Heartworm Disease in Cats
Written by Small Door's medical experts
Although it is preventable, heartworm disease strikes millions of pets in the United States each year. The condition is caused by parasitic worms that spread through mosquitoes. The worms are called "heartworms" because of their tendency to reside in the heart, lungs, and nearby blood vessels of infected animals. However, when heartworms infect cats, they are more damaging to the lungs than the heart. As the worms grow, spread, and eventually die, heartworms can cause potentially fatal complications. Learn more about how to prevent and manage heartworm disease in cats.
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Parasites depend on a "host" animal to survive, exploiting the animal's resources to feed, grow, and reproduce. Parasites often have a primary host, a species most adapted to the specific parasite's needs. For heartworms, dogs are the primary host, meaning that they thrive, grow, and reproduce more readily in dogs than in other hosts.
Cats are also vulnerable to heartworm infection, but their bodies are more hostile to the worms. The young heartworms undergo constant attacks from the immune system as soon as they enter a cat's body and usually die before becoming adults. When adult heartworms are present in cats, there are generally only 1 to 3, while dogs often have many more. Adult worms are rarely able to reproduce larval offspring in cats. Larvae born within a host's body are called microfilariae, but infected cats usually will not have microfilariae in their bloodstream.
Cats' natural resistance to heartworms may sound encouraging, but cats are actually more likely to die from heartworms than dogs. Cats are generally smaller than dogs, and they have smaller blood vessels and organs. Their immune systems also mount stronger reactions in response to heartworms. As a result, even a few heartworms can cause significant damage to your cat.
Severe complications from heartworm disease usually occur at several points in the worm's life cycle.
The first is when immature heartworms travel to the blood vessels near the heart or lungs (pulmonary arteries) and start to cause damage. Most worms will die at this point because of the relentless attacks from the feline immune system.
Next, the death of young heartworms triggers a powerful immune reaction, leading to lung inflammation. This inflammation and damage may cause symptoms, including difficulty breathing and coughing. When these adult worms die, they release harmful toxins, and their death generates an even more intense inflammatory response, often causing severe lung damage, blood clots, and death. Cats that survive the death of adult heartworms will likely live with chronic lung damage.
What do heartworms do to a cat's heart?
Heartworms get their name from their ability to live in the heart, but they primarily reside in the pulmonary arteries that carry blood from the heart to the lungs. Both dogs and cats can develop problems in their lungs, hearts, or other organs due to heartworms. However, feline heartworm infections primarily affect the lungs rather than the heart. The lung damage cats can sustain from heartworms usually stems from inflammation generated by the immune system in response to the worms or their death. A cat's immune system will usually kill off heartworms before they can grow into adults. If the worms do manage to reach the arteries or heart and mature into adults, the death of these worms can cause similar effects as in dogs, such as damaging blood vessels, obstructing blood flow near the heart, and interfering with the heart's ability to pump blood.
How common is heartworm disease in cats?
The prevalence of heartworm disease in cats is often estimated to be about 10% of the number of infected dogs in a given region, says Veterinary Information Network (VIN). According to the American Veterinary Medical Association (AVMA), approximately 1 million dogs tested positive for heartworms in 2019. Based on these calculations, one could estimate that about 100,000 cats were diagnosed with heartworms in the same year. However, the rate of heartworm disease in cats is likely underestimated because heartworms are much harder to diagnose in cats, many infections go undetected, and some rural veterinary practices may be underreported.
The signs and symptoms of heartworm disease in cats vary widely depending on how the infection evolves. When symptoms of heartworm disease in cats do occur, they may include:
Difficulty breathing
Coughing
Fast breathing
Nosebleeds
Weight loss
Vomiting
Lack of energy
When symptoms are present, they generally occur during specific phases of the infection, such as when immature worms migrate to the pulmonary arteries, when immature worms die, and when adult worms die. The arrival and death of immature worms in the pulmonary arteries usually occur about 3 to 4 months after the initial infection. At this stage, cats often develop heartworm-associated respiratory disease (HARD), which comes with symptoms such as difficulty breathing, fast breathing, and coughing. These symptoms are easily confused with conditions like asthma or other respiratory diseases.
