What To Do When Your Cat Has Heartworms

Ron Hines DVM PhD

  Heartworm disease is caused by a blood parasite, Dirofilaria immitis. This is actually a parasite of dogs and their wild relatives. But, given the chance, they will attack your cat as well. The best way to protect your cat from catching heartworms are not the pills or drops that veterinarians like me sell. It is to keep your cat indoors. The chances of an indoor cat catching heartworms or the many other diseases and dangers that lurk in the outside world are extremely small. If you let your cat roam, if your cat is inclined to sneak out the door when it’s open, if your dwelling has no screen and is full of mosquitoes, then it’s a different story. 

 How Common Is It For Cats To Catch Heartworms?

I mentioned that Nature designed heartworms to survive and proliferate in dogs, not cats. You can read about those much more common canine infections and how veterinarians eliminate them in dogs here. But when by mistake, a mosquito that bites an infected dog later bites your cat, one or more of the microscopic heartworm larva (microfilaria) can survive in your cat too. However, the heartworm’s life cycle in cats does not progress in its normal, orderly fashion. That is because the genetic road map that guides heartworms to the heart’s of dogs does not work correctly in cats. After being injected into your cat by the mosquito, a lot of the heartworm larva get lost on their way to a cat’s heart. Most are then destroyed by the cat’s immune system. The few that might reach your cat’s heart do not live as long as they do in dogs, nor can they mate to produce their next generation of microscopic heartworm larva. So, cats are dead-end hosts (also called aberrant hosts) that are unable to transfer heartworms to other cats or dogs. We humans are not entirely immune to this problem, either. (read here)

No one knows for sure how often this happens. When it does, it will most likely occur in cats allowed outside where mosquitoes are present in large numbers or in feral cat colonies. One Florida study found antibodies against heartworms that indicated exposure to the parasites in 0.4% of the cats they tested. The highest prevalence was found in the southern United States. The cat’s risk of being in that 0.4% antibody-positive group increased when the cat had outdoor access, when it was ill at the time of testing, or when it was also infected with the feline leukemia or feline immunodeficiency virus. (read here) Others guessed that for every 10–12 dogs that develops one or more mature heartworms in their heart or surrounding blood vessels, perhaps one cat does. In one Iowa study, about 6% of outdoor cats were antibody-positive for heartworm exposure. In Spain, about 7% of outdoor cats were. (read here & here)  These immunologic blood tests look for a protein found only in mature female heartworms. So, the actual number of cats exposed to the parasite could be much higher. For example, the feline immunodeficiency virus was found in 17-23% of outdoor feral cats (read here &  here) FIV is spread through cat bites, not mosquitoes. 

 Why Are Cats More Resistant to Heartworm Infections Than Others?

There are several reasons cats are more resistant to heartworm infection than dogs. The first, which I already mentioned, is that heartworm larva (microfilaria) are genetically designed to find their way through a dog’s body – not a cat’s body. The second is that heartworms probably employ a stealth technology to avoid being destroyed by a dog’s immune system cells. (read here) It appears that the immune system of cats finds and destroys heartworms much easier. When 100 heartworm larva/microfilaria were injected into a group of dogs, an average of 75 adult heartworms developed. When 100 larvae were injected into a group of cats, only 3–10 heartworms developed. However, because maturing heartworms are more inflammatory in cats than in dogs, and because the average size cat is smaller than an average size dog, each worm that does develop in your cat has the potential to do significantly more damage. That is particularly true if it is a female heartworm; female heartworms can be as long as 12 inches (~30 cm), males only a maximum of 7.5 inches (~19 cm). The girth of mature female heartworms is considerably wider as well. So, blood flow through the critical blood vessels supplying the cat’s lungs is considerably more restricted than in an average size dog. One or two adult heartworms are unlikely to cause ill health in a dog, but one or two adult heartworms in a cat can be life-threatening. 

  What Signs Might I See If My Cat Has Heartworms?

