All About Heartworms In Dogs

Mosquito bites are not only annoying, they can begin the cycle of heartworms in dogs.

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A chronic cough, exercise intolerance and decreased appetite are some signs of heartworm infection, but some dogs may have no signs of it at all, which is why testing for it is important. Michelle Kaufman Lyman/iStock/Thinkstock
A chronic cough, exercise intolerance and decreased appetite are some signs of heartworm infection, but some dogs may have no signs of it at all, which is why testing for it is important. Michelle Kaufman Lyman/iStock/Thinkstock
Dr. Jerry Murray

Heartworm disease is a serious and potentially fatal disease for dogs, cats and ferrets. It has been reported in all 50 states and in numerous countries all over the world, according to the American Heartworm Society. Heartworms are actual worms (Dirofilaria immitis) that live in the heart and lungs of canines. Adult heartworms can grow to be 10 to 12 inches long and cause damage to the heart and lungs of dogs.

Dogs can get heartworms from a mosquito bite. Mosquitoes are not picky and will bite any dog. Mosquitoes will even bite dogs with very thick coats, like Chow Chows. I once had a client who was convinced that mosquitoes could not bite through his Chow Chows’ thick coats. Unfortunately, he was wrong, and his two dogs had heartworms. In my opinion, every dog is at risk for heartworms.

The Spread Of Heartworms In Dogs
Heartworms are transmitted directly by mosquitoes. Mosquitoes pick up microfilaria (“baby” heartworms) when they bite a dog or a wild canine like a fox, coyote or wolf that already has heartworms. The microfilaria mature inside the mosquito to the infective larvae stage (“juvenile” heartworms) in roughly two weeks. When the mosquito bites the next dog, it will deposit the infective larvae. The larvae will travel to the heart and blood vessels of the lungs.

It will take six to seven months for the larvae to mature into adult heartworms. The adult heartworm will become sexually mature and produce microfilaria. The adult heartworm can live for five to seven years inside your dog and cause damage to the heart, lungs and blood vessels. An infected dog cannot pass heartworms directly to another dog. This is because the microfilaria requires a mosquito to develop into the infective larvae stage. Likewise, the microfilaria cannot directly develop into more adult heartworms inside your dog, but they are in the bloodstream for more mosquitoes to acquire and spread to other dogs.

Symptoms Of Heartworms In Dogs
Signs of heartworm disease in dogs will depend on the number of adult heartworms, how active your dog is, and if your dog has any other medical problems. The more heartworms and the more active your dog is the more severe the signs will be. I have seen a few inactive dogs with heartworms and no symptoms at all; however, the majority will have some clinical signs.

In mild to moderate cases a chronic cough, exercise intolerance and a decreased appetite are common. In more severe cases, congestive heart failure can develop. This can cause persistent coughing, a heart murmur, abnormal lung sounds, an enlarged liver, fluid accumulation in the abdomen (ascites), anorexia, weight loss and even death. I examined a young dog with all of these severe signs (except death) just last week.

Diagnosing Heartworms In Dogs
Your veterinarian can run an antigen test with a just a few drops of blood to test for heartworms. This simple test takes less than 10 minutes to run. The antigen test is detecting a protein produced by the adult female heartworm. The test will not detect cases with only male heartworms; however, it is rare for dogs to have just male heartworms. A dog with a positive antigen test should also be tested for the presence of microfilaria in his blood.

Your veterinarian may want to do further testing on a positive dog before starting treatment. This can include radiographs (X-rays) of the chest to look at the size of the heart and for changes in the lungs and an ultrasound exam of the heart. A complete blood profile may be done to check for anemia and any problem with the kidneys or the liver.

Treating Heartworms In Dogs
How a veterinarian treats a dog with heartworms depends on the severity of the clinical signs. Consult with your veterinarian to determine the best treatment for your dog.

In mild to moderate cases, I start a dog on an antibiotic called doxycycline and a heartworm preventive (ivermectin). The antibiotic will eliminate a bacterium called Wolbachia that is inside of the heartworm. Wolbachia contributes to the inflammation in the lungs and blood vessels when adult heartworms die, so treatment with doxycycline helps to reduce the inflammation and damage to the lungs when the adult heartworms are killed. After 30 days on the antibiotic, I give the first injection of melarsomine. Melarsomine is the only medication approved by the FDA to treat for adult heartworms. Some veterinarians will also start the dog on prednisone (cortisone) to help further reduce the inflammation in the lungs from the dying heartworms. One month later, two doses of melarsomine are given to kill the rest of the heartworms.

In more severe cases, I like to stabilize the dog by treating the ascites and congestive heart failure before starting to treat for the heartworms. This can be done with prednisone, a diuretic and a heart medicine such as an ACE inhibitor. When the dog is more stable then I start him on doxycycline and a heartworm preventive. After a month on doxycycline, the injections of melarsomine are started.

The three doses of melarsomine will cure roughly 98 percent of the dogs with heartworms. The most important thing is to really reduce the activity level of the dog during the treatment and for a month or more after the last melarsomine injection. This will help prevent clots in the lungs as the heartworms die.

Heartworm Prevention
Heartworm disease is a terrible condition that can cause permanent damage to the heart and lungs, but it is very easy to prevent! There are numerous brand name and generic once-a-month, flavored tablets and flavored chewables that prevent heartworms. Most of these will also control intestinal parasites, and some will even control fleas, too. There are two different liquid medications that you just apply directly to your dog’s skin once a month. Both of these will also control fleas and mites. There is even an injectable product that is given once every six months. All of these products work very well, so talk to your veterinarian about which heartworm preventive is best for your dog, and keep your dog on it year-round.

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Dogs · Health and Care

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