What To Do If Your Cat Has Heartworms

Ron Hines DVM PhD

Current Heartworm Diagnosis, Treatment & Prevention 

Heartworm disease is caused by Dirofilaria immitis . It is a dangerous parasite of dogs that occurs worldwide – but it can also attack your cat.

How Common Are Heartworms In Cats?

Heartworms were designed to live in dogs. (ref) When a baby heartworm is transferred to a cat by mistake, it is not as happy as it would have been in a dog. Many of the heartworm larva get lost and die before they reach the cat’s heart. And the few that do reach the heart do not live as long and do not reproduce. So for every ten dogs that develop heartworms, one cat does. Looking at it another way, probably as many cats have heartworms as have feline leukemia.

How Would My Cat Catch Heartworms?

For your cat to catch heartworms, it must be bitten by a mosquito. So cats that go outside are much more at risk. If heartworm disease is common in dogs in your area, mosquitoes that bit your cat are carrying heartworm larva. If you have an indoor-only cat and mosquitoes get into your home in various ways and annoy you, then your cat is also at risk. The more mosquito bites, the more the risk.

There are several reasons cats are more resistant to heartworm infection than dogs. The first is that heartworm larva are genetically designed to find their way through a dog’s body – not a cat’s. The second is that heartworms probably employ a stealth technology to avoid being destroyed by a dog’s immune system. The immune system of cats finds and destroys them much easier. When 100 heartworm larva are injected into a dog, 75 adult heartworms develop. When the same 100 larva are injected into a cat, only 3-10 heartworms develop. However, each worm that does develop in a cat does significantly more damage.

What Signs Would I See In My Cat If It Had Heartworms?

In dogs, the signs of heartworm disease are primarily the signs of heart failure due to mechanical blockages caused by the worms. But in cats, the signs are primarily due to the inflammation the parasites cause.Your cat’s immune system reacts much more violently than dogs to any heartworm it encounters. These worms locate themselves in the pulmonary arteries going from the heart to the lungs.

So the signs you will see at first are are all related to lung inflammation. These include coughing, vomiting, and difficulty breathing.   These signs are technically called H.A.R.D. (heartworm-associated respiratory disease) – signs one might easily mistake for asthma

These signs are apt to occur about three months after the cat was bitten by an infected mosquito which is the time that they arrive at the heart and lungs. These breathing difficulties then often reoccur several months later when the heartworms die prematurely and begin to break apart.

If your cat develops H.A.R.D. and has a history of going outdoors, your veterinarian may make a quick diagnosis. But often, the signs are muddled and it is unclear what the problem is. Everything from collapse, convulsions, mopyness, blindness, not eating, weight loss, fainting to sudden death have all occurred with heartworms as the unknown root cause.

How Are Heartworms Diagnosed In My Cat?

It is easier to diagnose heartworms in dogs than in cats. There is no single test that allows your veterinarian to identify every cat that is infected. So screening with combination of several tests is usually best. Here are some of the ways we decide:

A Physical Exam:

There is very little that your veterinarian will see in a routine physical exam that would suggest a heartworm problem in your cat. Occasionally, we pick up a heart murmur. But until the cat is in crisis, the cat usually appears very normal.

Bloodwork Analysis:

If certain blood cells, eosinophils or basophils are abnormally high in your cat’s blood, your veterinarian may think about heartworms. However, many cats with heartworms do not have high eosinophil or basophil counts and when they do, there are many other causes .

Microscopic Blood Examinations For Heartworm Larva:

Heartworms can not complete their normal life cycle in cats. In dogs, heartworms pass living larva into the blood stream in great numbers and they are often easy to see in a drop of blood. But in cats, the immune system usually kills these larva shortly after they are born. Since there are rarely baby heartworms (microfilaria), heartworm diagnostic tests that rely on finding them rarely work in cats.

A Heartworm Antigen Test:

These are currently the best screening tests available for heartworms in cats. They detect products in the cat’s blood stream that are released by heartworms. The most sensitive one your veterinarian can perform in the office is probably the Idexx Feline Heartworm Antigen Test which is sold combined with tests for feline leukemia and feline AIDS . If the test is positive, your cat has heartworms. However, It may take 5-8 months after the bite of an infected mosquito for this test to become positive. It also pushes the limits of the test’s sensitivity to detect an infection of a single female worm or multiple (smaller) male worms.

Chest X-rays:

When a thoracic x-ray of your cat shows that the arteries leading from the heart into the lungs are enlarged, twisted (tortuous) or inflamed, your veterinarian may become suspicious of heartworms. These changes can be very subtle and difficult to detect, so it is always best if a board-certified veterinary radiologist or cat internist reviews unclear results.


Echocardiography is a sophisticated test that allows veterinarians to see the blood vesicles within the cat’s lungs. If heartworms are present in these blood vesicles, they can sometimes be seen by a veterinarian experienced in using this technique. When they are seen, we know heartworms are the cat’s underlying problem. But even when they are there, they can be hard or impossible to see – depending on how small the worms are and where they are located.

Anti-Heartworm Antibody  Test:

Your veterinarian can send a blood sample from your cat to an outside laboratory to be tested for antibody against heartworms. (ref) This test is very accurate and a positive result means the cat has been exposed to heartworms. But many cats whose immune system has killed off all the heartworms, still have a positive antibody test. So the test is most valuable when another method of testing for the actual worms is also positive.

What Do Heartworms Do To My Cat’s Heart?

