What To Do When Your Cat Has Heartworms

Ron Hines DVM PhD

Current Heartworm Diagnosis, Treatment & Prevention 

Heartworm disease is caused by a parasite, Dirofilaria immitis . This is a parasite of domestic dogs and their wild relatives; but given the chance they will also attack your cat.

How Common Is It For Cats To Have Heartworms?

I mentioned to you that Nature designed heartworms to live in dogs. You can read about those canine infections and their treatment in dogs here .  When by mistake an infected mosquito transfers one or more microscopic heartworm larval (microfilaria) to your cat, the parasite’s life cycle does not progress in its normal fashion. The parasite’s genetic code “road map” was designed to navigate and complete its life cycle successfully in dogs; and cats differ too much from dogs genetically for the parasites to successfully complete their life cycle in cats. After being injected into your cat by a mosquito, a lot of the larval heartworms get lost on their way to the heart and are destroyed by your 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 reproduce their next generation of microscopic heartworm larva. So cats are dead end hosts – unable to transfer any larval heartworms through mosquitoes to other cats or dogs. Veterinarians call this phenomenon (event) of wrong species or wrong location an aberrant host/aberrant location situation. ( read here  ) Us humans are not entirely immune to this problem either. ( read here )

For every 10-12 dogs that develop one or more mature heartworms in their heart and 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 )  In contrast the   feline immunodeficiency/AIDS   virus infection is found in 17-23% of outdoor feral cats (read here & here ) Neither of the cat heartworm studies determined how many of those exposed cats actually developed a mature heartworm. The blood of ten of the cats in the Spanish study contained proteins found only on mature female heartworms so some of them certainly did. And since their is no blood test for male heartworms, we only know that the total number carrying heartworms of either sex must have been greater. Whatever the numbers, your cat’s likelihood of being exposed to canine heartworms is directly related to the likelihood of it being bitten by mosquitoes and the prevalence of heartworms in the dogs and wild canids where you live.

How Would My Cat Catch Heartworms?

As I just mentioned, for your cat to catch heartworms, it must be bitten by a mosquito. So cats that spend much of their time out of doors are at a much greater risk. However, female mosquitoes are attracted to the carbon dioxide that we exhale as well as our body heat and perspiration. 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 you get, the more the risk that your cat will be bitten as well. ( read here )

There are several reasons cats are more resistant to heartworm infection than dogs. The first I already mentioned is that heartworm larva 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 them much easier. When 100 heartworm larva are injected into a dog, 75 adult heartworms have been known to develop. When the same 100 larva are injected into a cat, only 3-10 heartworms developed. However because maturing heartworms are more inflammatory to cats than to dog and because cats are smaller in size than dogs, each worm that does develop in your cat has the potential to do significantly more damage to your cat than it would to a dog. That is particularly true if it is a female heartworm; female heartworms can be as long as 12 inches/31 cm, males only as long as 7.5 inches/19 cm. The girth of mature female heartworms is considerably wider as well. So blood flow around mature heartworms in cats is considerably more restricted.

What Signs Might I See If My Cat Has Heartworms?

In dogs, the signs of heartworm disease are primarily the signs of heart failure due to the mechanical blockage of blood flow through the heart and lungs 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 do to any heartworms it encounters. These worms generally locate themselves in the pulmonary artery or its branches going from your cat’s heart to its lungs. So the signs you will see at first are 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) – signs one might easily mistake for asthma or hairballs. These signs are apt to occur about three months after your cat was bitten by an infected mosquito which is the time it takes for immature heartworms to arrive at the cat’s heart and lungs. These breathing difficulties then often reoccur several months later when the heartworms die prematurely and begin to break apart. ( read here )

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 Can Heartworms Be Diagnosed In My Cat?

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

Your Veterinarian’s Physical Exam

There is very little that your veterinarian will see in a routine physical examination that would suggest a heartworm problem in your cat. Occasionally your veterinarian might pick up (hear) a heart murmur. But until a cat is in crisis it usually appears quite normal.

