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Heartworm Disease in Dogs

An ounce of prevention beats a pound of cure

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 classified as nematodes (roundworms) and are but one of many species of roundworms. The specific roundworm causing heartworm in dogs and cats is known as Dirofilaria immitis.

Heartworm life cycle

Adult female heartworms, which could be as long as 14 inches, live in the infected animal and produce baby worms, called microfilariae which get infected their host’s bloodstream. The microfilariae are ingested by a mosquito during feeding. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. When the mosquito bites another host (dog, cat or other susceptible animal), the infective larvae enters a new host through the bite wound. It then takes a little over six months for the infective larvae to mature into adult worms that may live for five to seven years (in a dog). The microfilaria cannot mature into adult heartworms without first passing through a mosquito. The adult heartworm is fairly large, several inches in length, and it prefers to live, not in the heart, but in the pulmonary arteries. It swims into a cozy tubular artery, where it is massaged and nourished by the blood coursing past it. In the pulmonary arteries of an infected dog, the worm’s presence generates a strong inflammatory response and a tendency for blood to inappropriately clot. If enough worms are present, the heart must work extra hard to pump blood through the plugged up arteries.

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.

Epidemiology

Ontario continues to be the principal center for heartworm infection in Canadian dogs.

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.

Testing

Testing is an important step before placing an animal on heartworm protection. Some medications are not approved for use in a heartworm positive dogs and may actually be dangerous if high numbers of worms are present. Heartworms can be detected by a simple blood test. The tests are usually highly specific and highly sensitive. The results are usually known in less than 12 hours. If a result is positive, then the test is repeated. If a second test is positive, a blood smear is done to detect micorfilariae.

Treatment

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.

Prevention

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.

Medication

Mode of application

Frequency of administration

Additional coverage

Safe age for administration

Comments

Intercepter Flavored Tablet Once a month for 6 months, starting in June Hookworm, roundworm, whipworms 8 weeks Safe for collie breeds
Heartguard Chewable Once a month for 6 months, starting in June None 6 weeks Not safe for collie breeds
Heartguard Plus Chewable Once a month for 6 months, starting in June Roundworms, hookworms 6 weeks Not safe for collie breeds
Proheart 6 Injectable Once, end of May – early June Hookworm 6 months Safe for collie breeds, Local inflammation can happen at the site of the injection
Revolution Topical Once a month for 6 months, starting in June Sarcoptic mange mites, ear mites, fleas, hookworms, ticks 6 weeks Not affected by swimming, safe for collie breeds
Sentinel Flavored Tablet Once a month for 6 months, starting in June Fleas, hookworm, roundworm, whipworms 8 weeks Safe for collie breeds