Heartworms are parasites, carried by mosquitoes, that can live in an animal’s heart, lungs and blood vessels. They can grow to be a foot long, wreaking havoc on internal organs and causing lung disease and heart failure. Needless to say, your pet’s monthly dose of heartworm medication is as non-negotiable as his daily bowl of kibble. His health, even his life, depends on it. 

Symptoms and treatment 

Typical symptoms of heartworm infection are coughing, unusual tiredness after exercise, shortness of breath and weight loss. But some dogs can be infected with as many as 250 worms and not have symptoms. This is one reason yearly heartworm testing is so important.

The main treatment for heartworm infection are injections of a drug called Immiticide (melarsomine), which is the only medication approved by the Food and Drug Administration (FDA) for clearing up heartworm in a dog. Steroids, doxycycline (an antibiotic) and NSAIDs also may be prescribed.

There’s no FDA-approved drug for treating infected felines, though. "For this reason it’s extremely important to prevent heartworm in cats," says Moorhead. A vet will monitor an infected cat's condition with several tests and may prescribe medications to treat symptoms.

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Heartworm prevention tips

As with most diseases in pets and humans, an ounce of prevention is worth a pound of cure. Most pet owners know their dog needs medication to prevent heartworm. , it’s important to make sure you’re doing it correctly, says Andrew R. Moorhead, DVM, PhD, director of the Filariasis Research Reagent Resource Center at the University of Georgia College of Veterinary Medicine.

If you’re not, it’s vital that you make an appointment with your vet so you can start. In either case, here are some important things you should know to protect your pet from heartworm.

1. Cats need heartworm medication Although canines are the most common host for the parasite, cats and even ferrets can become infected. It can take just three immature heartworms to permanently damage a kitty’s vital organs.

2. No place is safe. Heartworm infections have been reported in every state. Even if you live in the desert and rarely see mosquitoes, these pesky bloodsuckers thrive anywhere there’s standing water, such as on a golf course or in a man-made pond.

3. Pets must be tested before being given heartworm medication. Never purchase heartworm medication online and give it to a pet without having him tested by your vet first. A dog or cat that tests positive for heartworm can develop serious, life-threatening complications if given a heartworm preventative. However, depending on blood test results, your vet may prescribe pretreatment with a preventive drug, says Moorhead.

Related: When to Take a Sick Pet to the Vet

4. Regular testing is vital. Because heartworms can live for five to seven years in a dog and two to three years in a cat, a clean bill of health one year doesn't mean an all clear for a lifetime. Every year there's another mosquito season.

Also, testing is the only way to know for sure that preventive meds are working. Any pet that’s 7 months old or older should be tested. The American Heartworm Society recommends retesting six months and then one year after the first test. From that point on, testing should be done once a year.

5. If you miss a dose of heartworm medication or you don’t give it on time, your pet must be tested. As soon as you realize your oversight, call your vet. He or she will likely have you bring your pet in to be tested before giving you the go-ahead to resume giving the medication. It’s a good idea to put a reminder on your calendar or smartphone so this doesn’t happen. Testing is also necessary if you switch to a new heartworm preventative.

Related: How to Keep an Outdoor Cat Safe

Dianne Lange is a Lake Tahoe-based freelance writer specializing in health and travel. She is the author of four books on cancer and a former editor at SELF, Health, Natural Health and Prevention. Her work has appeared on websites such as RealAge.com, SymptomFind.com, WebMD and Everyday Health.