Bloodwork Analysis

If certain blood cells, eosinophils or basophils , are abnormally high in your cat’s blood, your veterinarian might think about heartworms as one of the possible causes. However, many cats with heartworms do not have high eosinophil or basophil counts and when they do, there are many other causes . Elevated eosinophil counts (eosinophilia) are said by some to be found in only 30% of heartworm-infected cats and it tend to drop back to normal as time goes by. ( read here ) Asthmatic cats can also have elevated eosinophil counts ( read here ) as can cats that have recently been exposed to round worms

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 worm, heartworms pass their living larva (microfilaria) into the blood stream in great numbers and they are often easy for your veterinarian to see microscopically thrashing about in a drop of blood or concentrate on porous filters to identify. But in cats, it is more common for their immune systems to kills these larva shortly after they are born. ( read here ) Since there are rarely any of these juvenile heartworms present in your cat’s blood, heartworm diagnostic tests that rely on finding them are rarely helpful in cats.

A Heartworm Antigen Tests

Although not as accurate in cats as they are in dogs, these tests are currently the best screening methods veterinarians have available to detect heartworms in your cat. They detect products (antigens) in your cat’s blood stream that are only released by heartworms. The most popular one is probably the  Idexx Feline Heartworm Antigen Combo Test  If the test is positive for heartworms, your cat has one or more adult worms. However, It can 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 even multiple (smaller) male worms.  But a negative test result is always comforting and does decrease the level of suspicion. Some suggest these tests are perhaps 60-80% effective in detecting heartworm-infected cats. Tests that rely on the detection of antibodies your cat might produce in the presence of a heartworm infection become positive earlier but sometimes miss infections later on. Antibody tests can also remain positive after the parasite(s) have already been destroyed by your cat’s immune system.  

Chest X-rays

When a thoracic x-ray of your cat’s chest shows that the arteries leading from your cat’s heart to its lungs are enlarged, twisted (tortuous) or inflamed, your veterinarian might become suspicious of heartworms. These changes can be very subtle and difficult to detect. They include changes in your cat’s lung density, possible heart enlargement, lung over-inflation, and chest fluid accumulation. So it is always best if a board-certified veterinary radiologist or cat internist reviews the results. None of these changes, when present, are specific for heartworm disease – but they do add to suspicions.  ( read here & here )

Echocardiography

Echocardiography is a sophisticated test that allows veterinarians to see the movement of blood within the heart, blood vessels and cat’s lungs. If heartworms are present, they can sometimes be detected by veterinarians experienced in using this technique. If they are seen, we know heartworms are the cat’s underlying problem. But even when present, heartworms can be hard or impossible to visualize in cats depending on how small they are and where they are located. ( read here )

What Damage Do Heartworms Cause To My Cat’s Lungs And Heart?

Many of the scientific article links I gave earlier describe in detail how heartworms in cats exert their damage. Because cats are smaller than most dogs and because their immune systems are more sensitive to the presence of heartworms, each worm has the potential to cause significantly more damage in your cat than it does in a dog. Heartworms primarily produce a lung disease in cats – not a heart disease. The symptoms veterinarians see in cats are due to the blockage of important arteries in and to the lungs and the lung inflammation their presence induces. When a living heartworm and the inflammation it causes blocks one of the pulmonary artery or one of its major branches, the lung tissue downstream and upstream from the blockage is injured and the cat’s ability to breath is affected. That is what 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, your cat might feel much better – for a while. 

What Treatment Might Cure My Cat?

Successfully treating heartworm infection in your cat is much more difficult for your veterinarian than when his/her patient is a dog. That is because of the more violent ways cats react to dying and dead heartworms in their circulatory system. As importantly, cats do not handle the only dog-approved medication ( melarsomine ) well. ( read here )

Supportive Care and Time

Because of the high likelihood of melarsomine toxicity in cats and lack of another drug option, your vet might suggest medications that help prevent inflammatory reactions cause by the heartworm(s) along with drugs that are thought to weaken heartworms and speed their demise.  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 2-4 years it takes for the heartworms to die naturally. ( read here )  During that 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 those symptoms be controlled with prednisolone.  Every six months, another x-ray and a heartworm antigen test can be run. A less intense or even a negative antigen test and improvements in chest x-ray pathology are good indication that your cat is eliminating the heartworm(s) on it’s own.

