Cats Susceptible To Heartworms; Diagnosis Can Be Devastating

It used to be a problem only dog owners had to worry about: deadly heartworm disease.

Now, unfortunately, we know cats are susceptible as well. Even indoor cats.

In some parts of the country, almost a third of cats are infected by heartworms, according to VeterinaryPartner.com. And although a heartworm preventative treatment exists for cats, only 3 million are on it. That's only 3 percent of the U.S. kitty population.

Feline heartworm disease is caused by the same parasite that affects dogs, and they become infected the same way dogs do - mosquitoes. But because of cats' physiology, the disease can be a more devastating diagnosis for them.

To complicate matters, feline heartworm diagnosis can be tricky.

"Cats usually exhibit coughing, vomiting and respiratory distress, which is often mistaken for feline asthma," says Diane Perry, a veterinarian at Broadway Cat Hospital in Dunedin. Until recently, she added, the veterinary profession overlooked feline heartworm disease because it was considered mostly a dog disease.

Heartworms do not lay eggs like other worm parasites; instead they bear live baby worms called microfilariae. Those are released into the circulatory system where mosquitoes slurp them up and take them to a new host. In order for a heartworm to reach adulthood, it must be passed through a mosquito. Once a carrier mosquito bites your pet (the new host), the microfilariae are released into the bloodstream and begin to mature into an adult worm.

The adult heartworm is several inches long and prefers to live in the pulmonary arteries. It swims into a cozy tubular artery, where it is massaged and nourished by the blood coursing past it. Its presence generates a strong inflammatory response in the host, and a tendency for blood to inappropriately clot. If enough worms are present, the heart must work extra hard to pump blood through the plugged-up arteries.

"Dogs can go for years before they show symptoms of having heartworms," says Perry. "Cats, however, can get very sick even in the early stages of the disease."

A dog can have several adult worms living in the heart before he begins to exhibit symptoms. A moderate heartworm infection in dogs would involve 25 to 50 adult heartworms up to 14 inches long.

In cats, one or two adult worms can be debilitating.

In cats, arterial openings are significantly smaller. Symptoms appear much earlier as the microfilariae migrate through those openings and cause inflammation. Also, immature worms in a cat's heart can be debilitating, whereas in dogs, it's the adult worms that cause problems.

Cats' immune systems are extremely reactive to heartworms, which makes it hard to detect microfilariae.

A cat's symptoms of infection tend to be more immune-related than heart failure-related. Cats develop more of a lung disease, complete with respiratory stress, and coughing or vomiting. Sadly, sudden death can occur.

Antigen testing is used to detect adult heartworms in cats, but one to three female worms must be present for a positive result. Keep in mind, a cat's system can be distressed with the presence of just one adult worm.

Antibody testing is more sensitive and may detect microfilariae, but is not adequate alone. A negative antibody test is good evidence that the cat is not infected; however, a positive antibody test may indicate several things. It may be the cat has a mature infection. It could be the cat is infected by immature worms. It could also indicate a past infection. So, it's best when a positive antibody test is accompanied by some other indication of heart disease such as symptoms, or radiograph or ultrasound.

"There's no treatment for cats, only supportive care," says Perry. "The drug that is used to treat heartworm disease in dogs is toxic to cats."

Worms can be surgically removed but it's generally not recommended.

In severe cases, a veterinarian may administer a treatment that kills adult worms. But about a third of the cats treated experience life-threatening embolic complications when the worms die suddenly. One month of cage confinement is typically recommended for recuperation after treatment.

If the cat does not appear sick, the American Heartworm Society recommends attempting to wait out the worm's 2- to 3-year life span and monitor chest radiographs every six months or so. A medication such as prednisone can be used to control symptoms.

"Prevention is very important to kill microfilaria before they imbed into the heart and lungs," Perry says.

She recommends a topical anti-parasite treatment, such as Revolution. Some topical brands ward off other parasites as well, including fleas, roundworms, hookworms and ear mites.

In Florida, mosquitoes frequently come right through an open door into your home, so preventing a mosquito from ever biting your cat or kitten is impossible.

As with most illnesses involving our pets, the best way to avoid heartache is prevention.