Listen as Dr. Karen Becker discusses heartworm infection in cats, how it differs in so many ways from heartworm disease in dogs... and what might be behind the sudden spreading concern about this relatively rare infection in cats.
By Dr. Becker
I'm getting a lot of questions lately from Mercola Healthy Pets readers and clients at my clinic about heartworm disease in cats.
It seems an increasing number of vets are starting to recommend heartworm preventives for cats right along with dogs.
Something that might be spurring this along is that in recent years there has been a lot of buzz that feline heartworm infections in heartworm endemic areas are occurring at the same rate as feline leukemia and FIV infections.
Another reason could be a recent study on the effect of immature heartworms on the health of cats (in dogs, it is mature heartworms that are the problem), and the coining of a new feline syndrome called Heartworm Associated Respiratory Disease or HARD.
Heartworm Infection in Cats vs. Dogs
Heartworm infection is a very different disease in cats than it is in dogs. Dogs are the natural hosts for heartworms -- cats are not.
In dogs, the disease is caused by the physical size of the worms and their ability to obstruct blood flow to the heart.
In cats, the disease is actually caused by an inflammatory reaction to the presence of the worms.
Since the cat is not the natural host for heartworms, very few larvae make it to adulthood in a kitty's body. In dogs, invading worms know exactly how to find their way to the pulmonary arteries. These same worms get lost inside a cat's body. Most larvae that do make it to the pulmonary arteries of a cat are wiped out by a massive immune response.
An average canine heartworm infection involves between 25 and 50 worms, each of which can reach an adult length of 14 inches inside the heart.
Infected cats usually have less than 6 adult heartworms ranging in size from 5 to 8 inches. Adult heartworms live on average about 5 years inside a dog, but only 2 to 3 years in a cat due to the strong immune system reaction.
Unfortunately, because the heart and blood vessels of cats are so small, even a few worms can cause serious and even fatal infections in cats.
Symptoms of Heartworm Infection in Cats
Symptoms of heartworm infection in cats are also very different from those in dogs. In cats, symptoms are the result of an immune system response and lung disease rather than heart-related issues.
The more common signs of heartworm disease in cats are coughing, difficulty breathing, and vomiting. In severe cases there can be a wide range of additional symptoms, including anorexia, diarrhea, weight loss, lethargy, blindness, rapid heart rate, fainting, convulsions, and even sudden death.
In cats there are two stages at which heartworm infection can cause symptoms. The first stage occurs between 70 and 90 days after infection as immature worms migrate to the lungs and pulmonary arteries. These smaller worms cause inflammation and disrupt circulation. The lungs can become severely inflamed, causing breathing problems.
The second stage occurs when the worms die. The cat's body mounts a massive immune system response to the dead worms in the lungs. There is widespread inflammation that can reach beyond the lungs and circulatory system to the GI tract, kidneys and nervous system.
Diagnosing and Treating Feline Heartworm Infection
Another difference between feline and canine heartworm infections is in the diagnosis.
A dog's blood is tested for proteins that are only found in adult female heartworms, because only mature worms present a problem for dogs.
However, cats can get sick from the presence of young and dead heartworms in addition to the adults. So the test used for dogs doesn't always pick up the infection in cats. There might be too few adults or only adult males, so the blood test doesn't pick up the telltale presence of proteins.
Dogs are also tested for microfilaria to confirm a heartworm diagnosis. Microfilariae are the offspring of adult heartworms born inside a dog's body.
The test is useless for cats for a few different reasons: there are too few adult worms to produce offspring, or all the adults are the same sex (this is common in feline heartworm infections), or the immune system kills off the microfilariae very quickly. So there's nothing to be gained by doing microfilaria tests on cats.
Tests for the presence of heartworm antibodies are only partially helpful in cats. A positive test can mean a number of things. There could be a mature infection. There could be an immature worm present. Or there could be a past infection that is being cleared.
A positive antibody test on a feline should be followed up with x-rays or an ultrasound of the heart to look for any signs of heart disease, or with a (positive) antigen test to make sure a definitive heartworm infection is truly present.
Low heartworm counts (less than 3 worms), all male worm infections, or immature infections can result in false-negative test results. When evaluating the results of an ELISA or heartworm antigen test on a cat, a negative test result doesn't rule out infection. But a positive test result is a very strong indicator of an active heartworm infection.
Several studies, which include echocardiographic studies, experimental, and necropsy studies, suggest that less than 40 percent of cats with adult worms are actually antigen-positive.
Treatment of heartworm disease in cats is challenging.
If a kitty doesn't seem sick, the American Heartworm Society recommends trying to wait out the 2 to 3 year lifespan of the heartworms.
If there is concern about underlying disease potential in the heart, periodic monitoring with x-rays is a good idea.
Sometimes steroid therapy is given to control symptoms of inflammation and immune system overreaction in cats, however, because of their potential for serious, long-term side effects, steroids should be given only as absolutely necessary
The drugs given to dogs with active heartworm infection are extremely toxic and dangerous for cats, so they are used only as a very last resort. I don't recommend their use at all.
About a third of cats given these drugs experience life-threatening complications as the worms begin to suddenly die off. A month of cage rest is necessary to help control circulation problems if these drugs are given. Again -- I don't recommend using them at all.
Is Feline Heartworm Disease Something You Need to Worry About?
Now let's take a look at what could be prompting all the sudden interest in feline heartworm disease -- the supposedly new syndrome called HARD or Heartworm Associated Respiratory Disease.
This new syndrome describes "vascular, airway, and interstitial lung lesions caused by the death of immature worms and which is virtually impossible to diagnose definitively." HARD, as it turns out, was coined by a giant pharmaceutical company that happens to make a heartworm preventive for felines.
Other than one abstract1 published in 2007 describing an experiment that used a very small sampling of cats exposed to a very large number of infected mosquitoes, there has been no further substantiation of this supposedly new heartworm-related respiratory syndrome in cats.
A study2 in 2010 on a separate aspect of heartworm disease involved 30 cats with no heartworm exposure, 30 cats naturally exposed and naturally cleared of the infection, and 30 cats naturally infected with adult heartworms.
Changes in airways, vessels and lung tissue were analyzed for all the cats. There was virtually no difference noted in the airways or lung tissue of the never-exposed cats and the naturally exposed and cleared cats.
The results of this study indicate exposure and clearance of heartworm infection in cats does not result in significant changes in the lungs, pulmonary tissue or pulmonary function.
That would seem to challenge the HARD syndrome theory that dead and decomposing worms are wreaking havoc in the lungs of the majority of cats exposed to infected mosquitoes.
There's also been no substantiation of an increased rate of adult heartworm infections in cats. In a fairly recent comprehensive study3 of over 2,000 kitties, 3 percent tested positive for heartworm infection.
Further, the study shows that cats that spend time any time at all outdoors have a two-fold greater chance of exposure than indoor-only cats.
Keep in mind that a cat's increased risk of exposure does not necessarily equate to an increased risk of acquiring an adult heartworm infection. Cats, as I mentioned earlier, are not the natural hosts of heartworms -- dogs are.
Can an aberrant infection -- an out-of-the-blue feline heartworm infection -- occur? Sure it can. But I really believe those rare infections are the exception and not the norm.
Tips for Prevention
How can you reduce the risk of a rare heartworm infection in your own kitty?
- First, keep his immune system strong and healthy. Parasitic infections are less likely in strong, resilient animals.
- Feed a balanced, species-appropriate diet. Don't allow your kitty to be over-vaccinated or overmedicated.
- Finally, keep your cat indoors and away from high mosquito populations during the summer months