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Wednesday, July 14, 2010

Parasites in Cat Hearts

When I was in vet school, I was taught that cats don't get heartworms. Well, that was good, because there was one less thing to worry about in my feline patients. Unfortunately, they taught us wrong. Cats DO get heartworms, and probably more often than we realize.

The problem is, heartworms behave very differently in cats than in dogs. In the cat, heartworms rarely become adults. It is a tiny larval form of the worm that usually causes all the trouble. And it does cause trouble! These larvae reside in the cats' airways where they may cause significant damage, even though the cats may show no symptoms at all. After a few months, these immature worms die and cause a much greater inflammatory response in the lungs resulting in coughing, difficult breathing, and sometimes death. Cats with these symptoms may be misdiagnosed as having asthma.

You see, there really is no good diagnostic test for heartworms in cats. They can even be missed at autopsy. I wonder how many cats over the years I thought had asthma that really had heartworms. To make matters worse, there's no good way to safely cure a cat that has heartworms. We just treat the symptoms and hope for the best.

The solution is to prevent cats from getting heartworms to begin with. Since heartworms are spread by the bite of a mosquito, all cats (EVEN INDOOR CATS!) are susceptible. I recommend that all my feline patients take monthly heartworm preventative. My favorite is Revolution. It's a topical spot-on product applied once a month which also does a great job at killing fleas and intestinal worms. It is a prescription product, so you can't get it at Rural King, the pet store, etc... My opinion is that all cats should be on Revolution year-round.


  1. Great blog, doctor. Just a couple questions related to heartworm. Since heartworms are spread by the bite of a mosquito, why should cats (or dogs) be on heartworm preventative in the winter months in Illinois when temps are too low for mosquitos? I've read some studies stating that the infective mosquito larvae can only mature when there are sustained temps about 57 degrees F for at least two weeks? So, why would we use year round heartworm meds? Seems sort of like putting on sun screen at nighttime.

  2. Excellent question! First of all,you are correct about the necessity of sustained mild (not warm!) temperatures being necessary for the larvae to become infective. In fact, up until about 10-15 years ago the AVMA and American Heartworm Society recommended that dogs be taken off HW prevention in cold winter months and restarted in the spring. So that's what we recommended as well.

    Unfortunately, an unacceptably high number of dogs were getting heartworms. We believe that this was mostly due to the fact that once a dog is off preventative they must be retested before starting again (a heartworm-infected dog can have a lethal reaction to HW preventatives), and many people would not get their dogs retested and restarted on prevention until well into the warm months. By this time, a number of these dogs had already become infected.

    Less common scenarios involve dogs who might actually get infected during a warm streak in the winter, and dogs who travel to warm climates with their families during the winter.

    At any rate, it became obvious that keeping dogs on year-round prevention was a pretty reasonable and effective solution. So the AVMA and AHS now make that recommendation.

    It's a little less complicated for cats since they can be started on preventative without a test. I am more comfortable with cats being on preventative during the warm months (April-November in Illinois), but I still think year-round is the way to go since HW prevention is relatively inexpensive for cats and an infection could be fatal.

    I hope this isn't too confusing! Let me know if it doesn't make sense to you.

  3. That makes sense. Thanks for the thoughtful response.

  4. Hey, it's good to know someone is reading this!