What Is Heartworm Disease?
Heartworm is a parasitic disease found in the blood of dogs and cats. It is transmitted by mosquitoes, known as a ‘vector’ – the mosquito bites an infected animal, carries the infected blood and injects it into the next animal it bites. The infected blood carries many tiny baby heartworms, called ‘microfilaria’. These tiny worms eventually make their way to the great vessels surrounding the heart, where they live and grow to adults – which can be up to 12 inches in length! Adult heartworms can cause a variety of health problems, most of them life-threatening: heartworm disease left untreated is 100% fatal. Clinical signs of heartworm disease include cough, lethargy, exercise intolerance and general malaise. However, it can take a long time – months to years – for the worms to grow large enough to cause these signs, so you may not see ANY signs at all until the disease is quite advanced.
How is Heartworm Disease Diagnosed?
There are very sensitive, quick tests available to detect microfilaria in blood, as well as the body’s immune response to the presence of the baby worms – these are the tests used to screen dogs and cats for the presence of heartworms. If an animal has clinical signs, additional diagnostic tests such as x-rays and ultrasound may be performed, along with blood testing.
How is Heartworm Disease Treated?
If detected, heartworm disease IS treatable – however, treatment can be painful, costly, dangerous, and is not guaranteed to be successful. If the disease is detected early enough, the risks associated with treatment are considerably lower. Treatment involves a series of injections to kill the adult worms, and then a treatment to kill the immature worms in the bloodstream.
Prevention is KEY!!!
Your veterinarian will begin by examining a sample of your dog (or cats) blood for the presence of adult parasites. The appropriate testing frequency will be decided by your veterinarian. If your dog is not infected, a preventive program should be started. The preventive program involves giving your pet a pill (trifexis) or applying a solution to the skin (revolution) once a month during mosquito season, which (in Ontario) typically runs from June to mid-October. This medication destroys the immature heartworms transmitted by the mosquitoes and stops the cycle of the parasites.
Here is a comparison chart of the two heartworm preventative medications we recommend
Q. My pet was on heartworm medication throughout the last mosquito season. Why do I need to have my pet tested for heartworm this year?
Of the 564 dogs that tested positive for heartworm in Ontario in 2010, more than 80 per cent were not on any kind of preventive treatment. In cases where preventive treatment appeared to have failed, owners reported that they had forgotten doses or that their pet had refused to take a dose. By testing your dog regularly, you increase the chance of detecting the disease early, before serious damage is done to your pet’s heart or lungs and while prognosis for treatment is still good. Also, depending on the type of heartworm preventive medication used, if given to a dog already infected with heartworm, a mild to severe allergic reaction may occur. For this reason, most heartworm preventive medications are prescription products. Annual testing is more important than ever.
Q. How prevalent is heartworm in Ontario? Do I really need to worry about it in my pet?
The actual number of heartworm positive pets in Ontario varies from year to year. While there is currently no system in place to track every case of heartworm in the province, a survey conducted in 2010 found that the number of dogs with heartworm in Ontario increased by 60% between 2002 and 2010. Of the dogs that tested positive for heartworm in Canada, nine per cent of them were confirmed as having been imported from the Southern United States (Katrina dogs) and 12 per cent had been imported from other parts of the United States or other countries. Fifty-one per cent had never left their local area.
The take home message is that Ontario pets are vulnerable from a variety of sources, and prevention is the best approach.
Q. Why does my pet need preventative medication? Why not just treat the disease if my pet develops heartworm?
The signs of heartworm disease are usually only detectable after the disease has progressed and damage has already been done to the internal organs, which may be irreversible. Although treatment for heartworm disease is available, it is costly and is not without dangers. Waiting for symptoms to develop is not a realistic alternative to prevention, nor is it particularly ethical.
Q. If my dog is always indoors and never gets bitten by a mosquito, does it need a heartworm prevention program?
Mosquitoes are a fact of life during the summer months in Ontario, and we have yet to see an effective method of avoiding all contact with the insect. Remember – it only takes one mosquito bite to infect a dog with heartworm.
Other resources for pet owners:
American Heartworm Society: www.heartwormsociety.org
Veterinary Partner: www.veterinarypartner.com/Content.plx?P=A&C=&A=1196&SourceID