In dogs, the signs of advanced heartworm disease are primarily the signs of heart failure due to the mechanical blockage of blood flow through the dog’s heart and lungs. But in cats, the signs are primarily due to the inflammation these parasites cause. Your cat’s immune system reacts violently to any heartworms it discovers. Heartworms that survive to maturity in cats generally locate themselves in your cat’s pulmonary artery or one of its branches leading to your cat’s lungs. So, the signs you would notice first are usually related to lung inflammation. Those signs include coughing, vomiting, and difficulty breathing. These signs are technically called H.A.R.D. (heartworm-associated respiratory disease). They are signs one might easily mistake for feline asthma or hairballs. Those signs are apt to occur about three months after your cat was bitten by an infected mosquito. That is about the time it takes for immature heartworms to arrive at a cat’s heart and lungs. Those respiratory difficulties might fade, but then reoccur several months later when the heartworms die prematurely and begin to break apart. (read here)

  How Can Heartworms Be Detected In My Cat?

The fact that your cat is not a natural host for heartworms makes it much harder for your veterinarian to detect their presence than in dogs. That is because infected cats have fewer of the worms present. That means that in cats, the heartworm antigen detecting tests your veterinarian uses are less accurate than they are in dogs – even if they specifically say on them that they are designed for cats. Those tests include the Witness® Heartworm Antigen test sold by Zoetis, the SNAP® Heartworm RT test sold by IDEXX Laboratories, and the DiroChek® Heartworm Antigen Test also sold by Zoetis. I marked them in the photo above with a green dot. If any of those tests are positive, your cat is almost certainly infected. But if the results are negative, it could be that your cat has only one or more mature male heartworms. Only mature female heartworms contain the specific protein that these tests look for. It could also be that it is too early in the infection for a female heartworm to produce the specific female heartworm protein that these tests are designed to detect. It could also be that some protein still exists from a female heartworm that died. Heartworms do not live as long in cats as they do in dogs. 

So, if your cat is an outside cat, or if it lives in an area where many dogs are infected with heartworms, or if your cat is showing asthma-like symptoms, or if your veterinarian has other reasons to be suspicious that heartworms might be the underlying cause of a health issue, your vet might suggest performing a test that looks for antibodies that cats infected with heartworms produce. Those testes are more difficult to perform, and more difficult to interpret. So, other than perhaps the HESKA Solostep® test kit, your vet will probably suggest that a blood sample from your cat be sent to a regional or national veterinary diagnostic laboratory. Heska was recently purchased by Mars, which adds to their capture of a large chunk of the veterinary industry. The Mars stable current includes all VCA Animal Hospitals,   Blue Pearl Veterinary Hospitals,   Banfield Pet Hospitals, and Antech Diagnostic Laboratories.  

Other Things That Might Give Clues:

Your Veterinarian’s Physical Exam?

There is very little that your veterinarian will observe in a yearly, “wellness” examination that would specifically indicate that a cat has heartworms. Occasionally, your veterinarian, using his/her stethoscope, might hear a heart murmur. But until a cat is in crisis, due to a heartworm obstructing its heart/lung circulation, it usually appears quite normal. Think of it as something similar to a human stroke or embolism, you feel fine until you don’t feel fine. 

Your Cat’s Bloodwork Results?

If certain of your cat’s white blood cells, eosinophils and/or basophils, are abnormally high in its blood laboratory report, your veterinarian might think about heartworms as one of the many possible causes. However, many cats with heartworms have normal eosinophil and basophil counts. Elevated eosinophil counts (eosinophilia) are said by some to be found in only 30% of heartworm-infected cats, and they tend to drop back to normal as time goes by. (read here) Asthmatic cats often also have elevated eosinophil counts. (read here) Cats that recently ingested a large number of round worm eggs might also have elevated eosinophil counts, as might cats with EGC. 

A Microscopic Blood Examinations For Heartworm Larva?

I mentioned to you that heartworms cannot complete their normal life cycle in your cat. In dogs with at least one mature female and one mature male heartworm, the female worm(s) pass their larva (microfilaria) into the dog’s blood stream in great numbers. They can be easy for your veterinarian to see microscopically when they are present. But in cats, it is more common for their immune systems to kill heartworm microfilaria shortly after they are born. (read here) Since there are rarely any of these juvenile heartworms present in your cat’s blood, these tests are rarely helpful in cats.

Chest X-rays?

