Ask Dr. Richards: 04/04

Question: I live in North Carolina and my veterinarian suggests I start my cats on heartworm preventative medication.  I have a dog, too, and he takes a heartworm preventative just like all the other dogs Ive had over the years. My veterinarian tells me that theres no cure for cats and it can be a very serious disease. What do you think about heartworm preventative medication in cats? All my cats stay indoors, so does that make a difference?

Answer: I answered a similar question about feline heartworm disease a little over a year ago in this column, but I think the topic is serious enough to deserve revisiting. Many cat people are unaware of the danger of heartworm disease in their pets. Awareness is high in dog people for good reason: Dogs living in areas where this mosquito-borne disease is present are usually tested for it annually and placed on preventive medication either seasonally or all year round. But ignorance is not bliss. Even though cats are innately more resistant to infection than are dogs (theyre estimated to have only about five to 20 percent of the infection rate in dogs in the region in which they reside), the disease they experience can be at least as devastating, and in some cases may lead to death. 

Sometimes Theres No Warning
Adult heartworms live about half as long in cats as in dogs (typically about five to seven years in dogs, and two to three years in cats). Thankfully, many cats, perhaps the majority, never show serious outward signs of disease and they just go on their merry way. When the heartworm parasites do cause problems, the signs may mimic those seen in a multitude of other feline disorders: intermittent vomiting, coughing, breathing difficulty, neurologic signs (fainting, circling, blindness, seizures), or simply the aint doin right signs of listlessness and a loss of appetite and weight. One of the saddest consequences is sudden death with absolutely no warning. The first case of feline heartworm disease I ever saw came to me as an emergency at 2 AM. This beautiful fluffy cat lived with a doting young couple, and he seemed the picture of health until he suddenly developed severe breathing difficulty shortly before I saw him. His breathing progressively deteriorated over the next several hours and he succumbed in spite of our best efforts to save him. Ive come to believe this is a fairly rare turn of events, but a tragic one nonetheless.

What About a Cure? 
It depends on how you define the word cure.  If your question is whether there are ways to help your cat get through the disease, the answer is yes. Supportive therapy carries the day for most of those in need of a little help (for example, corticosteroids, bronchodilators, antibacterials, and others based upon the cats needs). The worms dont live forever, and once theyre gone, normalcy or near-normalcy eventually returns.

If your question is whether there is a way to kill the worms living in the blood vessels of the heart and lungs, the answer is again yes, but the jury is still out as to whether doing so is helpful or even safe.  When heartworms die, they often induce a severe, life-threatening reaction in the lungs; thats what killed the cat I referred to earlier. Proponents of worm-killing treatments claim that its better to know when the reaction will occur (usually a fairly predictable time after the medication is given) so measures can be taken to assure the best outcome for the kitty.  Among other concerns, though, opponents cite studies in which there is no difference in survival between cats receiving heartworm killing medications and those receiving only supportive care.

An Ounce of Prevention
So, diagnosis is difficult (usually relying on a combination of physical examination findings, laboratory tests, X-rays, and ultrasound examination) and treatment is dangerous. Therefore, I believe heartworm prevention is the way to go. There are now three heartworm preventative drugs approved for use in cats: two as flavored or chewable tablets and one in a liquid formulation placed on the skin. If your cats live in an area where the disease is present in dogs (which is certainly the case in North Carolina), then they should be receiving preventative medication – even if they stay indoors. Both outdoor and indoor cats can become infected.