Heartworm Medications for Dogs

Heartworm is a disease which can easily kill your pet. Animals suffering from it can have worms the size of spaghetti living in their hearts and surrounding blood vessels, and at the very least can become very ill.

Heartworms are transmitted by mosquitoes, so animals of almost any age, sex and habitat are at risk of infection. While there are treatments for infected dogs, prevention is always better than cure and there are effective monthly or daily preventative medications which are ideal. Treating the disease is unpleasant and can be hazardous.

Prevention does not equal treatment, with preventative medications being quite different from treatment medications. Most preventative medications are tablets, chewables or topical liquids prescribed by veterinarians but applied by owners at home. Treatment medications are muscular injections given in a clinic, often in conjunction with a course of tablets at home.

The exact treatment for a dog with heartworm will vary depending on the severity of the disease. Dogs with a positive heartworm test but no other signs of disease are much simpler and safer to treat than dogs which are very sick.

"It may be necessary to perform surgery to remove some worms from the heart prior to killing the remainder with medication. This surgery is dangerous and many dogs will not survive it."

In very severe cases, with heavy infections (such as caval syndrome in which blood backs up into the liver and causing extensive damage), it may be necessary to perform surgery to remove some worms from the heart prior to killing the remainder with medication. This surgery is dangerous and many dogs will not survive it.

There is only one drug currently registered which kills adult heartworms in dogs. This is an arsenic-based product containing an ingredient called melarsomine. The drug is called Immiticide and kills adult heartworms over the age of four months.1 However it does not affect immature worms.

There are two treatment regimes used with this medication. The first is indicated for dogs without signs or with very mild signs of heartworm disease, while the second is for dogs with more severe signs. The injections need to be administered by a veterinarian and the animal observed for any ill-effects afterwards.

The standard regime involves two injections of the medication into the muscles of the lumbar area of the back, 24 hours apart1. This is designed to kill all the adult worms quickly. The alternate regime involves one injection of the medication into muscles of the lumbar area of the back, then one month later, two further injections into the same area,24 hours apart1. This will kill the adult worms more slowly. The dog will need to remain in hospital during treatment and for a short time afterwards for close observation.

When the worms die, they can dislodge and be carried as an "emboli" in the blood and block a blood vessel, particularly in the lungs. This can cause symptoms such as coughing, breathing difficulties, depression, collapse, or sometimes death. The second regime is designed to minimize the likelihood of these signs occurring. After treatment, it is important that exercise is severely restricted for a period, to avoid embolism. Blood clots can also develop and travel in the blood as a "thromboembolism", with similar consequences.

Heartworm treatment effects
The most commonly seen signs after the injections include:

  • pain or swelling at the injection site
  • reluctance to move
  • coughing and gagging
  • depression and lethargy
  • anorexia and loss of appetite
  • fever
  • vomiting
  • panting
  • excess salivation.1

The dying worms can cause inflammatory reactions too. Some veterinarians will prescribe either corticosteroids or aspirin as anti-inflammatory or anti-clotting agents along with the treatment.

Following treatment, there are often baby worm larvae, or mircofilariae, in the blood which must also be cleared. There are no products currently registered for this purpose, however most of the monthly heartworm preventatives are effective in killing the microfilariae and are used in an extra-label manner for this purpose.

There is certainly a benefit in the dog being on heartworm preventative medication which will prevent any new infections from developing. Some vets will start this medication even before the treatment to kill the adult worms, or continue it, if the dog is already on preventative medication.

To avoid the need to treat for this disease, ensure your dog is on heartworm preventative medication throughout the whole heartworm transmission season. Consult your veterinarian for further information as to when this occurs in your state and to get a prescription for appropriate medication.

1. Product label - Immiticide. http://us.merial.com
American Heartworm Society. http://www.heartwormsociety.org/heart.htm
Payne, P.A., Dryden, M.W., Carter, G.R. External Parasitic Diseases of Dogs and Cats. In: A Concise Guide to Infectious and Parasitic Diseases of Dogs and Cats International Veterinary Information Service, Ithaca NY. www.ivis.org
Tilley, L.P., Smith, F.W.K. The Five Minute Veterinary Consult Canine and Feline. Second Edition. Lippincott Williams & Wilkins, Baltimore, 2000.