Heartworm (scientifically known as Dirofilaria immitis) is, unfortunately, exactly what it sounds like. The heartworm is a parasite that can grow up to a foot long. It lives primarily in the blood vessels in the heart and lungs and can damage both of them, often fatally.
Heartworm disease is transmitted by mosquitos and can be indirectly transmitted between certain animal species. When a mosquito bites an infected animal, they carry the heartworm larvae from that animal to the next animal they bite.
Once transmitted, the larvae develop into adult worms that will produce more larvae increasing the number of worms in the carrier.
One of the dangers of heartworm is that the worms damage the tissues and vessels they live in, which means even once the worms are killed or removed, the damage can remain.
Given the potential for permanent damage, prevention is key, but we’ll get to that in a bit.
In the meantime, check out this video for a full explanation of the heartworm infection process:
Does Heartworm Affect Cats?
Yes, it does. Truthfully, almost any mammal can get heartworm. While dogs are what is known as the "definitive carrier," meaning it's a disease that primarily affects dogs, heartworms can spread to other animals as well.
Affected animals include cats, wild canids like wolves and coyotes, jackals, foxes, ferrets, and even creatures like bears, seals, and sea lions. In extremely rare cases, even humans can get it! However, not from pets. More on this below.
Luckily, heartworm is relatively rare in cats, and it doesn't affect cats in the same way that it affects dogs. For some reason, heartworms don't generally thrive in cats and almost never reach the adult stage.
“Heartworm disease in cats is a bit different than in dogs. Heartworms in cats do not live as long (average lifespan is only 2 to 4 years) or grow as long, and fewer of them mature into adults.” – US Food and Drug Administration (FDA)
While larvae heartworms can still cause damage, cats rarely have adult worms, and when they do, it's only a couple of them instead of the dozens or hundreds like a dog can end up with.
It's estimated that cats get heartworm at about 5-20% of the rate that dogs in the same area do.
"Cats are relatively resistant to heartworm infection when compared to dogs, with the infection rate in cats reported to be 5-20% of the rate in dogs in the same geographic location. Typically, cats have fewer adult worms than dogs, usually less than six. Many pet owners are surprised to learn that approximately 1/3 of infected cats live indoors only." – VCA Animal Hospitals.
Larvae heartworms do still cause damage, though.
“you can start to see signs of pathology in an affected animal’s body, even though the parasite has not yet reached maturity.” – Dwight Bowman, DVM, PhD
Heartworm infections invoke an inflammatory response in the vessels and arteries even when they die. Because heartworms primarily affects the lungs in cats, veterinarians often refer to it as HARD (or Heartworm Associated Respiratory Disease).
Can Heartworm in Cats Be Treated?
Yes, heartworm in cats can be treated. However, treatment is only effective at the early stages of the infection while heartworms are still babies. There isn’t a treatment for heartworms once they become adults in cats. So monthly heartworm prevention medicine is critical.
Current treatments for heartworm are intended for dogs and they do not work very well in cats. Melarsomine is a medication used to treat heartworms in dogs. This medication kills the worms in dogs quite effectively.
Unfortunately, melarsomine doesn't work very well in cats. The medication has severe side effects in cats, so much so that it's considered toxic.
It does work to kill worms present in the cat, but those dead worms can obstruct the lung's blood vessels and, in many cases, causes sudden death in the poor feline fur baby taking the medication. Obviously, you don't want that, which is why most vets won’t prescribe it.
“There is no known safe and effective adulticidal treatment available for feline heartworm disease. There is no scientific evidence that any treatment intended to kill adult heartworms in dogs will safely do so in cats and increase their infection survival rate” – Maddie’s Fund
There are two-ish treatment options for cats with heartworm.
The first is to treat the symptoms and hope. Worms don't live as long in cats as they do in dogs (only about 2-3 years, instead of the 5-7 they do in dogs.) That means it's possible to give your fuzzy feline treatments like oxygen in a crisis and corticosteroids to keep them fighting through the infection.
The goal of this option is to help the cat fight to survive the infection while preventing further worms from growing in the kitty. Unfortunately, there's always the risk of a worm blocking an artery and causing sudden death, and there's not much that can be done about that.
The second option is surgery. Open heart surgery can be performed to remove the worms in the cat's system, and since there are usually less than half a dozen adult worms present, it's easier to get them all than it would be in a dog.
"Studies have shown that up to 40% of cats may die during or after this procedure, so surgical heartworm removal is typically reserved for those cats who have severe disease and a poor prognosis without surgery." – VCA.
Sadly, there are no other treatment options currently available for cats with heartworm.
The only saving grace is that, since heartworms don't thrive in cats the way they do in dogs, many cats infected by worms show very few symptoms, such that you might never notice they have an infection in the first place. It's only a small fraction that advance enough to show symptoms and end up in a life-threatening position.
Can You Prevent Heartworm in Cats?
Yes, you can. Extremely effective heartworm prevention is not only possible, but it’s readily available for pet parents from several companies.
While severe heartworm disease isn’t common in cats, the fact that there are really no treatment options specifically intended for cats means prevention is key.
Preventative medications circulate through a cat's body and kill off any young larval worms that may have been deposited by infected mosquitos.
These medications are completely effective at killing the early larvae worms. The caveat is that if the worms are allowed to molt into their adult stages, the medications no longer work on them.
Since heartworms can molt to their adult stage in as little as 51 days, you need to give your fur baby a preventative medication more frequently than that.
Heartworm prevention medications are generally safe and can be given to cats even younger than 8 weeks of age, which is great because even kittens can get heartworm.
This is a lifelong treatment; once your kitten is old enough to get the medication, you give it to them on a regular schedule for their entire lives.