When a thoracic x-ray of your cat’s heart and lungs shows that the arteries leading from its heart to its lungs are enlarged or twisted (tortuous), your veterinarian might become suspicious of heartworms. That is particularly true if your cat’s lifestyle fits, or used to fit, the pattern of cats at most risk for heartworms, and it is of an age that is younger than when other heart/lung related problems are likely to occur. These changes can be very subtle and difficult to see. They include changes in your cat’s lung density, possible heart enlargement, lung over-inflation, and chest fluid accumulation. So, it is always wise that a board-certified veterinary radiologist or cat internist reviews the films. None of these changes, when present, are specific for heartworm disease – but they do add to suspicions.  (read here  & here)

What Treatment Might Cure My Cat Of Heartworm Disease?

Unfortunately, melarsomine (Immiticide®, Diroban®) which is FDA and EMA-approved for use in dogs, is quite toxic to cats. So, your veterinarian is left with few treatment options. Even if your cat is positive on both the antigen and the antibody tests for heartworms, and even if the worms have been identified on an echocardiogram; but your cat is not having respiratory difficulties, It is often best to just provide it with a stress-free life and good nutrition and hope that it will eliminate the worms on its own. We call those “subclinical” cases. I mentioned that heartworms do not live as long in cats as they do in dogs. You might ask your veterinarian if he/she thinks a period of time taking the drug doxycycline might speed that heartworm death. We know that drugs in that class weaken heartworms that contain a bacteria called wolbachia. Eliminating that bacteria with drugs like doxycycline is beneficial to heartworm-positive dogs. We do not know what the effect is in cats. Then, your veterinarian might suggest that the antigen heartworm test be repeated in 6–12 months, in the hope that it is negative. At that time, improvement in your cat’s x-ray or ultrasound examination would be a wonderful event as well. There might be benefit in putting your cat on a monthly moxidectin dose as well. The problem is that anything that destroys heartworms that are present in your cat liberates the toxins that they contain. That can be fatal, and there is no way for your veterinarian to predict if that will occur. 

If your cat is already showing signs of heartworm-related illness, most vets would suggest a course of oral prednisolone. Prednisolone is a corticosteroid. Although prednisolone can have a number of unwanted side effects, it is highly effective in reducing inflammation. If your cat is already quite ill, or becomes seriously ill during its therapy, corticosteroid drugs are best given by injection. In those cats, bronchodilators such as albuterol (Ventolin®) heart support medications, and even oxygen therapy might prolong your cat’s life or get it through a crisis. 

Surgical removal is an option for some heartworm positive cats. The procedure is delicate and expensive, and the owners of cats and their veterinarians know that outcomes are uncertain, and the risks are high. There are a very limited number of veterinarians performing this procedure. In involves passing a snare similar to the ones in my diagram through your cat’s right jugular vein or, alternatively, surgically opening the left side of your cat’s chest (a thoracotomy). Which is preferred depends on the heartworm’s location and the surgeon’s preference. Tearing a worm in the process of its removal has led to fatal anaphylactic shock. 

How Can I Prevent My Cat From Catching Heartworms?

Indoor Living Is The Best Way

As I mentioned at the beginning of this article, keeping your cat indoors greatly decreases its risk of becoming infected with heartworms. It also lessens the chances of your cat developing a host of other maladies. Allowing your cat out of doors (on a leash) only when the average temperature is below 57 F/13.9 C would also be helpful – but less certain. (read here)

Provide Your Cat With A Monthly Heartworm Preventative

It is true that indoor living does not entirely eliminate heartworm risk, as long as the chances for mosquito bites remain. If mosquitoes frequently bite you inside of your house, they might bite your cat as well. Most of us live our lives attempting to minimize our risk, not entirely eliminate risks. But if you decide to give your cat a monthly heartworm preventative, My preference is for a monthly topical moxidectin product. It sold in combination with imidacloprid (for fleas) as Advantage Multi® aka Advocate® as well as generic equivalents with the same formula. Whichever product you purchase, be sure you see a picture of a cat on the package.

The Heartworm Society would prefer you choose option 2. That is because they are sponsored by the veterinary industry that makes these lucrative tests and preventatives. But if you would like to read their take on things, ask me for American Heartworm Society 2016. 

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