Heartworm prevention is a fact of life for pet parents in much of North America, with heartworms having been diagnosed in all 50 of the United States and southern areas of Canada. Heartworms can be found in countries on every continent of the world, except Antarctica. These pesky parasites don’t exclusively affect our dogs though! Heartworms can also be found in wolves, foxes, coyotes, ferrets, sea lions, and other species, which unfortunately includes our friendly feline companions. More and more pet parents are becoming aware of heartworms in cats.
Heartworms are one of the more dangerous parasites in cats, so it’s important for you to understand what these parasites are, how they affect cats, and what you can do to keep your cats safe.
Can Cats Get Heartworm?
The simple answer is that yes, cats can get heartworms. However, heartworm disease in cats isn’t the same as it is in dogs. Before we get to the differences between heartworms in dogs and heartworms in cats, let’s go over what heartworms actually are.
Dirofilaria immitis (heartworm) is a parasite that is spread to dogs and cats by mosquitoes. The name “heartworm” gives you two important pieces of information about these parasites. One, they are worms, and two, they affect the heart. An adult heartworm looks like a thin, cooked spaghetti noodle, with adult worms ranging from 4 inches to over a foot in length. The adults typically live in the pulmonary artery (which carries blood to the lungs from the heart) and right ventricle of the heart. As a result, heartworms mostly impact the heart and lungs.
While the heartworms themselves aren’t thought to cause pain, the secondary conditions that result from the worms can cause severe distress and death. Cats are considered an atypical host for heartworms while dogs are the preferred host. As a result, dogs can have 30 to over 100 adult worms in their heart and lungs while cats who have adult heartworms typically only have one to three. This doesn’t mean that heartworms are any less severe in cats, with heartworms more likely to cause severe respiratory disease or sudden death in cats than in dogs.
How Common Is Heartworm in Cats?
A 2020 study in Florida found that the prevalence of adult heartworms in shelter cats was 4 percent compared to 28 percent in shelter dogs, meaning for every seven dogs with adult heartworms there was one cat with adult heartworms (1). Overall, the prevalence of heartworm disease in cats is suspected to be about 5 percent to 15 percent of the prevalence in dogs in any given area (2).
What Causes Heartworm in Cats?
Cats get heartworms through mosquito bites. Currently, mosquitoes are the only known vector of heartworms. When the mosquito pierces the cat’s skin while biting, heartworm larvae that were picked up from another animal (usually a dog) can enter the cat. Most heartworms that are passed to a cat will not make it to adulthood, but those that survive will migrate into the pulmonary artery and right ventricle of the heart, developing into adult worms over six to eight months. Adult worms live in cats for around two to four years. While this is the overall gist of a heartworm’s life cycle, more specifics about the lifecycle can help us understand their effects on cats.
Because dogs are the preferred host of heartworms, adult male and female heartworms will mate while parasitizing a dog, producing microfilariae (baby heartworms). Microfilariae are microscopic and move throughout the bloodstream. These microfilariae are ingested by mosquitoes when they bite the dog. Within a few weeks, the microfilariae will develop into stage three larvae within the mosquito. This stage is the infective stage that can be passed on to cats or other dogs when the mosquito feeds.
Over two months, the larvae that pass into the cat will develop into immature worms in the animal’s subcutaneous tissue, fat, or muscle. Immature adult heartworms will enter the bloodstream, which will allow them to move into the heart and pulmonary artery which they call home. Once these immature worms reach the heart and lungs, you may begin to see signs of heartworm disease in cats. These immature worms can cause a severe inflammatory response that affects the arteries, small airways (bronchioles), and air sacs of the lungs (alveoli).
If the cat survives the initial migration of the worms into the heart and lungs, the worms will mature into adult heartworms over three to five months. While adult heartworms live for up to five years in dogs, they typically have a shorter lifespan (two to four years) in cats. Because the cat is an atypical host, these worms don’t usually reproduce within the cat, meaning the cat usually doesn’t have circulating microfilariae and isn’t infectious to other animals. When adult worms die, some cats have an extreme inflammatory response that can manifest as respiratory distress, shock, or sudden death of the cat.