Can I Buy Heartworm Cat Medication Over the Counter or On the Internet?
Unfortunately, no. Heartworm prevention medications are prescription-only.
FDA WARNING: “Heartworm prevention medications are prescription only—so beware of internet sites or stores that will sell you these medications without a prescription.” - US Food and Drug Administration (FDA)
The added hurdle exists for two reasons:
Dosage is very important, and if you don't give your cat enough of the medication for their weight class, it might not be effective.
If a cat has an already-active heartworm infection, the medication has a rare but serious chance of causing dangerous reactions or death. Any cat other than a newborn kitten will need to be examined before the prescription can be written.
All of that said, cats are generally safer than dogs when it comes to heartworm, so while you should keep them on preventative medications, you don't necessarily have to worry quite as much about it.
Note: Keeping your cat indoors doesn't prevent heartworm. Infected mosquitos can get in through doors, windows, and cracks, and there's no way to tell whether or not they're infected until it's too late.
Even indoor-only cats should be given heartworm prevention medications on a regular schedule.
Are There Home Remedies for Heartworms?
No. There are no home remedies capable of preventing heartworms, nor are there home treatments that can cure a cat with worms.
While I'm normally fine with trying home remedies when they have a chance of helping, this is one case where there's absolutely nothing that can be done safely outside of FDA-approved preventative medications, and it would be ethically wrong of me to tell you otherwise.
"There are no home remedies for heartworm in cats. The only "natural" or "organic" home remedy for heartworm in cats is to consider treatments that center on preventing or warding off mosquito bites." - Chewy.com
Dogs may have some options, but that's a topic for another time, and since there are treatment medications that can work in dogs, the risk of a home remedy failing isn't nearly as bad as it is with cats.
What Are the Symptoms of Heartworm in Cats?
Some of you may have adopted a stray cat, taken in a cat with an unknown medical history, or are just getting worried because you didn't know how dangerous heartworm can be for cats, and now you're starting to wonder. Does your cat have heartworm?
Sometimes, a heartworm infection won't show any symptoms at all.
Other times, some symptoms can show up, so it’s good to know what to look for.
If you've read these and thought to yourself, "But that's just what any illness does to a cat," you're unfortunately correct.
Most of these symptoms are the generic "something is wrong" with a cat, so it can be difficult to tell the difference between a heartworm infection and a cold.
If your fur baby is sick, the best thing to do is take them to the vet. The vet can then do an exam, including blood tests, x-rays, and antigen tests, to see if there are any signs of heartworm.
If there aren't, you can just ensure that your fur baby is on a preventative medication and move on to treating whatever is causing their symptoms.
What Else Should You Know?
There are some additional details that might be helpful to know.
Heartworm isn't directly contagious between cats.
A cat with heartworm isn't going to infect another cat or dog for that matter. Because the disease needs an intermediary host, the mosquito, removing any stagnant water from around your home and treating for mosquitos can reduce the risk of infection.
Why are cats more immune to heartworm than dogs?
Well, it's not entirely clear, other than the fact that heartworm is a dog-centric parasite and evolved to infect canines, not felines. The fact that it can infect non-canines at all is an oddity, in a way.
Will a cat recover from heartworms?
Maybe. Some cats with heartworm infection do spontaneously resolve. If a cat can outlive a heartworm infection, they can then be put on preventatives to keep it from happening again.
However, the damage done by the worms before they die is permanent, so your cat might be stuck with difficulty breathing or a reduced energy level for the remainder of their life.
Unfortunately, there's no feline vaccine for heartworms yet. Preventative medications are antiparasitics that kill the larvae if they show up, but like any medication, they wear off over time.
A vaccine would train the cat's immune system to detect and fight off the worms instead, but no such vaccine has yet been developed. Maybe one day!
Can People Get Heartworms from Their Pets?
No. People cannot get heartworms from their pets. As discussed above, heartworms are only transmitted by the bite of an infected mosquito. In rare cases, people may get heartworms via a mosquito bite.
However, given the fact that humans are not a natural host for heartworms, the larvae usually die before becoming adult worms.
How Can I Learn More About Heartworms in Cats?
Ask your veterinarian.
Call the FDA’s Center for Veterinary Medicine at 240-402-7002 or email firstname.lastname@example.org
Finally, heartworms are historically most common in the American South, but unfortunately, they have spread.
Various factors, like climate change and cross-country animal adoption have contributed to the spread. Consider the number of animals that have been adopted across the US after major hurricanes have hit the southern states.
Even though mosquitos might not be around all year 'round, at this point, it's a big enough risk that vets recommend preventatives all year. From the American Heartworm Society:
"For a variety of reasons, even in regions of the country where winters are cold, the American Heartworm Society is now recommending a year-round prevention program... Mosquito species are constantly changing and adapting to cold climates, and some species successfully overwinter indoors as well. Year-round prevention is the safest and is recommended." – American Heartworm Society.
Keep your fuzzy feline friend safe and remember, an ounce of prevention is worth a pound of cure.
Do you have any non-urgent questions about heartworm or how you can prevent it in your fur baby? If so, you're always more than free to leave a comment down below, and I'll do my best to get back to you as soon as possible!
K Marie Alto
K. Marie is an animal lover, wife, kitty mom, dog auntie, writer, and co-founder of Toe Beans, a proud American family-owned online boutique pet supplies store focused on the improvement of the life of furry family members via pet parent education, better products, and advocacy. She has over 20 years of experience as a pet momma. She loves sharing her personal journey and experience as a pet parent via her blog and Facebook page where she currently has more than 50K followers (@furrytoebeans) and counting :-). Read more