Heartworm Testing, Prevention, and Treatment

Spread by the bites of mosquitoes, heartworms are a deadly kind of parasite. The life cycle of the heartworm begins when a mosquito carrying larvae bites a dog. As the mosquito sucks the dog’s blood, it also injects the heartworm larvae into the vessel from which it is drinking. Once in the dog’s bloodstream, the larvae mature through several stages and eventually take up residence in the heart, from which they release offspring called microfilaria into the bloodstream of the dog. If a mosquito bites a dog whose blood is filled with microfilaria, the mosquito will become a new vector of the parasite, the microfilaria of which turn into larvae inside the mosquito.

There are cases in which adult heartworms have reached over a foot in length. Parasites of such considerable size located in such a vital area do very serious damage, not only to the heart but also to the lungs and the surrounding major blood vessels. A dog can, in some cases, live for months or even years harboring heartworms, depending on the number of heartworms present. However, untreated heartworm disease is always fatal after this window has passed.

The physical presence of the heartworms irritates the vessel and heart walls, and, depending on the number present, occlude vessels, causing the heart to pump harder against increased resistance to blood flow. Eventually, the heart will fail. Additionally, heartworms can damage the lungs of an animal afflicted with them. This damage often comes in the form of inflammation of the lung tissue, known as pneumonitis. This inflammation impairs both the dog’s ability to breathe and to exchange oxygen into the blood. Both heart failure and pneumonitis can cause the dog to cough.

In the final stages of heartworm disease, a large number of heartworms will sometimes block blood flow to the heart almost completely. This is known as caval syndrome. Dogs afflicted with caval syndrome will soon die barring very rapid emergency surgery to clear up the clogging by removing the worms from the heart and major vessels. Even when surgery follows caval syndrome rapidly, treatment often fails, and a dogs chances this late in the disease’s progression are slim.

Even when heartworms are treated before the dog’s heart fails, whatever damage they have done prior to treatment will remain after the heartworms perish. This is why the amount of worms present and extent of damage present at the time of treatment will have huge ramifications for the dog’s quality and quantity of life. If a dog has a low number of heartworms and a less severe amount of damage to its organs, that dog has a good chance of living a normal lifespan after it is treated. On the other hand, if a dog has significant damage, it may require medication for the remainder of its life.

The good news is that heartworms are extremely easy to prevent. Preventative Care requires only a monthly heartworm preventative chewable medication, or a newer semi-annual heartworm preventative medication which your veterinarian can administer by injection. Both methods serve to kill any heartworms which may be present while they are still in the larval form, effectively preventing them from ever reaching maturity. Both approaches are, in most cases, both safe and effective, and some of these medications also provide protection against other parasites.

Heartworms in Cats

Heartworms are not as well adapted to the bodies of cats, and so heartworm infestation is less prevalent among them. However, they can also become infected by the bite of a carrier mosquito. Often, heartworms will be unable to reach maturity in cats, but this does not mean it is impossible for them to do so. If heartworms do mature in cats, they will often take up residence in both the heart and the lungs of the cat. Generally, the symptoms of a heartworm infestation in a cat will manifest differently than would be the case with a dog, and the nebulous nature of this difference makes a diagnosis in cats particularly difficult. Worse, one of the most common signs of heartworm infestation in cats is sudden death. Furthermore, even if a diagnosis of the infection is achieved, there are currently no treatments available for cats which can kill the worms without a great risk of also killing the cat. The best we can do, then, is to provide relief which addresses whatever symptoms occur. Fortunately, there are safe means of heartworm prevention for cats.