By: Jody Smith | Reviewed by Dr. Clayton Greenway, B.Sc., DVM | Apr 12, 2017
Your pet doesn’t have to look sick to have heartworm. It is all too common for a sick dog or cat to carry on without showing any signs or symptoms until well into the progression of the disease. It’s a serious, sometimes life-threatening condition. Fortunately, when handled with an ounce of prevention, you and your pet may be able to avoid its dangers completely.
I was in a freak accident some years ago, in which I hit the big collie dog of my best friend as he raced on up out of a ditch on a country road. He went spinning in one direction, while I and my kids in my station wagon went spinning in the other. I had the horrible task of knocking on my friend’s door and telling her I had just hit her dog. The dog lived, and recovered very well, all things considered. But along with his injuries, the vet found that the collie had heartworm. My friend said that when I hit her dog I probably saved its life. I thought that was a bit of a stretch but I appreciated her attempt to relieve my guilt. And I took her point. If not for that accident, her dog’s disease might not have become visible until it was too late.
Heartworm is caused by the Dirofilaria immitis parasite. The heartworm life cycle is as follows: it can be passed along via mosquito bites when a mosquito takes blood containing microfilariae, which are immature forms of Dirofilaria immitis. When the mosquito bites another animal, it will also become infected. After the initial bite, larvae travel through connective tissue beneath the animal’s skin for about two months, into the bloodstream and on to the lung’s arteries.
Over a period of six months or so, the microfilariae grow to adulthood in the dog’s blood vessels and heart. The adult form usually settles into the right side of your pet’s heart and the blood vessels nearby. An animal with heartworm will have the adult form living in pulmonary vessels if the disease progresses unchecked. The parasite can live for five to seven years.
Symptoms of heartworm in cats and dogs don’t generally appear until the disease has advanced to the point where treatment becomes difficult. That’s why heartworm prevention is critically important.
Wondering how to tell if your dog has heartworms? They may be experiencing exercise intolerance, fatigue or weight loss. It may experience a decrease in heart function and lung capacity. It can develop a chronic cough. Blood supply to the brain can be reduced due to a lack of oxygen. In severe cases, this disease can kill. Death can occur suddenly.
Other domestic animals susceptible to heartworm are cats and ferrets. Symptoms of heartworm in cats is breathing difficulties, gagging, vomiting, lethargy and weight loss to name a few. Heartworm associated respiratory disease (HARD) is a recently defined condition which can be recognized by signs that look like feline asthma or allergic bronchitis.
Success in treatment after infection can be iffy, so heartworm prevention for cats and dogs is always the best way to go.
Ultrasounds and X-rays of heart and lungs may reveal what’s going on. Blood tests can detect antigens or microfilariae, usually about seven months into the infection. Adult heartworms in dogs may be killed with a series of adulticide injections into the muscle. Because there is no approved medication for heartworm in cats, prevention is critical.
A veterinarian may test for heartworm depending on an animal’s general health, its lifestyle, and where you live geographically. According to the American Heartworm Society, all dogs should be tested yearly for heartworm, even if they have been prescribed a year-round heartworm preventative.
Monthly medications may be effective. This may include chewable tablets or topical medications once a month unless otherwise specified by the veterinarian. Every dog should be on a heartworm prevention product at least 6 months out of the year, or longer if you’re travelling south during the winter.
There are no universally accepted guidelines for prescribing heartworm preventatives to cats. Cat owners should discuss prevention with their cat’s veterinarian. Considering the potentially dire consequences of the disease, vets may recommend the use of a preventative even for indoor-only cats within endemic areas.
Keeping your pets away from mosquitoes can be helpful in preventing a case of heartworm. That can mean staying inside in the early mornings and evenings during mosquito season. For a pet who has heartworm and is currently being treated for it, exercise should be limited during the recovery period of a month or two months.
The good news is that, with a little education and a few precautions, you may be able to protect your pets from this virulent disease.