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Heartworm Disease

Heartworm disease mainly affects dogs but sometimes affects cats. It’s found in warmer and temperate climates and it caused by the bite of a mosquito. It affects dogs in all 48 continental states, and Hawaii, Puerto Rico, the US Virgin Islands, and Guam. It is not usually found in Alaska because the weather isn’t consistently warm enough for the worm to grow in the mosquito and cause disease in dogs. It is also found in wild animals such as wolves, coyotes, and foxes. In North America, heartworm disease is most easily spread in July and August.


Heartworm is caused by a worm called Dirofilaria immitis.  It has a life cycle where the mosquito picks up the early worm stage called Larva 1 from the blood of an infected animal and then the worm goes through several stages in the mosquito and is spread to a new animal when the mosquito bites it. It then takes at least six months before the dog can be tested to show that it has heartworm. The worms can grow to be 10 inches long and there can be many worms. They live in the heart and lungs of dogs and cause severe damage. Testing is done annually to prevent this damage so treatment can be initiated quickly.


There are several products that can prevent heartworm. These medications should be given monthly year round. The medications also contain a preventative for intestinal worms, such as roundworms and hookworms. Giving the medication continuously is best even in northern climates that do not have mosquitoes year round. It also helps to get in a routine.   Even forgetting one month of prevention can cause heartworm to develop.


Heartworm can be treated safely in most dogs, but it has some risk and is expensive. Once a dog tests positive, we will do additional testing to see if there has been some damage. Radiographs sometimes show damage to the heart and blood vessels. The heart can enlarge on the right side of the heart where the worms live. This x-ray picture is called a Reverse D because the heart looks like a backwards letter D.


Blood work is done to see if there is any organ damage. There is a strict protocol for treating heartworm in dogs that is recommended by the American Heartworm Society. On the first day of treatment the dog is started on heartworm prevention monthly. Prednisone, an anti-inflammatory, is started for several weeks to prevent lung damage. An antibiotic, doxycycline, is started to help with complication that can be caused by a bacteria that lives in the worm, called Wolbachia. The dog must be kep very quiet, no running at all while it is being treated. As the worms die they can block the blood vessels to the lungs and cause very serious lung disease and even death.


On day 60, the medication that kills the adult worms, called melarsomine is injected into the dog’s back muscle. Prednisone is started again to protect the lungs. The next month, two shots of melarsomine are given a day apart. Prednisone is started again. The dog must be kept very quiet for 2 months after this last injection to prevent thromboemboli of the dead worms from affecting the lungs. This is a very important part of the treatment as the dog can die from the dead worm breaking off in large pieces in the lungs. During the fourth month, the blood is tested for microfilariae (immature worms). If the immature worms are present, a medication is given. We must wait 6 months before testing for heartworm again to make sure the worms are gone.


Heartworm can occur in cats, although they are not the normal or natural host. If the cat does contract heartworm, it will only have a small number of worms. There is testing for cats also, but there is no safe treatment to remove the worms. The worms will usually die in 1-2 years and do not reproduce. The cat can have a severe lung reaction to the worms, similar to asthma. The drug prednisone can help with the symptoms. There is a monthly preventative for cats also.

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Westgate Pet Clinic
4345 France Avenue South
Minneapolis, MN 55410
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