Heartworm

About Heartworms

Heartworm in dogs has been a serious problem for many years in New South Wales and Queensland however the incidence of heartworm within Victoria has appeared at a low but increasing rate and your dog could contract heartworm without ever leaving Melbourne.

What are Heartworms?

Heartworms, (Dirofilaria immilis), are parasites of dogs which occasionally affect cats and, very rarely, humans. Infections in cats and humans are usually, sterile and harmless, but in the dog, their main host, they can cause great harm.

The adult heartworm is a long thin worm 12 - 30 cm long, which lives inside the right chamber of the heart and pulmonary artery, which leads up into the lungs. Occasionally adult worms can be found in other tissues, such as the brain, eye1 lungs, and peritoneal cavity.

The parasite feeds on the blood of the dog and the harmful effects are as follows:

The heartworm larvae (microfilaria), migrate through the tissues of the dog's body as they develop into adult heartworms and cause damage to organs such as the kidney, which can lead to irrepairable renal failure. At any stage of the heartworm disease sudden death may occur due to the blockage of arteries or capillary beds in the lungs, brain, heart, bowel or kidney, either migrating larvae or adult worms.

What are the Symptoms of Heartworm?

The disease is very slow in its progress, usually over many months, and the symptoms are similar to an old age heart, which can't pump efficiently. No symptoms are evident in the early stages of heartworm infestation and symptoms such as a chronic cough, shortness of breath, low exercise tolerance and a swollen distended abdomen only become evident when the disease is well advanced.

How Is Heartworm Spread?

The disease is transmitted from infected dogs by the bite of a mosquito, heartworms are not passed directly from dog to dog, a mosquito must first bite an infected dog and then pass this on to another dog by biting that. The mosquito sucks the blood of an infected dog into its body, taking with it the microscopic immature forms of the adult heartworm. These larvae, called microfilaria are now present in the mosquito and are subsequently injected into another dog when the mosquito feeds again, thereby infecting the next dog.

Once a dog is infected, the heartworm larvae develop through a series of immature stages over a period of 5 to 9 months into adult heartworms, which then commence laying new live larval heartworms (microfillaria) into the bloodstream and the cycle repeats itself. In this way, since an adult heartworm lives for several years, an initially infected dog is capable not only of spreading heartworm to other dogs (via mosquitoes) over a prolonged period, it is also capable of reinfecting itself many times over, resulting in a large population of heartworm within its own system. Dogs have been known to harbour over one hundred adult heartworms alone, but even one or two adult worms can cause severe disease.

Heartworm cycle

How Can Heartworm Be Prevented?

As it is very difficult to eliminate the possibility of your dog being bitten by an infected mosquito, and the effects of heartworm can go undetected for many months while causing irrepairable damage, it is obviously preferable to prevent infection rather than be faced with a seriously ill dog. Once your dog is infected, the disease is a difficult and potentially fatal disease to treat; on the other hand however, heartworm disease is 100% preventable.

Heartworm disease can be easily and safely prevented by the administration of either a daily or a monthly drug both of which are perfectly safe when used in correct doses at correct intervals. The daily medication is a drug called Diethylcarbamazine citrate, or D.E.C., and it is available in tablet or liquid form. D.E.C. acts by killing microfilaria, and therefore these larval forms are prevented from developing into the adult form of the heartworm which is principally responsible for the harmful effects of the disease.

Importantly, however, to be totally effective, the correct dosage is dependent upon your dog's weight, and the minimum effective daily dose is between 6 to 11 milligrams per kilogram of weight. Many commercial preparations of D.E.C. give only the lower dose range and it is therefore important to have your dog accurately weighed and to consult your veterinarian.

Secondly, it is important that D.E.C. be given strictly on a regular daily basis as the drug is rapidly absorbed and excreted within 24 hours. There is no residual action and cover is for a 24 hour period only, any larvae picked up in a 24 hour period not covered by D.E.C tablet will escape the initial action imperative at that crucial stage and will go on to develop into an adult heartworm. Having started on a course of D.E.C., when you realise you will run out of tablets soon, it is necessary to have a new supply ready to go on with, especially if your vet clinic requires notice when you order drugs to be picked up over the counter.

Finally the last important point regarding the administration of D.E.C. is that it must never be given to dogs already infected with heartworm. Because D.E.C. kills microfilaria these may be present in such large numbers in an infected dog (possibly millions of larvae in the bloodstream), that to kill them all would cause a toxic reaction and sudden death to the dog. To avoid this it is absolutely necessary to have your dog tested for the presence of heartworm before commencing any preventative tablets or if you have lapsed in your regular administration of this drug. However if you dog is under the age of 3 months, due to the life cycle of the heartworm, it will not require testing before commencement of the D.E.C. Your dog can easily be tested for the presence of heartworm by a simple blood test performed by your vet.

The second form of medication used to prevent heartworm is a monthly tablet containing the drug Invermectin and dispensed under the product name of Heartgard. Heartgard is available only from veterinary clinics, and comes in three weight specific size categories. It has a wide safety margin and can be used in all dogs over 6 weeks of age. Again you need a blood test before a vet will authorize usage. Your dog must be weighed and administration must be at regular 1 month intervals. Heartgard kills the larvae at a specific stage in their development which lasts several weeks and this makes the once a month dosing possible. D.E.C. however kills larvae at a stage which lasts only one or two days so it is therefore necessary to give this tablet daily to ensure no larvae survive beyond this point. Heartgard does work out to be relatively more expensive yet some people prefer its convenience, especially if their dog is difficult to give tablets to. Heartgard is also handy to use if you have lapsed in the administration of D.E.C. because it covers your dog for the month prior to giving the tablet. However if you plan to recommence with the daily D.E.C. this must be given the day after a Heartgard tablet.

Heartworm Treatment

If your dog is unfortunate and becomes infected with heartworm, there are treatments for both the heart disease and eradication of the worms themselves. The treatments are not always simple or fully effective and can be potentially toxic to some dogs. In the case of such an event it is best to discuss the options fully with your veterinarian and be guided by him as to the most appropriate form of action and the most suitable type of treatment should you decide to go ahead.

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