Dirofilaria immitis, the heartworm, can grow over a foot long at maturity. The heartworm larvae spreads by mosquito bites, when the insect bites an infected canine and spreads the larvae to another dog. In infected dogs, the heartworm invades the heart and pulmonary arteries. It may take five to seven months from the time the dog is bitten to the time a blood test detects the presence of heartworms, so veterinarians use another type of test.
Jane Meggitt has been a writer for more than 20 years. In addition to reporting for a major newspaper chain, she has been published in “Horse News,” “Suburban Classic,” “Hoof Beats,” “Equine Journal” and other publications. She has a Bachelor of Arts in English from New York University and an Associate of Arts from the American Academy of Dramatics Arts, New York City.
Your vet conducts antigen testing on your dog to ensure he is heartworm-free before prescribing medication. Dogs given the medication who have heartworms may experience severe reactions. According to the American Heartworm Society, millions of “baby” heartworms circulate in the bloodstream of affected dogs. Giving medication to a dog with these microfilaria, as theyre technically called, can cause a shock-like reaction in the dog as the microfilaria die off.
While its convenient to go to the pet store and make all your dogs food, treat, flea control and other purchases at one time, you cant buy heartworm medication over-the-counter in the United States. Theres a good medical reason for that — its not safe if the dog is heartworm positive.
If your dog test negative for heartworm, your vet prescribes a monthly or daily preventative, either in pill or chewable tablet form or applied topically. Depending on your location, your vet will advise you whether to give the medication year-round or only during the seasons in which mosquitoes are active. Each year, when you take your dog for his annual check-up, your vet conducts another test to make sure he is still heartworm negative before prescribing medication.
Myth: Why bother with prevention? I can just treat heartworm if my dog gets it.
Treating a heartworm infection, especially a severe one, isnât as easy as you might think.
Early on, you probably wonât notice any symptoms. But as the number of worms increases, your dog will develop inflammation in their lungs, and theyâll probably start coughing or become very fatigued. If it gets bad enough, your dog can have trouble breathing and die. Heartworm may also lead to liver damage, which can cause bloody or dark urine and a swollen abdomen and become fatal.
Assuming you find an infection in time, your dog will need several months of an injectable treatment called melarsomine. This drug has to be given slowly and carefully because if you kill off the worms too quickly it can be dangerous for your dog, Bowman says. He explains that the worms that die slowly decay in the lungs. (They donât come out in poop like intestinal parasites would.) Meanwhile, youâll need to  restrict your dogâs activity because overtaxing their lungs can be dangerous during treatment.
âHeartworm treatment requires a minimum of 2 months of strict rest and can be quite expensive,â Mandese says. Â
In severe cases, heartworm infection will call for emergency surgery: A veterinarian may have to anesthetize your dog and try to pull worms out of their blood vessels in an effort to save their life. Thatâs costly, highly invasive, and your petâs life will be in jeopardy.
What if my cat tests positive for heartworms?
Like dogs, cats can be infected with heartworms. There are differences, however, in the nature of the disease and how it is diagnosed and managed. Because a cat is not an ideal host for heartworms, some infections resolve on their own, although these infections can leave cats with respiratory system damage. Heartworms in the circulatory system also affect the cat’s immune system and cause symptoms such as coughing, wheezing and difficulty breathing. Heartworms in cats may even migrate to other parts of the body, such as the brain, eye and spinal cord. Severe complications such as blood clots in the lungs and lung inflammation can result when the adult worms die in the cat’s body.
Here’s what to expect if your cat tests positive for heartworm:
So Your Dog Has Heartworms, What Now?
Heartworm — a worm that can take up residence in a dogâs heart, blood vessels, and lungs — might seem like an unlikely threat to your pet. But itâs not uncommon, affecting about 1 in 100 dogs in the U.S. every year.
âThey look like spaghetti in the heart and blood vessels,â says veterinarian Wendy Mandese, DVM, clinical assistant professor at the University of Florida College of Veterinary Medicine. âEnough worms will cause potentially irreversible damage to the heart and vessels, and can eventually lead to heart failure and pulmonary hypertension,â which is high blood pressure in the arteries that supply blood to your lungs.Â
That means protecting your pooch from this parasite is more important than you might have realized, no matter your dogâs age, breed, or where you live in the country. Yet many myths about heartworm persist, and they might lead otherwise responsible dog owners to neglect this aspect of their petâs health.