Canine Heartworm

Heartworm is a serious parasite of the dog which causes severe damage to the blood vessels of the lungs and the heart.

Heartworms are thin worms up to 30 centimeters long that live in the major blood vessels of the lungs and the right ventricle of the heart. The adult worm produces larvae which circulate in the blood throughout the animal’s body.

These small immature forms, or microfilariae, require another host to complete their life cycle. This is achieved when a mosquito feeds on the dog and sucks the blood containing the immature parasite. The microfilariae complete their development over two or three weeks in the mosquito and eventually migrate to the mouthparts of the insect.

They are then transferred to a new host when the mosquito feeds upon another dog. The infective larva rests in a site under the skin of the animal and undergoes further development for another three months. It then finds its way to the heart, where it grows into a full-size worm.

The effects of the worms on the health of the dog depend upon the number of worms present, which vessels they occupy and the length of time they have been present. Large numbers of worms have been found, in some cases up to 200.

When such a massive infection occurs there is a serious impairment to the circulation of the blood through the heart and lungs and the dog may die from cardiac insufficiency. Even a small number of worms can bring about thickening of the walls of the blood vessels and loss of function which causes the dog to tire more easily when exercised.

What are the first signs of heartworms in dogs? A dog may develop a chronic cough, lose weight and eventually develop a chronic heart disease with accumulations of fluid in the abdomen. Other vital organs of the body, such as the liver and kidneys, may suffer because of diminished blood supply.

The diagnosis of the condition is achieved by examining blood samples for the presence of the microfilariae circulating in the blood. This test is simple and your veterinarian can carry it out in his own surgery.

Sometimes x-rays of the chest are taken to try to establish changes in the size of the heart and major blood vessels. Other tests such as electrocardiograms and blood analysis may help to discover if other organs are affected.

Treatment of heartworm is difficult unless only very few adult worms are present. There is no problem in killing the actual parasite. Quite a number of substances do this very efficiently.

Unfortunately, the presence of dead worms in the major blood vessels can cause complete obstructions in the vessels of the heart and lungs, with fatal results.

One solution is to hospitalize the patient and use small doses of an organic arsenical compound to bring about a gradual destruction of the adult worms. Where the parasite has been present for a long time there may be such serious and permanent damage to the heart and lungs that the extra stress of treatment may not be considered a worthwhile risk.

Fortunately, canine heartworm disease can be prevented.

For dogs living in areas where the disease is endemic, tablets of a drug called diethylcarbamazine citrate, or DEC, given daily will prevent the development of the immature form of heartworm. If daily treatment is not possible the dog may be treated twice a year with the organic arsenical treatment already mentioned.

For owners taking their animals temporarily into infected areas, DEC tablets should be started on entry and continued for 80 days after leaving the infected area. This period is necessary to kill the parasite that may be in the resting stage in the tissues of the dog at the time of leaving.

As the presence of even small numbers of heartworms can produce serious effects in the dog, it would be ideal if all owners had their dogs’ blood tested annually. If it is planned to take dogs to an endemic area it is most important to have this test done to ensure that a dog already harboring adult worms is not given the DEC tablets.


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Hannah Elizabeth is an English animal behavior author, having written for several online publications. With a degree in Animal Behaviour and over a decade of practical animal husbandry experience, Hannah's articles cover everything from pet care to wildlife conservation. When she isn't creating content for blog posts, Hannah enjoys long walks with her Rottweiler cross Senna, reading fantasy novels and breeding aquarium shrimp.

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