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Heartworms in Dogs

Heartworms..... even if you don't know anything about them, the word is enough to send you scadaddling.  Before you do, take a few moments to learn about them.  This information is taken from articles written by Mary Straus and may be read in their entirety at www.dogaware.com/heartworm.html

Catching It And Preventing It

First, understand that all dogs are subject to heartworms if they are not on a preventative.  The larvae is transmitted by mosquito bites, and in Florida that is a year round possiblity.  The two most common (and generally considered safe) heartworm preventative ingredients used today are ivermectin (used in Heartgard by Merial, and other products) and milbemycin oxime (used in Interceptor by Novartis).   These monthly pills are available from your vet.  Although there are some holistic preventives, they have not yet been proven effective.

Rarely, some dogs may have a genetic fatal reaction to Ivermectin, the active ingredient in Heartgard.  Breeds known to be affected include Collies, Australian Shepherds, Shelties, Border Collies, Old English Sheepdogs, English Shepherds, McNabs, Long Haired Whippets, and Silken Windhounds.  If your dog is one of these breeds, we recommend your vet test him for the gene prior to treating with Ivermectin.


Stages of Heartworm Infection

Heartworm infection is divided into four or five stages (depending on the model used), based on the severity of the infestation and the age and health of the dog.

Stage 1 (mild) consists of young, healthy dogs with no symptoms and minimal changes evident on X-rays.

Moderate (stage 2) infection will show heartworm disease that is evident on X-rays, but symptoms are minimal, mostly coughing. Stage 3 is a severe infection, with weight loss, coughing, difficulty breathing, more damage visible on X-rays, along with liver and/or kidney damage.

Stage 4 and 5 are considered critical, with the dog often collapsing in shock. These dogs will not survive ordinary heartworm treatment, and must have the worms surgically removed if they are to have any hope of survival.

Treatment of Heartworms 

There are three conventional methods of treating heartworm: a "fast kill" method using Immiticide (melarsomine); a "slow kill" method using Heartgard (ivermectin); and a surgical method, where the worms are surgically removed from the arteries. In addition, there are so-called holistic treatments, such as Paratox homeopathic or herbal preparations.

Immiticide (Fast Kill)

This is the method preferred by us for dogs beyond stage 1 of the infection.  Standard treatment with Immiticide consists of giving two injections 24 hours apart, then keeping the dog strictly confined for the next four to six weeks. The injections must be given in a painful location — the muscle close to the dog’s spine in the lumbar (lower back) area. The worms start to die immediately. As their bodies begin to decompose, pieces are “shed” into the dog’s bloodstream and filtered out through the dog’s lungs. This can cause the dog to cough and gag, or lead to a fatal pulmonary embolism.

The dog must be kept confined and his physical exertion kept to an absolute minimum, in order to prevent pieces of the dead worms from being forced by a rapid heart rate and/or increased blood pressure into clogging the tiny blood vessels in his lungs, causing embolisms. This generally means that the dog must be kept crated or penned and allowed out to potty only on a leash. Aspirin may be prescribed to lower the risk of blood clots, though this is controversial. Remember that it’s dangerous to combine aspirin with any other nonsteroidal anti-inflammatory drug (NSAID) or with prednisone, and to give it only with food.

Heartgard (slow kill)

The "slow kill" method, which is a newer approach, is the one prefered by us for treatment of dogs who are in Stage 1 of the infection.  It consists of giving the dog Heartgard on a monthly basis. This heartworm preventative medication has some effect against the adult worms and should gradually eliminate them over a period of one to two years; without treatment, the worms can live up to five years. The earlier the treatment is started after infection, the more quickly it will work to eliminate the adult worms. Note that only Heartgard (ivermectin) should be used, as Revolution (selamectin) affects far fewer adult worms, and Interceptor (milbemycin oxime) almost none at all.

Although this method is gentler than the use of Immiticide, the danger from the dying worms is still present, and for a much longer period. A recent Italian study showed that pet dogs (as opposed to the caged laboratory dogs this method had been tested on before) did get pulmonary emboli and some of the dogs died of it. The more active the dog, the higher the risk.


Surgical methods

Surgical methods of heartworm removal require specialized training and instrumentation, and are generally reserved for high-risk patients who would not otherwise be expected to survive. The surgery is followed by one of the more standard treatments a few weeks later to kill any remaining worms.


Alternative methods

Alternative methods to kill heartworms, such as Paratox, are no safer than conventional drugs, since they rely on the exact same action -- they kill the larvae/worms in the bloodstream. It is the death of the worms that causes the greatest danger to dogs during treatment.

In addition, no studies have been done to show that alternative treatments are effective. If these treatments do have any effect, they would be comparable to the slow kill method, with the same drawback of continued damage to the body while the worms remain present.

Finally, some of the herbs used to treat heartworm are considered dangerous and may be toxic in the amounts used to try to kill the worms.

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