Because cats are smaller than most dogs and because their immune systems are more sensitive to the presence of heartworms, each worm causes significantly more damage in a cat.

In cats, heartworms are primarily a lung disease – not a heart disease. The symptoms we see in cats are mostly due to blockage of important arteries in the lungs.

When a living heartworm and the inflammation it causes blocks one of these pulmonary arteries, the lung tissue downstream from the blockage is injured and the cat’s ability to breath is affected. This causes the coughing, wheezing and gasping that we commonly see. It is common for the cat’s body to find new ways to get blood past these blockages. So within a few days of the initial attack, the cat may feel much better – for a while. When the heartworm eventually dies and disintegrates, inflammation in the lungs is more widespread and severe.

What Treatments Might Cure My Cat?

Successfully treating adult heartworms in cats is much more difficult than in dogs. This is because of the violent way cats react to dead heartworms in their circulatory system and the fact that cats handle the only approved medication (melarsomine designed for dogs) poorly.

Supportive Care and Time:

The only approved medicine (Immiticide) that kills heartworms well in dogs is very toxic to cats. So if your cat is not showing signs of illness, it is often best to just support the cat’s general health and wait the 2-3 years it takes for the heartworms to die naturally.

During this time, periodic chest x-rays can monitor the situation. If the x-rays show an increasing problem with lung blockage and inflammation, the Heartworm Society recommends they be controlled with prednisone Every six months, another x-ray and an antigen test can be run. A less intense or negative antigen test is a good sign that the cat is eliminating the heartworm(s) on it’s own.

If the pet becomes ill during the monitoring period, more steroids (prednisone, etc.) ,intravenous fluids, bronchodilators, oxygen and cage rest are often enough to get the pet through rough periods.


This corticosteroid is very effective in cats in relieving the lung inflammation caused by heartworms. Too much corticosteroid causes its own set of problems so the dose and period of time they are given must be closely worked out.

Immitacide®/Diroban® (melarsomine dihydrochloride):

Unfortunately, cats just don’t handle this drug well. About one out of three heartworm-positive cats that receive Immitacide will get very ill or die after receiving it. This is not so much from the drug itself, but from the lung reactions the cat has to the dying heartworms.


It has been  found that ivermectin, when combined with doxycycline, can be effective in destroying heartworms. In dogs, moxidectin a drug similar to ivermectin, seems even more effective. Probably because moxidectin remains in circulation longer than ivermectin. It may turn out that these drugs are  preferable to immitacide in treating heartworms in cats. Not enough feline cases have been closely followed yet. However, cats will still have to overcome the severe anaphylactic-like reactions that occur when the heartworms die. When either of these drugs is given to cats, the dose must be exact. Dogs can tolerate much larger amounts of ivermectin in relation to their body weight than cats can.

Surgical Removal Of The Heartworms:

If your cat’s cardiovascular system is failing, you can not wait for the heartworms to die naturally. In some cases neither you nor your veterinarian feel that the pet will survive immitacide treatment. In that situation, the alternative is to attempt to physically remove the heartworm(s). A very skillful veterinarian can visualize the heartworm(s ) using ultrasound. While viewing the ultrasound image, it is sometimes possible to enter the cat’s right jugular vein and extract the parasites. Many veterinary specialists have refined and used this procedure successfully. (ref)

What is Wolbachia?

Wolbachia is an organism that lives inside of heartworms. Some veterinarians now believe that Wolbachia is responsible for some of the blood clots and and malaise (sickness) that heartworms cause. Because of this, more and more veterinarians are pre-treating with an antibiotic, doxycycline, to destroy Wolbachia prior to administering Immiticide or ivermectin.

How Can I Prevent My Cat From Catching Heartworms?

Several medications are effective in preventing heartworm infections in cats.

Keeping your cat indoors certainly decreases the risk of heartworms. But it does not entirely eliminate it. The American  Heartworm Society  refers to a study in which 25% of the cats that tested positive for heartworms were considered “indoor cats”. If you give a monthly preventative, it is easiest to give it year round. Using it only during mosquito “season” when average daily temperature is above 57 F. is a less sure method.

There are 4 heartworm-preventing products approved in the USA for use in cats. My personal preference is for ivermectin – but they are all effective.

Heartgard® For Cats (oral ivermectin):

Do not use Heartgard tablets that were sold for dogs. Even if your cat weighs the same as the dog for whom the pill was designed, it will not be effective in your cat. That is because the amount of ivermectin needed to prevent heartworms in cats is about 4 times as much as required to protect dogs. Heartgard comes in a flavored chewable tablet. It is always wise to have your cat tested for heartworms before beginning a heartworm preventative.

Revolution® (selamectin ,topical):

Revolution kills a number of parasites including heartworm larva. It also prevents fleas, ear mites and a number of intestinal parasites. It is a liquid that you drop between the fur of your cat’s neck – which is much easier than giving your cat a pill. Be sure that you place it high on the scruff of the neck where the cat can not lick it off. An occasional cat will loose fur at the application site. A few others will develop diarrhea.

Advantage Multi® aka Advocate® for Cats (imidacloprid and  moxidectin)

Produced by Bayer Pharma to compete with Pfizer Pharma’s Revolution, Advantage Multi®(aka Advocate Spot On®) is a product which combines imidacloprid for flea control and moxidectin for heartworm prevention in one product. The image link is to their dog product. You need their cat product. It has not been widely used long enough for there to be much feedback on its safety and effectiveness.

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