The second symptomatic stage of infection occurs when the adult heartworms die and release toxins that damage the lungs and trigger an inflammatory response. This stage usually causes severe lung damage or even sudden death and may come with symptoms such as incoordination, falling, seizures, and coughing up blood.
It's also possible for a cat with heartworms to exhibit no symptoms, but even asymptomatic infections can sometimes cause sudden death or serious complications. More often, cats with no apparent signs fight off the infection spontaneously and are never diagnosed.
Feline heartworm disease is caused by an infection of parasitic worms. The scientific name for heartworms is Dirofilaria immitis. These organisms infect "host" animals in order to grow, survive, and reproduce. They also need mosquitoes, which act as intermediate hosts, to help them spread and infect new hosts.
After a mosquito bites an animal infected with heartworms, it can pass on the infection (heartworm larvae) to the next animal they bite. Many wild animals and pets are susceptible to heartworm infections, but some are better hosts than others. While cats can be infected by heartworms, they don't often spread the disease to other animals because they are not adapted to accommodate the worms.
What is the life cycle of the heartworm?
The life cycle of heartworms is much shorter in cats than in dogs. Following a bite from a heartworm-infected mosquito, the heartworm larvae will move into the cat's body and try to mature despite constant attacks from the immune system. Then, surviving larvae will grow into slightly more mature worms and migrate toward the heart and lungs. It usually takes about 3 to 4 months after the initial infection for the worms to reach the arteries near the heart and lungs, and then another couple of months for the immature worms to become adults. Many of the worms will die before becoming adults, and only 1 to 3 worms, on average, are present in cats with adult heartworm infections. Heartworms in cats can rarely reproduce microfilariae because it is unlikely for a worm of each gender to survive into adulthood. This means that when a mosquito bites a heartworm-infected cat, it is unlikely to take in heartworm larvae and pass them on to another host. If the worms manage to reproduce, their larvae are often quickly destroyed by the cat's immune system. Adult heartworms can only live for 2 to 4 years in cats, compared to an average lifespan of 5 to 7 years in dogs. The most severe complication of feline heartworm disease, such as fatal lung injury, tends to occur when these adult heartworms die.
Your veterinarian will likely diagnose heartworm disease with the use of blood tests and imaging scans. Blood tests are used to look for microfilariae in the blood or to identify specific blood proteins that indicate the presence of heartworms in the body. Imaging techniques such as X-rays and ultrasounds provide a visual of the heart, lungs, and pulmonary arteries and can help your veterinarian spot potential worms. Your veterinarian will also consider any symptoms when diagnosing heartworms, although infected cats may not always exhibit symptoms.
It is challenging to detect and diagnose heartworm infections in cats. Infected cats may or may not have adult heartworms, and they rarely have microfilariae in their blood. They can be sickened by infections with only immature heartworms, which are more difficult to detect. Cats with adult heartworm infections usually only have a few adult worms, which may not show up on tests.
To increase the chances of an accurate diagnosis, your veterinarian will likely utilize multiple types of tests.
Unfortunately, none of the currently available treatment options for heartworm disease are sufficiently effective or safe for use in cats. The U.S. Food and Drug Administration (FDA) has not approved any medications for this purpose.
Your veterinarians may recommend corticosteroids as a way to potentially ease symptoms and reduce the risk of complications. Still, these drugs will not kill the worms, and they are not always effective. Since adult heartworms can live for years, the delivery of corticosteroids must be well-timed with the worms' death for the drugs to provide much benefit.
In some cases, surgery to remove the worms is an option. Surgery is usually only used for severe infections because it can have life-threatening consequences if the worms are not removed entirely. Veterinarians may offer surgery if the cat's condition is critical and an ultrasound shows a significant number of heartworms.
Is there a cure for heartworm disease in cats?
Heartworm disease in cats cannot be cured by medication. Surgery to remove the worms may be an option for some infections, but it is highly risky. In some cases, cats may rid themselves of mild, asymptomatic heartworm infections without medical intervention.
Is heartworm disease in cats contagious for humans or other pets?
Heartworm disease is not contagious because only mosquitoes can spread it. It cannot spread from animal-to-animal or animal-to-person through proximity, bodily fluids, or any other interactions. Heartworms are not directly contagious because they can only mature and become infectious inside the body of a mosquito.
