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February 2000

This article is the second of a two-part series. Last month we talked about what heartworms are, what heartworm disease is, and what damage heartworm disease can cause. This month weíll discuss how problems resulting from heartworm disease are treated, and how heartworm disease can be prevented.

 

Treatment of heartworm disease and its symptoms

If a cat is diagnosed with heartworm disease, the choices are either to kill the worms or to manage the disease conservatively. This situation is different from that for a dog with heartworm disease. In dogs, heartworm disease is normally treated by killing the adult worms with an adulticide like melarsamide. However, because the death of the worms can cause severe, life-threatening problems or sudden death in cats much more often than in dogs, the adulticide treatment is rarely used in cats. If the adulticide is given to cats, about 30% of them will experience life-threatening symptoms within 3 weeks of the treatment as the worms die.

Cats that are given the adulticide must be watched 24 hours a day at a veterinary hospital. The complications from the treatment and subsequent worm death are respiratory failure, embolization causing severe lung inflammation, coughing blood, and dyspnea, and other serious problems, including sudden death. Oxygen therapy, corticosteroids, and IV fluids can help the cat live through the crisis. But death can occur before therapy can be started. The only advantage to killing the worms with treatment is that the time of greatest danger is known, and the cat can be watched continuously during the weeks after the administration of the adulticide. If the heartworms die naturally, no one knows when the critical time (if any) will occur. Despite this advantage, adulticide treatment has not been consistently shown to increase survival time compared to conservative management of the disease.

Because of the possibly dire consequences of heartworm disease treatment, conservative management and not treatment is recommended for cats. That is, the symptoms of the disease are treated but not the underlying cause (the worms). The most common symptoms of heartworm disease are vomiting followed by coughing and sudden death. And most cats have no symptoms at all.

Conservative management involves the use of prednisolone (a corticosteroid). Prednisolone is an anti-inflammatory agent that works throughout the body. It reduces the bodyís allergic response and inflammation in the lungs, which are the typical reactions of the catís body to the worms. The prednisolone can ease the catís symptoms and make handling the disease easier. It can also reduce the risk of a severe reaction when the worms die naturally.

Unfortunately, however, this treatment does not prevent a severe crisis from happening. The onset of acute respiratory distress is an emergency, and the cat must be taken to the vet immediately. At the hospital, the cat may be given oxygen therapy, cage rest, IV fluids, and prednisolone as treatment for the crisis. The worst problems usually occur when a worm dies (as we described last month). This fact explains why adulticide treatment is so risky, because multiple worms will die within a short time period. (Although, even the death of just one worm can cause the cat to have a severe reaction or even die.)

On the other hand, some cats will have no symptoms when the worms die. There is no way to predict which cat might have life-threatening problems and which cat might not. And there is no relationship between the clinical signs (vomiting, coughing, and their severity) and the risk of acute complications.

 

Prevention of heartworm disease

Because the consequences of heartworm disease can be so terrible, and because treatment of the disease (killing the worms) is not recommended, prevention makes sense, especially in areas where dogs have a high risk of heartworm disease. (This region includes the Southeastern United States, along the tributaries of the Mississippi River, and in many other places in the world.) Outdoor cats seem to be most at risk, as the disease is carried by mosquito; however, about 1/3 of cats with heartworm disease are considered indoor cats. Consequently, all cats are candidates for heartworm disease prevention.

As described last month, cats are infected by being bitten by a mosquito that carries the heartworm larvae. The larvae develop inside the catís body into adult worms. Prevention involves killing the larvae with macrolyde antibiotics Ė ivermectin or milbemycin. These antibiotics are given once a month in pill form. The usual medication is Heartgard ® for Cats, which contains ivermectin and is disguised as a treat so your cat will be more likely to eat it. The prevention program can be started for a cat as young as 4 Ė 6 weeks of age, and it should continue for the life of the cat.

Before the prevention program is begun, cats are normally not tested for heartworm disease or the presence of the larvae in the bloodstream (microfilaremia). This situation is different than that for dogs; dogs are tested before treatment. Cats are not tested because it is more difficult to diagnose heartworm disease in cats, and there are no complications if the cat is placed on the prevention program even if it already has heartworm disease.

 

How the prevention program works

The preventative medication is not a vaccine. That is, it will not prevent future larval infections. If a mosquito carrying the heartworm larvae bites a cat, the cat has a chance of getting infected with the larvae. But, the antibiotic does kill the larvae after the infection and prevents them from developing into adult worms. The heartworm prevention medication kills any larvae that were introduced in the past 30 days. The prevention medication does not kill adult worms; consequently the medication must be taken while the heartworms are still in their larval stage. This means the medicine must be given consistently once a month.

In addition, the prevention medication has only been studied and approved by the FDA for regular, monthly usage. It is possible that the protection would extend a little longer than that. If you forget to give your cat its medicine one month, contact your vet. Another reason for the monthly dosage is to make it easier for the owner to remember to give the medication. If you always give your cat its heartworm preventative on the 1st of the month, it will be much easier to remember. And, because heartworm disease can be so serious in cats, it is important that you donít forget it.

 

Sources:

"Clinical Significance of Feline Heartworm Disease," by Ray Dillon, DVM, MS, Volume 8, No. 6, November 1998, pp. 1547-1563.

® Heartgard is a registered trademark of Merial Limited, Iselin, New Jersey.

Special thanks also to Dr. Oltman of Countryside Veterinary Clinic in Ellicott City, MD (410-461-0517) for her helpful information. Any errors in this article are the responsibility of the author.

Read last month's article.