| Introduction:
Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are roundworms, in dogs and cats known as Dirofilaria immitis.
If the worm infection is a heavy one (over 25 worms for a 40 lb dog), the worms begin to back up into the right ventricle (the chamber which pumps blood through the lung). The worms actually take up a significant amount of space within the heart, leading to less blood being pumped.
When over 50 worms are present, the ventricle is full and the atrium, the chamber receiving blood from the rest of the body begins to contain worms.
When over 100 worms are present, the entire right side of the heart is filled with worms and there is very little room for any blood to be pumped. This drastic phenomenon is called "Caval Syndrome" and most dogs do not survive it.
Ontario continues to be the principal center for heartworm infection in Canadian dogs. They were over 380 cases just last year in Ontario. Many more didn't get reported because the tests were done in-house.
Dr. Owen Slocombe, veterinary parasitologist from the University of Guelph, has been studying the incidence of heartworm disease in Canadian dogs for more than two decades. Despite the increasing numbers of domestic dogs being tested and put on preventive medication, Dr. Slocombe has seen the number of infected dogs rise. In his 1997 poll of Canadian veterinarians, there were 647 diagnosed cases of canine heartworm disease that year.
In considering these results, it should be noted that only about a quarter of Ontario's canine population is ever tested for heartworm infection. So the vast majority of dogs out there are of unknown infective status, and represent a reservoir of infection. Dr. Slocombe's survey then surely underestimates the real number of infected dogs.
Heartworm disease is able to spread widely because infection can be introduced into an area by a visiting dog.
Heartworm disease has been diagnosed in foxes, wolves, coyotes and raccoons. These animals are thus a permanent source of infection for our domestic dogs, since no preventative medication or treatment for the disease is given to the local wildlife. Although rare, the disease has even been reported in the domestic cat.
A simple blood test is all that is needed to detect the presence of heartworm in an infected dog. Once a negative result is obtained, preventative medication, which interrupts the lifecycle of the heartworm, can be administered. These medications come in a variety of convenient formats (such as chewable treats or topical liquids) and are given once a month during the high-risk mosquito season. In Southern Ontario, the initial dose must be given by June 1st and should be continued for 6 months.
It is absolutely critical that we ensure that your dog is not infected with heartworm before beginning preventative medication. Giving a heartworm preventative to a dog that is positive for heartworm can cause serious and severe adverse effects. Even dogs who take medication every year must be re-tested before beginning another season of preventatives. While the preventative drugs are highly effective, and when regularly administered will protect more than 99% of dogs from infection, failure of protection can occur. Most failures of protection result from irregular and infrequent administration of the drug, discontinuing medication too early in the fall, vomiting medication after ingestion and the dog exceeding its recommended dosage due to growth or weight gain. Most monthly preventives all have a reasonable margin for error in their administration so that if a single month's dose is accidentally missed, adequate protection is usually provided so long as the next two monthly doses are administered on schedule, although no guarantee is possible.
Heartworm is a devastating disease. The outcome of heartworm disease depends on the severity of the infestation and how long a pet has been infected. Most patients are hospitalized for the treatment. An adulticide medication called melarsomine dihydrochloride is administered to kill adult worms. This medication is not available in Canada, so it has to be specially ordered from the States. After adulticide treatment, a pet is to be confined for 5-6 days. Severe restriction of activity then follows for 4 to 6 weeks.
After killing adult worms, a medication is prescribed to kill microfilariae or baby worms. Usually Ivermectin is used for this purpose.
If heartworm disease is untreated, it will eventually kill an animal. A pet can be re-infected if not placed on appropriate preventative medication.
There are a number of medications available to safely and effectively prevent your pet from becoming infected with heartworm disease. The following table is a heartworm preventive comparison for dogs in alphabetical order. |
