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What you need to know about heartworm

May 1, 2018

It is estimated that heartworm affects around one million pets at any time in the US. Learn more about prevention, signs, and treatment of this disease in pets.

What you need to know about heartworm

We all care about the health and wellbeing of our pets. That’s why it’s so important to remain vigilant about heartworm disease. April was designatedNational Heartworm Awareness Month by the American Heartworm Society (AHS) to raise awareness about the disease, as well as to promote early detection, accurate diagnosis, and effective treatment. Here we’ll look at the causes of heartworm disease and provide some guidance on diagnosis, treatment, and prevention.

What Is Heartworm?

Heartworms are a parasite (Dirofilaria immitis) that is estimated to affect approximately one million pets at any time in the US. The worms themselves live and grow within the heart and large blood vessels of an infected animal. Adult heartworms range in size from 4 to 12 inches and can impede the proper functioning of the heart, valves, and blood vessels. If left untreated, heartworm can be fatal.

How Is Heartworm Transmitted?

Heartworm is a blood-borne parasite transmitted by mosquitoes. When a mosquito bites an infected animal, it draws tiny immature heartworms into the insect’s body. When the mosquito then bites an uninfected animal, these larval worms enter through the bite wound and make their way through the circulatory system to the heart. Heartworm cannot be transmitted directly from dog to dog and is most commonly found in areas where mosquitoes are abundant, such as the Southern US, the central and lower Mississippi Valley, and the Southern and Mid-Atlantic coastal regions.

How Do I Know if My Pet Has Heartworm Disease?

In its early stages, heartworm often has no symptoms, but antigens produced by the female parasite can be detected by your veterinarian using a simple blood test. The AHS recommends annual testing of pets to prevent spread and maturation of the parasite. In more severe cases, an infected animal may display a soft dry cough, lethargy, loss of appetite, weight loss, trouble breathing, or a bulging chest. A positive blood test result for heartworm can be confirmed by microscopic examination of a blood sample, by EKG, or by x-ray. It’s important to note that while cats are more resistant to heartworm than are dogs, they can and do get the disease, so remember to have them checked as well.

Treatment for Heartworm

Treatment for heartworm disease varies depending on the animal’s overall health and age, and on the severity of the disease. Animals with advanced disease may not be treatable, except symptomatically. Those with moderate disease may need to be stabilized before they are strong enough for your vet to treat the heartworms themselves. Animals with less severe disease can be treated by your veterinarian, using an injectable drug called melarsomine. Within days of treatment, the worms will begin to die and decompose, usually in the lungs, eventually being reabsorbed into the body.

During the post-treatment period (which may last from weeks to months), it is essential that your pet rest. Exercise should be severely limited as your pet’s body heals. At a follow-up visit, your vet will then administer a second medication to kill any immature heartworms that may remain in your pet’s system. 

Heartworm Prevention

There are preventive treatments for heartworm, but none are available over-the-counter, so you must see your vet to have these treatments administered. It is recommended that heartworm preventative be given year-round for maximum protection, despite seasonal climate changes and mosquito prevalence.

Vigilance and regular screening can also help. Being aware of heartworm’s symptoms and having your pet screened using an annual blood test can help stop heartworm before it becomes severe.


Cecily Kellogg is a pet lover who definitely has crazy cat lady leanings. Her pets are all shelter rescues, including the dog, who is scared of the cats. She spent eight years working as a Veterinary Technician before becoming a writer. Today she writes all over the web, including here at Figo.

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