New Mexico's Pet Resource FALL 2007


by Nancy Marano

The first time I saw what a dog’s heart looked like with an untreated heartworm infection was on the TV show, “Emergency Vets.” I was so horrified I thought I might throw up seeing all those worms crawling in and out of a heart. But the shock factor worked. I never forgot to give our dog her heartworm medicine after that.

No one talked about feline heartworm, though, because no one considered it to be much of a problem. Recently that view has changed.

Heartworm infection occurs when a mosquito carrying microscopic-sized heartworm larvae bites a cat. The cat isn’t a natural host for heartworm, according to Dr. Wendy C. Brooks, of the Veterinary Information Network. This means that the infection is not likely to make its way to the cat’s heart. The cat’s immune system reacts strongly to heartworms by eliminating them when possible but it takes far fewer worms to cause problems in a cat’s system than in a dog’s.

In those cats who do not eliminate the worms, the larvae travel through the subcutaneous tissue directly under the skin. Cats have fewer and smaller worms than dogs and the worm’s life span is shorter than those worms in a dog. But the heartworms don’t need to develop into adults to cause severe pulmonary distress for a cat.

A new study done at Auburn University proves that feline heartworm disease can develop from larvae rather than adult worms. The larvae affect the cat’s airway passages, which can lead to a diagnosis of feline asthma or acute bronchitis instead of the newly defined pulmonary syndrome, Heartworm Associated Respiratory Disease (HARD). Researchers have learned that even one worm is sufficient to cause death in a cat.

The American Heartworm Society and the American Association of Feline Practitioners have teamed up to save cats’ lives by spreading the word about this disease. Their efforts are sponsored by an educational grant from Pfizer Animal Health. Pfizer also manufactures Revolution®, one of the products used for the prevention of feline heartworm.

According to Dr. Charles Thomas Nelson, past president of the American Heartworm Society, the cat-owning public and the veterinary community have some work to do when it comes to developing awareness of the risks of feline heart disease. Studies show that only 5% of households with cats regularly use heartworm prevention drugs whereas 59% of the households with dogs use a heartworm prevention product.


1. Dogs are the only ones who get heartworm disease. FACT: It affects cats differently from dogs but is just as serious.

2. Indoor cats are safe from feline heartworm disease. FACT: It only takes one mosquito to infect a cat. Since mosquitoes can get indoors, both indoor and outdoor cats are at risk. If a disease carrying mosquito bites a cat, the heartworm larvae enter the cat’s system through the bite wound. A North Carolina study proved that 28% of the cats diagnosed with feline heartworm were indoor only cats.

3. This disease affects a cat’s heart. FACT: The name implies the disease affects and damages only the heart and not the lungs. The disease is often mistaken for feline asthma, bronchitis or other respiratory diseases.

4. It is adult heartworms that cause the disease. FACT: Cat’s don’t need to have an adult heartworm present to show clinical signs of the disease. Larvae are the main cause of feline heartworm. Studies show cats infected with heartworm larvae have significant disease of the small arteries supplying blood to the lungs.

5. It is easy to diagnose feline heartworm disease. FACT: It is difficult to make an accurate diagnosis because negative antigen and antibody tests don’t always rule out the presence of the disease.


Some infected cats may not show any outward symptoms while others do. When the cat doesn’t display symptoms, it may just suddenly die. Then it takes an autopsy to determine that heartworm infection caused the death.

Symptoms associated with HARD include but may not be limited to: coughing, collapse, fainting, anorexia, difficulty breathing, diarrhea/vomiting, weight loss, seizures, rapid heart rate, convulsions, lethargy, and sudden death.


The American Heartworm Society says currently there are no products approved for treatment of feline heartworm infection in the United States. Most cats with heartworm infections are allowed time to have a spontaneous cure. If there is evidence of the infection in the lungs and blood vessels, they can be monitored with chest X-rays every six to twelve months. Supportive therapy with small, decreasing doses of prednisone is recommended if there is X-ray evidence of lung disease.

Severe manifestations of the disease may mean the cat will require additional therapy. This could include intravenous fluids, oxygen therapy, cage confinement, bronchodilators to expand air passages, cardiovascular drugs, antibiotics and nursing care.


Usually the veterinarian will test for antibodies and antigens before prescribing preventatives. Four products currently are FDA approved for use in cats. They are Heartguard® for Cats, from Merial, Interceptor® from Novartis, Revolution® from Pfizer and Advantage Multi™ for Cats.

Cat owners need to be aware of the possibility of feline heartworm disease whether they have indoor-only or indoor-outdoor cats. It is difficult to get exact numbers of cats affected by this disease because there was no accurate test for feline heartworm. Often the first sign was sudden death. There is need for more study into this disease. However, it is safe to say that where canine heartworm is prevalent, feline heartworm is, too. The top five states for heartworm disease are Texas, Florida, North Carolina, Georgia and Louisiana. While New Mexico doesn’t have a high prevalence of this disease, there are reported cases in our state. According to Dr. Nelson, this is a preventable disease. But to prevent something, a person first needs to be aware of it. For further information on feline heartworm disease and prevention please check these websites: and

Nancy Marano is an award-winning author who is owned by two cats, Sammy and Rocky, and a Westie named Maggie May.

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