While people cannot contract heartworm disease from an animal, it is possible for humans to be infected through a mosquito bite. However, heartworm disease in humans is rare and is often asymptomatic and non-serious.
What is the cost of treating heartworm disease in cats?
In addition to fees associated with an exam and diagnostic testing, the cost to treat heartworm disease in cats can vary, depending on factors such as the severity of symptoms and the disease process at the time of diagnosis. Costs can vary from a few hundred dollars to a few thousand dollars.
If your cat is diagnosed with heartworms, he or she will likely be monitored throughout the infection and managed with medical interventions. As a cat parent, you’ll be advised to limit your cat's movement and activity. Your veterinarian may offer corticosteroids to potentially lessen the immune system response that will occur when the heartworms eventually die. Similar medications may also be used to ease symptoms, such as vomiting or lung inflammation.
Infected cats may need to undergo regular testing to monitor their condition. Other therapies can be given depending on the symptoms and severity of the infection. For example, supplemental oxygen can help reduce respiratory symptoms. Cats that survive heartworm disease may require additional monitoring or testing to determine whether the worms caused any long-term damage.
How long can a cat live with heartworm disease?
Heartworm infections may be a passing, asymptomatic, and non-serious event for many cats. It's difficult to estimate how many cats have this experience because symptomless infections often go unnoticed, but as VIN states, approximately 80% of cats spontaneously rid themselves of heartworms.
Cats can experience heartworm-associated respiratory disease (HARD) and may sustain long-term lung damage. This can develop when they have a heartworm infection and immature worms reach the pulmonary arteries and some worms are killed by the immune system.
However, fatal injuries often arise during the next stage – when adult heartworms die. According to VIN, about 10% to 20% of heartworm-infected cats experience sudden death, and these incidents are typically driven by adult worms dying within a cat's body. Because many feline deaths due to heartworms occur upon the death of adult worms, cats can often live with heartworm disease for years. While the death of mature heartworms is often fatal for cats, some cats can survive these episodes, especially with medical management. As the Merck Veterinary Manual states, approximately 25% to 50% of cats survive if they are monitored and treated appropriately.
Fortunately, medication can be used to help prevent heartworm disease in cats with nearly 100% certainty, according to the AVMA. Cat owners have several FDA-approved options to choose from, including topical solutions that are applied to the skin and oral medications that are swallowed. All of these medications must be administered on a once-monthly basis. You can get your cat a prescription for heartworm preventatives by consulting your veterinarian.
Because there is no effective treatment for heartworm disease in cats, prevention is of the utmost importance. All cat owners should give their cats medicine to prevent heartworms, regardless of the season or the age of your cat.
What is the best preventative medication for heartworm in cats?
Recommended topical brands for heartworm preventatives in cats include Revolution Plus or Advantage Multi. These involve applying a small tube of liquid to the cat’s skin (typically on the skin between the shoulder blades while parting the fur) and is done consistently on a once-monthly basis, all-year round.
Do indoor cats need heartworm preventatives?
Regardless of whether they live indoors or outdoors, all cats need heartworm preventatives. Mosquitoes are well-known to get inside the house by following you through an open door or sneaking in through a broken screen. VIN notes that in one study, researchers found that within a sample of cats with heartworm disease, between 25% and 30% were indoor cats.
Does my cat still need heartworm testing if they are on preventatives?
Even if your cat takes medication to prevent heartworm disease, your veterinarian will likely still recommend annual testing. Preventatives will not kill off existing worms, so a previous, undetected infection can develop while taking preventatives. Testing is particularly important if the monthly doses are missed or administered incorrectly.
Is there a vaccine for feline heartworm disease?
There is no vaccine for feline heartworm disease at this time. But, according to the American Heartworm Society, there may be one in the future. Currently, the only options for preventing feline heartworm disease are the highly effective, FDA-approved preventative medications.
While heartworms are more commonly associated with dogs, cats can also contract these potentially fatal but preventable infections. In fact, heartworms are more deadly in cats than in dogs. Cat owners should be aware that feline heartworm infections look different because the worms harm your cat's lungs more than its heart. Not all cats show signs of heartworm disease, but some of the potential symptoms include coughing, difficulty breathing, and vomiting. There is no effective treatment for feline heartworm disease, so as a cat owner, consistently giving your cat preventative medications is the most effective way to protect your cat from heartworms.
