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Treatment of Heartworm Disease in Dogs:

Until recently, the only effective adulticide (drug to kill the adults) for dogs was the drug, thiacetarsamide sodium (Caparsolate®), which contains arsenic. It has to be given in the vein through a catheter. If any drug gets outside of the vein, severe tissue damage could occur. Some animals become quite ill from this drug, and therapy sometimes has to be stopped. Almost all animals have to be hospitalized for the several days of treatment. (Note from Dr. McKee: The drug is no longer available and all of our stocks went out of date some time ago. The use of this drug is no longer an option here.)

A few years ago the drug, melarsomine (Immiticide®), became avaiable in the US. It too contains arsenic but can be given deep in the muscles of the back instead of intravenously. It is less likely to cause side effects than thiacetarsamide and is more effective. The treatment protocol depends on the severity of infection. Dogs with mild cases are treated once daily for two days. For more severe infections, the dog is given one injection, then thirty days later is treated once daily for two days. Four months after the treatment, the dog should be tested for microfilariae using the antigen test. Some animals may need to undergo a second round of injections at that time. It is recommended that dogs also be on a monthly heartworm preventative during the treatment.

Regardless of which drug is used, when the adult heartworms die, they can obstruct blood vessels to the lungs (these are called pulmonary embolisms). If only a small part of the lung is involved, there may be no clinical signs. However, if the vessels to a large portion of the lung, or a small area of an already diseased lung are blocked, severe signs may result. These include fever, cough, coughing up blood and even heart failure. Because of the risk of these embolisms, any dog being treated with an adulticide must be kept very quiet during treatment and for 2-4 weeks thereafter.

In very severe infestations, adult worms are removed from the heart surgically.



Melarsomine is an injectable drug for in-hospital use to kill adult heartworms in dogs. Treatment (not prevention) of heartworm infection can result in fatal complications, but death from the untreated heartworm infection is far more likely. Dogs must have 4-6 weeks of cage rest following treatment.

Generic Name

Brand Name

Type of Drug

Form and Storage
Available as a powder which is reconstituted (mixed with sterile water) before use.
Store powder upright at room temperature. Once reconstituted, store in refrigerator protected from light and use within 24 hours.

Indications for Use
An arsenic-containing compound used to kill immature (4+ month old) and adult heartworms (Dirofilaria immitus) in dogs.

General Information
FDA approved for use in dogs. Used in a hospital setting. Heartworm disease can kill, but it is preventable using the daily or monthly heartworm prevention medications. Routine heartworm testing is recommended even for those dogs on heartworm prevention year round because the earlier heartworm disease is detected the better the chance of a full recovery. Heartworm disease is graded Class 1-4 with 4 being the most serious. A heartworm preventative can be given at the same time as treatment with Immiticide.

Melarsomine is preferred over another drug, thiacetarsemide, that kills adult heartworms because it is less likely to cause a reaction where injected, it is more effective, less likely to cause liver damage, and can be used more safely in dogs with severe heartworm disease.

Usual Dose and Administration
Given intramuscularly (IM) deep in the lumbar (back) muscle. Do not administer at any other site. Alternate sides. Usually given twice 24 hours apart for animals with Class 1 or 2 heartworm disease. May need second round of treatment 4 months later depending on repeated test results. Dogs with other classes of heartworm disease may receive different doses/timing to decrease risk of complications. Given in a hospital setting to allow for the necessary supervision of the patient.

Side Effects
May see pain, swelling, and tenderness at the injection site or reluctance to move due to pain at injection site. Firm nodules can persist indefinitely. May also see coughing, gagging, depression, lethargy, lack of appetite, fever, lung congestion, and vomiting. Less commonly seen are excessive drooling, panting, diarrhea, coughing up blood, abnormal heart rhythms, and death.


Drug or Food Interactions
None known, although with another drug (Caparsolate) used to kill adult heartworms, glucocorticoids have a protective effect on adult heartworms, i.e., Caparsolate is less effective if the dog is also receiving glucocorticoids.

Low margin of safety. Need to have an accurate weight before treating. May see damage to the lungs or kidneys. May see drooling, panting, restlessness, fever, vomiting, diarrhea, tremors, lethargy, staggering, and difficulty breathing which may progress to collapse, coma, and death. A possible antidote for an overdose is BAL in Oil Ampules (Dimercaprol Injection, USP). It is listed in reports as an antidote for arsenic toxicity. Contact your veterinarian or the Animal Poison Control Center if you think your pet may have accidentally received or been given an overdose of the medication.