By: Margot B. Schwag, VMD
** Permission of author is required for use of this article.**

 Heartworm disease has become a very serious problem in dogs in almost every area of the United States.  Because the infection is transmitted from dog to dog by mosquitoes, the disease was initially prevalent in the warm southern coastal areas.  However, with the freer movement of dogs today, due largely to a more mobile human population, the disease has spread widely and has become more rampant throughout the country.



Heartworm disease is a condition in dogs caused by Dirofilaria immitis, a nematode (worm) that lives as an adult in the right side of the dog’s heart and those blood vessels going to the lungs.  Damage to the dog’s body begins with the onset of the adult worms in the heart.  Although we can free the dog from infection, preventing further harm, we cannot reverse the damage already done.  Therefore prevention is essential.  If your dog tests positive, early detection and treatment are essential.  This infection can be lethal to your pet!



Severe infections of adult worms result in coughing, labored breathing, tiring, and general weakness.  In advanced stages, the heart becomes irreversibly damaged, as do the liver, kidney and lungs.



Heartworms are transmitted from one dog to another by mosquitoes.  The life cycle of heartworms can be broken down into three stages.  The adult female lays live immature worms called microfilaria in the dog’s bloodstream.  The microfilaria do not grow, but circulate and live in the dog’s bloodstream for as long as three years.  When a mosquito bites an infected dog, it takes in blood containing the microfilaria.  (The mosquito is essential to the life cycle of the worm.)  Within two weeks the microfilaria has progressed into an infected larva.  The infected mosquito then bites a dog and passes the infective larva into this dog’s body.  The larva migrates under the skin where it lives for the next three months and grows to about three inches in length.  It then migrates to the dog’s heart, where it settles into the right side and adjoining blood vessels.  There is completes its growth within the next three months.  When fully grown, it begins to develop microfilaria and the cycle is complete to repeat again.

Since adult female worms live about five years and microfilaria persists for three years, a single infected dog can be a constant source of infection to all dogs within a two mile radius for at least eight years.



To detect heartworm infection, a blood sample should be drawn yearly.  If a dog is found to be free of microfilaria (negative), it should be placed on a preventative medication.  In the past, dogs were given a daily dose of diethycarbamazine (DEC).  Heartgard (ivermectin) and Interceptor (milbemycin), to name two, are now available in tablet and chewable form.  Their advantage is that they are only required once monthly.  There are also topical, once monthly, preventatives as well.  Most have the added bonus of controlling several intestinal parasites as well.


The preventative is given throughout the entire year.  It also protects the dogs over the winter months as mosquitoes can emerge at temperatures over 40 degrees F.  For those owners who stop heartworm preventative in the winter, their dogs must be tested in the spring before medication is started.


Again, all adult dogs must be tested before the preventative is started.  Should an infected dog be given the medication, the dog could die from anaphylactic shock (a severe immune reaction).  Puppies can be started on the preventative without testing if they are older than eight to ten weeks and under six months of age.


If a dog is found to have heartworm disease, it can be treated.  Although the treatment is not without possible complications, most dogs suffer no ill side effects.  Even though heartworm is treatable, our aim is to prevent infection because once the dog is infected, the damage to the heart and blood vessels may be irreversible.


© 2008 Margot B. Schwag, VMD. All rights reserved. For permission to reprint contact author at Landisville Animal Hospital, 3035 Harrisburg Pike, Landisville, PA 17538.