to speak to your veterinarian about it.

The bottom line is that preventing heartworm is much easier than treating it. Treatment is painful, since we are injecting a reactive medicine into their muscle. Before treatment, we give pets pain medication and a light sedative, because it is uncomfortable. Treatment usually costs between one and two thousand dollars for the bloodwork, x-rays, hospitalization and medication, depending on the size of the dog and stage of disease. Confinement after treatment can be difficult as well. Not treating heartworm dramatically shortens the lifespan of pets and leads to serious heart and lung disease. Dogs can be affected by heartworm more than once, so even after treatment they must receive preventative pills. Most dogs look forward to their monthly “treat,” and there isn’t much better than preventing a deadly disease while making your dog happy.

The American Heartworm Society has a very informative website to answer other questions.

Dr. Anna Dunton-Gallagher

Dr. Anna Dunton-Gallagher is a veterinarian at All Points Animal Care in Rutland. Have a question on this or any animal health topic? E-MAIL:

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