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DVA 326 Bridge Street * Dedham, MA 02026 * 781.326.2800
Canine Heartworm disease
What is Heartworm (HW) Disease? The Heartworm, Dirofilaria immitis, is a round worm that lives in the blood of the heart and lungs of most canine and some feline species. This worm is transmitted by an infected mosquito when it takes a blood meal from your cat or dog. The mosquito injects a baby heartworm (larva) into your dog or cat’s body, then this baby worm develops into a teenager and finally an adult worm where it resides in the right ventricle and pulmonary arteries. It takes six months from the bite of the mosquito to the development of the adult heartworm. An adult heartworm can live up to seven years in a dog and two to three years in a cat.
Once the adult male and female heartworms reach the right ventricle of the heart, they mate and begin to produce more baby (larval) heartworms. These are called microfilariae. Anywhere from one to twenty adult heartworms can live in between the heart and lungs. These worms cause severe inflammation of the lungs and blood vessel walls leading to emboli (blood clots), respiratory difficulty, right heart failure, and if untreated, ultimately death.
What are the signs of heartworm disease? Some pets may not show any signs. This is especially true for early infections, infections with very few adult worms, or infections in the cat. The earliest clinical signs are exercise intolerance, cough and weight loss (rarely seen in cats, although some of this signs might resemble asthma). As the disease progresses, the cough becomes more severe and pets develop labored breathing, and right heart failure sings usually manifested as abdominal distension due to fluid building up in the abdomen. Once the disease has reached end stage, the prognosis is extremely poor. It is impossible to reverse the heart damage that is done by a heartworm infection. How is heartworm disease diagnosed? The most common method is a quick blood test done at the hospital that would show a positive result if a pet was infected with an adult female worm. Occasionally, a veterinarian may suspect heartworm disease if chest x-rays show a particular pattern. This can usually be confirmed by performing an ultrasound of the heart (the actual worms can be seen). We recommend the heartworm test on an annual basis.
How is heartworm disease treated? In order to ensure that your dog is a good candidate for heartworm treatment, we will need to run some basic blood work, a urinalysis, and take some x-rays of your dog’s chest before he or she is treated.
We currently use the safest published protocol for the treatment of heartworm disease in the dog. The recommended treatment is based on the slow killing of adult worms using an arsenic based drug called Melarsomine. It is administered via injection deep into the muscles of the lower back. Some of the more common side effects are local tissue irritation at the injection site, and damage to liver and kidney, therefore it is important to check your pet’s blood work prior to initiating treatment.
Because some dogs can form a severe and sometimes life threatening reactions to the dead adult worms, we recommend that pets be closely monitored for at least a couple of weeks after the treatment. Strict rest is mandatory for the duration of the treatment, as exuberant activity could release a mass of dead adult worms to the lungs or brain causing a dangerous clot to form.
Other therapies that might be indicated are steroids for reducing inflammation in the lungs and aspirin, which might reduce vascular damage and clot formation after the treatment.
It is currently recommended that all dogs continue their preventive therapy with Heartgard® despite a positive heartworm test. This will reduce the risks of this pet transmitting the disease to other animals via the mosquito as it works by killing any baby heartworms that get into your pet’s body by the bite of a mosquito.
There are no current, safe drug treatments for the treatment of adult heartworms in cats. Heartworms are not treated in the cat, but rather the disease is managed with heartworm prevention and anti-inflammatory medication.
All dogs and cats should be on regular, monthly heartworm prevention: A mosquito can get into your home any time you open a door or a window. Even if your animals never go outdoors, they are still at risk for a mosquito bite and potentially a heartworm infection. Prevention is the most important part of controlling the disease. Pets do not become immune to the disease after they have been infected, so prevention is always recommended. Prevention should be started between 6-8 weeks of age and should continue once per month for the rest of your dog or cat’s natural life. Products that prevent heartworm infection such as Heartgard®, Revolution® and Interceptor® can be used safely in untested puppies less than 6 months of age. After 6 months of age, testing is required. If your dog has been infected and you give preventive medication, an adverse reaction could occur. The American Heartworm Society recommends the use of heartworm preventive treatments given on a year round basis in the northeast portion of the USA.
At our hospital we carry Heartgard Plus®, which will not only help prevent heartworm disease, but will also help prevent intestinal infections from other worms. Heartgard Plus® is also safe to use during treatment of heartworm disease.
Heartworm disease is a very serious disease that is easier and safer to prevent than to treat. If your pet is not currently on any heartworm preventative medication, please talk to your doctor about a testing and prevention plan.
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