May 17 2013

Heartworm Disease By: Lynn McCarron DVM, DABVP

Heartworm disease is a serious and potentially fatal disease in dogs

Adult heartworms live in the heart and pulmonary arteries of infected animals, and produce millions of young (microfilaria) that live in the bloodstream. The worms can be approximately the size of a piece of spaghetti. Dogs may be infected with as few as three or four worms, or as many as 300. Heartworms are not contagious until they pass to another animal through a mosquito bite.

The female mosquito bites the heartworm infected dog and ingests the microfilariae during a blood meal. The infected mosquito then bites another dog and the infected larvae are passed on. When fully developed, the infective larvae enter the bloodstream and move to the heart and vessels, where they grow to maturity and start reproducing more microfilaria.

It takes a number of years before dogs show signs of infection. Consequently, the disease is diagnosed mostly in four to eight year old dogs. However, we can start testing for it as soon as dogs reach around one year of age. We generally don’t test for heartworm in puppies, as the young worms (larvae) take up to seven months to mature following an infected mosquito bite.

What do heartworms do to the dog?

Heartworms cause disease by clogging the heart and major vessels leading from the heart, which can decrease the blood supply to other organs of the body, particularly the lungs, liver and kidneys. Heart failure is often the end result.

Most dogs do not show any signs of the disease for several years. Unfortunately, by the time signs are seen, the disease is well advanced. The most obvious signs are: a cough, shortness of breath, weakness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint. Often the damage done to the heart is permanent.

How are dogs treated for heartworms?
Treating heartworms can be expensive and dangerous to your dog- which is why we recommend PREVENTION year round (with warmer temperatures some mosquitoes are surviving the winter). The drug we currently use to treat heartworms does not have appear to have toxic side effects, unlike some of the old treatments, however it can still be dangerous if the treatment is not followed through step by step. We are able to successfully treat more than 95% of dogs with heartworms.

Complete rest is essential after treatment: This is often the most difficult part of treatment for both the owner and the dog. Strict exercise restriction is necessary for at least one month following treatment. As the adult worms die, they break up and are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This can be a dangerous period, so it is absolutely essential that the dog be kept quiet and not be allowed to exercise for one month following treatment.

Preventing heartworm disease is much easier and safer than treating it. Many medications are available that prevent heartworm disease from developing in your dog, including once monthly chewable pills and topical liquids. Since prevention is not 100% we recommend an annual heartworm test, as part of your dog’s regular checkup, which allows early detection of the disease before symptoms have been noticed.

Can my cat get heartworm disease?
Cats may become infected with heartworm disease, although it’s not too common, and symptoms include coughing, vomiting, and even sudden death. Unfortunately the disease is much more difficult to diagnose in cats, and more difficult to treat. Ask your veterinarian if they recommend monthly heartworm prevention for your cat.

Can I get heartworm disease?
Humans can potentially get infected if they are bitten by a mosquito that is carrying heartworm. However, human infections are very rare, as humans are not a natural host for the heartworm.

Do I have to worry about heartworm disease in Utah?
Canine heartworm disease occurs all over the world. In the United States, it was once limited to the south and southeast regions. However, the disease is spreading and is now found in most regions of the United States and Canada, particularly where mosquitoes are prevalent. In 2011, there were 85 reported cases of heartworm disease of dogs in Utah.

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3 thoughts on “Heartworm Disease By: Lynn McCarron DVM, DABVP”

  1. karen says:

    I’ve recently been told that heartworms don’t exist in Utah County, I just acquired a second dog, taking her to my vet next week for her first checkup. Have I been wasting money on heartworm medication all this time? Is there actually a threat of heartworm in Utah County for a dog that NEVER travels outside of our county?

    1. uvhdcadmin says:

      Karen, Although Heartworm Disease isn’t widespread in Utah, it is unfortunately here. Last year we diagnosed 3 cases, 2 of them were pets that had only ever been in Utah- never traveled out of state, which means that they contracted the heartworm here in Utah. Heartworm disease treatment is very expensive and is also very hard on the dog’s body as well, therefore we recommend to ALL of our clients that they give year round heartworm preventative- and not just in the summer months. All it takes is 1 infected dog to be bit by a mosquito to transmit it- and since we do have confirmed cases in Utah each year, that means there are mosquitos out there spreading the disease. Take a look at the American Heartworm Society’s incidence map and you will see how it’s quickly spreading everywhere. Hope this helps!

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