Can People Get Heartworms from Cats?
This all sounds very scary for your cat, and honestly, it is! A natural worry you might develop is if you or your family members can get heartworms from your cat. Humans can get heartworms, but this isn’t common. Because heartworms do not produce microfilariae in cats, human infections are usually carried by mosquitoes from dogs or wild canids. Your heartworm-positive cat is almost never at risk of transmitting heartworms to you. Humans are accidental hosts and aren’t suitable for the worms to thrive in. Heartworms that die in the pulmonary vessels result in nodule formation in the lungs, which can be hard to distinguish from lung cancer nodules and result in the need for a biopsy. Most of the time, humans don’t have any clinical signs of heartworm infection. In the United States, 116 cases of heartworms affecting humans have been reported (3).
Heartworm Symptoms in Cats
One of the most alarming characteristics of heartworm infection in cats is that you often don’t know your cat has heartworms until it’s too late. Unlike parasites that affect the stomach and intestines, you’re not going to find heartworms or their eggs in your cat’s feces, so you’re probably not going to know your cat has heartworms unless they develop signs or you’re routinely getting them tested by a veterinarian.
Common clinical signs of heartworms in cats include:
- Intermittent vomiting
- Rapid and labored breathing
- Coughing, gagging, and wheezing
- Loss of appetite
- Weight loss
In some cats, these signs will appear around two months after the initial infection when the immature adult worms are migrating into the heart and lungs. If you’re noticing these symptoms in your cat, it’s important to have your cat examined. These clinical signs, especially the respiratory symptoms, are often mistaken for feline asthma. This severe inflammatory response is called heartworm-associated respiratory disease (HARD).
So, is heartworm in cats fatal? Unfortunately, the answer is often yes. For around 10 percent to 20 percent of cats (4, 5), the first sign of heartworm infection is sudden death, which can occur at around two months when the worms migrate to the heart and lungs or later on when an adult worm dies (sometimes two to four years after infection). Sudden death usually occurs due to severe inflammation in the lungs, which is not responsive to medications like steroids.
Stages of Heartworm Disease in Cats
Although there are more stages of heartworm disease in dogs, the American Heartworm Society (6) divides feline heartworm infection into two stages:
Stage 1 occurs when the immature worms arrive in the pulmonary arteries, at which point many of them die. This is the point at which cats can develop HARD. Some cats may die or be euthanized at this stage due to the severity of their illness.
Stage 2 occurs when adult worms die, resulting in an extreme inflammatory, anaphylactic response that is very often fatal. Dying adult heartworms can form a clot (embolism). Cats who do survive often have permanent lung damage and long-term respiratory disease. Adult worms can die at any point, but in cats they may live for two to four years. This stage can occur anywhere from around six to eight months after the initial mosquito bite up to around four years later. Again, many cats who experience this stage will be euthanized due to the severity of illness.
Diagnosing Heartworm in Cats
Diagnosing heartworms in cats is a bit tricky. After performing a physical exam and collecting your pet’s history, additional testing your veterinarian may do to determine if your cat has heartworms include:
Heartworm antigen test: The usual SNAP test that your veterinarian runs for your dog at the clinic specifically detects an antigen that is associated with adult female heartworms. This means the heartworms need to be at least 6 months old and that there needs to be adult female heartworms for the test to be positive. Recall that dogs can have well over 100 worms. The chances that at least one of those is a female worm is very high. But cats often have only one to three worms. If all their adult worms are male, they will test negative on this SNAP test. Furthermore, cats that are having HARD symptoms at two months after infection will still test negative. So, veterinarians often must rely on other testing to confirm a diagnosis of heartworms in cats.