Should your cat becomes worse during the monitoring period, more steroids (prednisolone, etc.) , intravenous fluids (balanced electrolytes) , bronchodilators, oxygen and cage rest are often enough to get your pet through those rough periods.

Prednisolone

The efficacy (potency) of prednisolone and prednisone are thought to differ in cats due to differences between the two drugs in their amount of intestinal absorption when given orally. Their potency is also thought to differ because of how well the feline liver converts prednisone into prednisolone (its more active form) between dogs, humans and cats. That can be overcome by increasing the dose of prednisone given, but most veterinarians prefer to just give prednisolone to cats to avoid the problem. Corticosteroids like prednisolone are quite effective in cats in lessening the lung inflammation caused by heartworms. However too much corticosteroid causes its own set of problems. So the dose and period of time they are given to your cat must be closely considered. If there is x-ray evidence that significant lung pathology is occurring in your cat, your vet might decide that prednisolone, in decreasing doses over time, is a wise treatment addition. 

Moxidectin/Ivermectin

It has been  found that ivermectin, when combined with doxycycline, can be effective in slowly destroying heartworms in dogs. Moxidectin , a drug similar to ivermectin, seems even more effective because it persists in the cat’s body longer. It may turn out that these drugs are effective in treating heartworms in cats. However not enough feline cases have been closely followed as of yet to decide one way or the other. Even if effective in killing heartworms in cats, the cat might still have to overcome the severe anaphylactic-like reactions that can 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 these drugs in relation to their body weight than cats can. In dogs, the use of moxidectin or ivermectin to kill adult heartworms is often called the “Slow Kill” method. ( read here )

Doxycycline

Wolbachia is a bacteria that lives inside of heartworms. Many veterinarians believe that Wolbachia might be responsible for some of the blood clots and and malaise (illness) that heartworms cause and that eliminating Wolbachia weakens adult heartworms. Because of this, many veterinarians  pre-treat dogs with an antibiotic, doxycycline or minocycline, that destroys Wolbachia prior to administering other drugs. Heartworms and Wolbachia have a symbiotic relationship. I can not tell you how they help each other but it is assumed that they do. Similar to the use of moxidectin or ivermectin in heartworm-infected cats, I know of no controlled studies of its use in heartworm-infected cats. Discuss its use with your veterinarian. Doxycycline capsules and tablets have been known to lodge in the throat (esophagus) of cats. So they should always be accompanied by water and food.

Surgical Removal Of The Heartworms

If your veterinarian explains that your cat’s cardiovascular system is failing, you do not have the option to wait for the heartworms to die naturally. The only alternative is to attempt to physically remove the heartworm(s). Depending where the parasites are, a highly skillful veterinarian can sometimes visualize the heartworm(s ) using ultrasound imaging  . While viewing the ultrasound image, it is sometimes possible to enter a cat’s right jugular vein or chest and extract the parasites using various extractors that have been modified from those used in human cardiac procedures. A small group of veterinary specialists have refined the used this procedure successfully. ( read here  & here )

How Can I Prevent My Cat From Catching Heartworms?

Indoor Living

Keeping your cat indoors certainly decreases its risk of becoming infected with heartworms. It lessens the chances of your cat developing a host of other maladies as well. But indoor living does not entirely eliminate heartworm risk as long as the chances of mosquito bites remain. Allowing your cat out of doors (on a leash) only when the average temperature is below 57 F/13.9 C is also helpful – but less certain. ( read here )

Provide Your Cat With A Monthly Heartworm Preventative

My preference is monthly moxidectin , sold in combination with imidacloprid (for fleas) as Advantage Multi® aka Advocate® for Cats But ivermectin (Heartgard for Cats®) , milbemycin oxime (Interceptor for Cats & Dogs®) and selamectin-containing products (Revolution for Cats®) are all acceptable too as long as you see a picture a cat on the package. Please don’t assume that similar products you already have at home and which were purchased for your dog are safe to give to your cat based on your cat’s body weight. There are many drugs, including these, that cats do not safely tolerate in as large a dose/pound or /Kg body weight as dogs do. 

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