April is Heartworm Awareness Month and, as veterinarians, we are happy to have an opportunity to spread awareness about this devastating yet completely preventable disease. And while heartworm disease is often associated with dogs, cats can be affected with heartworms, too. The thing is, cats are considered atypical hosts for heartworms because most worms in cats do not survive to the adult stage. According to the American Heartworm Society, “While this means heartworm disease often goes undiagnosed in cats, it’s important to understand that even immature worms cause real damage in the form of a condition known as heartworm associated respiratory disease (HARD). Moreover, the medication used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the effects of heartworm disease.” The AHS also found that heartworm continues to be diagnosed in all 50 states, so no is immune to what they’re calling a “hearworm-demic”.
Because awareness and, most importantly, prevention of heartworm disease in cats is so critical, we’ve rounded up some frequently asked questions about it below and their accompanying answers below.
What is cat heartworm disease?
Cat heartworm is an internal parasite that causes significant lung damage. Cats become infected by being bitten by mosquitoes. A mosquito injects these worms into the body of the cat, and it migrates toward the lungs.
Can a cat pass on heartworm to another cat, a dog, or a person?
No, all heartworms have to go into a mosquito for part of their life cycle before being transmitted to another animal.
What is the cycle of the heartworm, and how is that information essential to a cat?
The heartworm cycle in a cat is when a mosquito bites the cat and injects a baby worm into the cat’s skin. These worms then migrate through the skin into the bloodstream, and they arrive at the lungs, and that's when all the damage is done. Months later, when the worms start arriving at the lungs, the cat develops an intense inflammatory reaction to destroy these worms before they continue to mature. The cat has these asthma-like symptoms—coughing, open mouth breathing, vomiting, and labored breathing.
How does a cat's lifestyle affect their risk for heartworm?
Cats that roam and are outside more are at a bit of an elevated risk of becoming infected with heartworms. It's much less than the risk of a dog. Even so, because heartworm disease is invariably fatal for a cat if it reaches maturity, prevention is paramount.
What's a sign that my cat has heartworms?
Cats develop symptoms much earlier than dogs because the immature stages arrive in the lungs earlier than in dogs. Dog symptoms are more geared towards adult worms being present. It's the immature or earlier stages that cause problems in a cat. Sometimes a few worms will make it to adulthood, and that's not a good scenario when that happens.
Signs that your cat may be infected with heartworms are as follows:
- Weight loss
- Open-mouth breathing
- Coughing
- Labored breathing
- Vomiting (cats tend to vomit with many different diseases)
Is heartworm painful for a cat?
Heartworm disease tends to cause asthma-like symptoms, so the cat can't breathe. They're not perfusing, so they're very panicked. Some of their tissues may be a little painful because they're not perfusing, although we can’t be sure.
How is a cat tested for heartworm?
Typically, if we suspect a cat has heartworm disease, we will recommend a test just like in a dog. It comes out negative most of the time because, once again, that test tests for adult heartworms, and it's not usually adult heartworms that cause problems in cats. There aren't tests out there for immature stages. We sometimes test for antibodies, though they can develop antibodies a couple of months after infection, as they are very transient. They go away really quickly, so we have a short window to catch them. That only means exposure and not necessarily that the cat is continuing to be infected as well. We will often look at x-rays, and we can see changes in the lungs that are highly suspicious of heartworm.
Most veterinarians will also recommend yearly retroviral screening, which are diseases like feline leukemia and FIV, which is a discussion for a different day. But that particular test also tests for heartworm disease. We recommend outdoor cats get screened for those diseases, so we're inadvertently screening for heartworm as well. It helps with preparedness for what might happen in the life of that cat if it is positive.
How often should my cat be tested for heartworms?
We only recommend testing when we think the cat has heartworm disease. Routine testing is typically not done, probably because there's no treatment for it once the cat has adult heartworms. They don't have microfilaria, which is the baby warm circulating in the blood, so all heartworm preventatives are safe for them. We recommend prevention in cats.
In a symptomatic or sick cat where we suspect heartworms, then we’re going to get an x-ray to check for antibodies and do the ELISA antigen test as well.
If my cat's on prevention, do I need to have them tested yearly like dogs?
No, because they don't have microfilaria, so we can go ahead and put them on preventatives. They so rarely test positive because they rarely get adult worms, so yearly testing is not recommended or needed.
How is heartworm prevented in cats?
Typically, there are some pills that you can give kitties like you do dogs that are labeled for a cat but are a bit different. There are also topicals. The vast majority of people use the topicals that you put on the skin because cats are so difficult to get to take pills. These prescription medicines prevent the dog or the cat from getting infected and, as we’ve mentioned, the key with a cat is to prevent them from getting infected.
How effective is heartworm prevention in cats?
Heartworm preventions are 95% or more effective if appropriately administered and given regularly.
Are there any holistic or over-the-counter treatments for heartworms?
There are not. Vector and mosquito control can help, but still, those preventative products are essential. Many of those preventative products also keep fleas and ticks away and help treat intestinal parasites.
Can I do anything in my cat's environment to reduce the risk of heartworm?
Preventing standing water is essential. You can use area mosquito sprays and things like that, and they are safe if you follow directions. Anything you can do to keep the mosquito population down in the area helps. Usually, you can combat that by dumping standing water from your flower pots, and other things like that.
What should I do if I miss a dose of my cat's heartworm prevention?
If you miss a dose, keep going. As soon as you realize you've missed it, go ahead and give the cat another one and go ahead with their regular monthly routine. If the cat isn’t showing any symptoms, then we don’t have to go any further with that.
If a cat does get an adult heartworm or two, there is no treatment available. They can't get the conventional treatment that dogs do. The medication is somewhat toxic in a cat. As the worm dies, the cat often dies as well, which is why prevention is so vital. If infected, the cat develops chronic asthma-like symptoms once infected with heartworm, which lasts for life. We treat those symptoms with things like corticosteroids and bronchodilators as needed, but there's permanent damage done to the lungs.
Say my cat has tested positive for heartworm. What would the next step be?
If your cat’s tested positive for heartworm disease, the next steps would be X-rays, monitoring, and bracing owners that this cat may go into a fatal crisis when this heartworm dies (they live about two years). There's not a whole lot you can do to prevent that. Despite the best of efforts, those cats die. Sometimes specialists can surgically/manually remove heartworms, but it’s a risky and costly procedure.
Is early detection and diagnosis of heartworm more difficult in cats?
Cats often test negative for everything, but they're infected with heartworm disease because it does different things in a cat. If a cat has asthma-like symptoms, and cats get asthma anyway, it's an allergic lung disease, but it can also be triggered by heartworm. It's an educated guess. The treatment is the same as treating asthma, with steroids and bronchodilators, but we’re still going to have to figure out if heartworm is the underlying cause. If we suspect heartworm, we may do antigen testing later to see if it was heartworm that caused the asthma attack, and we may never know.
The incidence in cats is much lower than dogs because it is a dog parasite for the most part. Cats’ lifestyles and things like that make them a little less susceptible as well. But it's the cat's immune system reaction to the parasite when they get infected that causes all the cat symptoms more than anything.
Are there side effects to the medications that we use in cats to prevent heartworms?
For the most part, these preventives are very well tolerated. Sometimes you’ll see local site reactions where you apply the topical; they'll lose hair and get itchy. On occasion, owners report that their cats don't feel well after they apply the topical. If we suspect that the heartworm preventative is causing some lethargy or depression, additonal options are available at your primary care veterinarian.