Canine Heartworm Disease

by Nancy McDonald, BSN, RN-retired

Since macrocyclic lactones (the parasiticides used in preventing heartworm infections) became available, perhaps heartworm disease doesn’t get enough attention. Let’s take a look at the cycle to understand why it is vital to remember to prevent this deadly disease.

THE BEGINNING: A mosquito bites an animal infected with heartworms. Within that blood meal are microfilariae (heartworm babies) that the mosquito ingests. (Micro means the microfilariae are so small a microscope is necessary to see them and so small they survive within a mosquito.) The microfilariae live in the mosquito for two to three weeks, maturing into larvae.

NEXT: The mosquito, needing another blood meal, deposits the larvae on the skin of the dog. The larvae screw themselves into the skin, then into the tissues, and stay there for two to four months. WHILE THE LARVAE ARE IN THE TISSUE IS THE ONLY TIME MACROCYCLIC LACTONES, SUCH AS IVERMECTIN, MILBEMYCIN, SELAMECTIN OR MOXIDECTIN (THE DRUG THAT PREVENTS HEART WORMS), CAN KILL THE LARVAE. If one dose of prevention is missed, the next dose can prevent the disease. However, if two doses are missed, it opens up a greater chance for the disease.

CONTINUING: The larvae grow while in the tissue. Locating a small vein, the small worms move into the blood stream where they locate in the right side of the heart, grow into adult worms that produce offspring. This whole process takes about 9 months. Heartworms, 5 to 12 inches in length, usually start in the right side of the heart but as they multiply, will migrate into other chambers, the pulmonary arteries causing inflammation and moving into the lower part of the lungs. Usually symptoms of the disease are barely detectable signs such as a slight cough. As the disease progresses a combination of problems resulting from the dysfunction of the lungs, heart, liver and kidneys occur. Dogs that are not exercised strenuously may not have apparent signs of the disease until there are a large number of heartworms in the heart and vessels.

IN THE MEAN TIME: A blood test made by a veterinarian is the only way to determine whether or not your dog has heartworm disease. Since microfilariae generally appear in the blood within 9 months after an infection occurs, mild cases of the disease can and should be detected and treated long before any symptoms are evident. This means a dog should be tested regularly by a veterinarian for the disease. It takes as much as nine months for heartworms to develop but during that time preventatives can stop the disease from continuing.

A DOG WITH HEARTWORMS: All but the most severe cases can be successfully treated. The goal of treatment is to kill all adult worms with an adulticide and all microfilariae with a microfilaricide and accomplishing this with a minimum of harmful effects from drugs and tolerable degree of complications created by the dying heartworms. Dogs with mild symptoms with a limited number of heartworms present have the easiest road to recovery. Dogs with a greater number of worms have a greater chance that complications might arise during treatment. All dogs should be thoroughly examined prior to treatment.

Killing Adult Heartworms: There is one organic arsenical compound, Melarsomine dihydrochloride, approved by the FDA to eliminate adult heartworms. It has demonstrated a higher level of effectiveness and safety than other adult heartworm treatments previously available. The primary post-adulticide complication is the development of severe pulmonary thromboembolism. This results from the obstruction of blood flow through pulmonary arteries due to the presence of dead heartworms and lesions in the arteries and capillaries of the lungs. If heartworm adulticide treatment is effective, some degree of pulmonary thromboembolism will occur. Clinically signs may include fever, cough, hemoptysis and potentially sudden death. It is vitally important that the dog is kept quiet, allowing minimal activity post treatment so the body can eliminate the dead worms.

Eliminating Microfilariae: A few weeks following the treatment for adult heartworms, the microfilariae that are circulating in the blood must be eliminated. The most effective drug for this purpose is the macrocyclic lactone drug (ML), the active ingredient in commonly used heartworm preventives. Although their usage as microfilaricides has not been approved by the FDA, they are widely used by veterinarians as there are no approved microfilaricidal drugs currently available. Although circulating microfilariae usually can be eliminated within a few weeks by the administration of the ML-type drugs, the most widely used microfilaricidal treatment is to simply administer macrocyclic lactone preventives as usual. Microfilariae will be cleared slowly over a period of about six to nine months. Following treatment, confirmation of the efficacy of adulticide should be verified

MACROCYCLIC LACTONE (ML): While treatment of heartworm disease is usually successful, prevention is far safer and economical. Macrocyclic Lactones are highly safe and effective parasiticides used in preventing heartworm infections when used in a timely manner. For greater compliance, the American Heartworm Society recommends giving a preventative every month, year round. Options for use include daily and monthly tablets and chewables, monthly topicals and a six-month injectable product. Each of the macrocyclic lactones brands can have additional intestinal parasite or external parasite activity, which could be the determining factor that a veterinarian uses to recommend a particular product for a certain region or an individual situation.

  • Ivermectin – Ivermectin (Heartgard plus by Merial, Iverhart plus, and Iverhart MAX by Virbac and Tri-Hart plus by Schering-Plough) was the first in this family of drugs to be approved for preventing heartworm infection. An infection with larvae as long as two months prior to the initiation of ivermectin treatment will be blocked from development.
  • Milbemycin – Milbermycin oxime (Interceptor & Sentinel by Novartis) has benefits, which are similar to ivermectin.


  • Moxidectin – Moxidectin (Advantage Multi by Bayer) is available in a topical formulation, in combination with a flea control product, immidacloprid Moxidectin is also available as a six-month injectable product for dogs (ProHart 6 (moxidectin) Sustained Release Injectable for Dogs, by Ft Dodge Animal Health).

Reviewed by Larry Letche DVM, 2013