Heartworm antibody test: The preferred method for screening cats is to run both an antigen and antibody test. While the antigen test detects adult female heartworms, the antibody test detects exposure to heartworm larvae. The antibody test can be positive as early as two months after initial infection. One issue with the antibody test is that antibodies can persist for years, so a cat who was exposed but cleared the infection on their own may test positive. However, if your cat’s physical exam and history are suggestive of heartworms and either the antigen or antibody test is positive, your veterinarian is likely to move forward with management of heartworm symptoms.
Chest X-rays (thoracic radiography): Your veterinarian is also likely to perform radiographs of the chest to look for changes to the heart and lungs that may be suggestive of heartworms.
Ultrasound of heart (echocardiogram): In some cases, an echocardiogram may be beneficial. Worms can sometimes be visualized within the right ventricle or pulmonary artery with an echocardiogram. Not all clinics have the capability to perform an echocardiogram.
Heartworm in Cats Treatment
There is no approved medical treatment for heartworms in cats. The injectable medication (melarsomine) used in dogs is toxic to cats at low doses, and the sudden death of the worms from the injection may also result in the sudden death of the cat itself. The only way a cat will be cured of heartworms is if they clear the infection and survive on their own, which occurs in around 80 percent of cases (5). Instead of treating heartworms, the goal is to manage the symptoms.
In cats who are experiencing respiratory distress or shock, they will need emergency treatment. If your pet is having difficulty breathing or collapses, you need to take them to the emergency veterinarian immediately. Treatment may include steroids, intravenous fluids, bronchodilators, and oxygen administration.
Long-term management of symptomatic cats may include slowly tapering doses of steroids such as prednisone to reduce inflammation in the lungs, as well as bronchodilators. A bacteria called Wolbachia lives within the heartworms, so your cat may be prescribed doxycycline to kill these bacteria and weaken the heartworms. No home remedies are shown to be effective, but it’s recommended to reduce stress in the home, as this could contribute to development of symptoms.
The disease can be monitored with repeat antibody and antigen tests. Your veterinarian may recommend this testing on an annual basis in asymptomatic cats but may want to test more often in cats with symptoms. In cats who have heart or lung changes identified on X-rays or an echocardiogram, repeat imaging may be recommended every six months.
Heartworm Surgery for Cats
In cats with severe symptoms, especially those rare cases with high worm burdens causing obstruction in the heart (caval syndrome), surgical removal of the heartworms may be recommended. This is a very rare procedure and usually reserved for severe cases due to the high risk associated with the surgery. Typically, instruments are introduced through the right jugular vein into the right side of the heart to remove worms (7).
Cost of Treating Heartworms in Cats
Cost will vary depending on what treatments you pursue. If your cat is in respiratory distress or shock and you elect to attempt treatment, emergency services can cost several thousand dollars. However, if your cat’s case is mild, management with steroids is usually very affordable and may be under $10 to $20 for the prescription itself after the initial testing. Initial diagnostics will usually cost under $500 for exam, antibody test, and antigen test but may increase to over $1,000 depending on if advanced imaging (echocardiogram) is performed. Chest X-rays are usually around $200. Surgical removal of heartworms is an extremely specialized procedure, which may cost over $6,000 and is not guaranteed to be successful.
Heartworm Prevention in Cats
Luckily, pet parents have multiple options for preventing heartworms in their cats.
Although indoor cats can definitely still get infected with heartworms (it only takes one mosquito bite!), they’re less likely to be affected than cats who live or venture outside. Keeping your cats indoors is one way to reduce exposure to mosquitoes. That being said, one study at North Carolina State University did find that about a third of heartworm-positive cats were housed exclusively indoors (4), so you’ll definitely want to keep your indoor cats on prevention.
The best way to protect your cat, whether indoor or outdoor, is to keep them on year-round heartworm prevention. As the climate changes, mosquito ranges are expanding, and mosquito season is beginning to include more time of the year. Current options include either topical solutions that are applied to the skin between the shoulder blades or oral medications. This medication is typically given to your cat once monthly for life. The long-term injectable preventative is currently only used in dogs. A prescription is needed for heartworm preventatives, and these medications are sometimes combined with prevention of fleas and internal parasites.
Examples of preventative medications that work against heartworms include: