by Rena Sherwood

Created on: November 06, 2011   Last Updated: November 07, 2011

Melarsomine, better known by the brand name Immiticide, is the first drug choice to treat adult heartworms in dogs. It is currently the only drug approved by the Federal Drug Administration for killing adult heartworms in dogs. Melarsomine contains arsenic, which kills the heartworms but may also kill the dog if not administered carefully.

Unfortunately, melarsomine is very hard to get a hold of in America. Merial, the US manufacturer has had to import melarsomine from Europe since September of 2011, but these shipments cannot keep up with demand. This is yet another reason why heartworm prevention medication is so important.

Confinement Helps The Drug

In order for melarsomine to be effective, dogs need to be confined to a cage from 4 to 6 weeks in order to assure that they get rest. Adult heartworms get inside of the dog’s heart and can also crowd into the lungs. If enough worms are present, the dog dies of congestive heart failure. Confinement assures that the dog’s heart and lungs are not stressed, will not become choked off and recover. This confinement is usually done at the veterinarian. 

Common Side Effects

The most common side effects are swelling and tenderness of the skin at the injection site; a reluctance to move because of the swelling and tenderness; firm nodules appearing under the skin and may last for the rest of the dog’s life, even when the heartworms are killed.

Other side effects include the dog appearing far more tired than usual; vomiting only once; gagging or “dry heaves”; fever, lack of appetite and noisy breathing due to lung congestion. 

Severe Side Effects

A vet needs to be alerted immediately if a dog on melarsomine drools far more often than usual; vomits more than once; pants far more often than usual (which indicates pain); fever, diarrhea; coughing up blood; problems breathing; abnormal heartbeat rhythms; problems in coordination including uncontrollable trembling and coma. Eventually, the dog dies from liver and kidney failure. These can be signs of an overdose. The antidote is an injection of Dimercaprol or BAL in Oil Ampules.

Side Effects From Dying Worms

As a secondary side effect, dying worms can also cause problematic side effects in dogs. These include coughing, fever and listlessness. If too many worms die at once, the dog could also die because dead worms my clog up arteries in the lungs. This possibility is another reason why a dog needs confinement at a vet’s in order to quickly treat an effected dog with the steroid prednisone.

Warnings

Melarsomine should never be used in dogs with a Class 4 heart disease known as caval syndrome. Dogs most prone to developing caval syndrome are males of sporting breeds that live primarily outdoors. The only way that these dogs can be treated is to have their heartworms surgically removed. Use of melarsomine could kill the dog before the worms are killed.

People, cats, pregnant or nursing dogs should never be given melarsomine.

This drug cannot be used as a heartworm preventative. It cannot kill juvenile heartworms, called microfilaria. It only kills heartworms aged four months or more.

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