Tracking An Insidious Yet Preventable Disease
Special Report
by Ray Valek

For nearly 20 years, Stephen Heckler, DVM, has performed necropsies on cats that suffered sudden death. His mission all along has been to find the reasons why these cats -- which seemed perfectly healthy one day -- were suddenly lost to their loving owners the next. "In one week, I had two sudden death cases where the owners thought their cats had been poisoned," he recalls. "Another time, an owner brought in her cat in the morning to be spayed. By noon, it was dead."

On more than 50 occasions, Heckler has found feline heartworm disease to be the cause of death in the cats on which he has conducted postmortem examinations. His findings echo similar findings by veterinarians showing that feline heartworm disease is a factor that should be considered in nearly half of all acute feline cardio-pulmonary illnesses. An insidious illness being detected and reported with increasing frequency by veterinarians across the country, feline heartworm disease is spread by mosquitoes. The mosquitoes take blood meals from animals, usually dogs, that are infected with the heartworm (Dirofilaria immitis) larvae and then transmit the infection to cats.

"If there are mosquitoes and heartworm in dogs in your area, your cats are at risk -- inside or outdoors," says Steve Peterson, DVM, a spokesman for Synbiotics, an industry leader in animal diagnostic products.
      Once heartworms have spread and cats are severely infected, there is not much a veterinarian can do to treat the disease. "You can treat the symptoms," Peterson explains, "which include respiratory distress, diminished appetite, coughing, listlessness and weight loss. "But unfortunately, treatment in cats carries considerable risk. The disease can be deadly to cats -- treated or untreated."

The good news is that feline heartworm exposure and infection can be detected with inexpensive tests. The American Heartworm Society recommends that cats periodically receive these tests at a veterinary clinic. The tests require only a small sample of the cat's blood, which is drawn at the clinic, and the cost is about $20. Results are available in a matter of minutes.

Peterson recommends first running the antibody test, which detects exposure to, but not infection with, heartworm larvae. If the antibody test is positive, he recommends performing the antigen test, which detects heartworm infection. If both tests are positive, the cat has heartworm disease.

If the cat does not have heartworm infection, both Peterson and Heckler recommend giving it a preventive chewable pill, Heartgard(R) for Cats, each month during mosquito season. Cats living near standing water in a region with mild winters should receive the pills, which cost about $5 per month, year round.

Carol Robertson-Plouch, DVM, director of veterinary professional services at Merial Limited, says taking this preventive step will keep the disease away by eliminating the incubation of heartworm larvae acquired during the prior 30 days. She emphasizes that Heartgard is safe for all cats, even those already infected with heartworms, and will provide effective removal and control of hookworms as well.

Robertson-Plouch says the preventive medication should be given to cats regardless of whether or not they spend time outdoors. The reason why indoor cats should take the preventive medication, Robertson-Plouch says, is because many of them spend time in areas accessed by mosquitoes, such as screened or unscreened porches or near windows. Also, at the American Heartworm Society's 1998 meeting, a Brazilian veterinarian, Norma Labarthe, reported the finding of a mosquito species (Culex quinquifasciatus) which carries heartworm larvae and prefers to feed indoors. This type of mosquito is found throughout the United States.

Sometimes, even without the aid of preventive medicine, a cat's immune system will destroy heartworm larvae by building antibodies against these foreign substances. But there are no guarantees. Once the larvae develop into mature adult heartworms, they will travel to the heart and pulmonary vessels. The worms will do more and more damage before symptoms first appear.

Cats infected with heartworms should be monitored closely for complications during treatment. "The worms can be killed once they are in the cat's system, but that may cause other problems," Peterson says. "Cats are very sensitive immunologically and may have severe allergic reactions to components of the worms in their bodies." One reaction is a high risk of thromboembolism -- obstruction of blood vessels. Surgery is sometimes an option to remove the worms, but is costly.

Prevention and early detection of heartworm exposure is eminently preferable. "Some vets are concerned about saving their clients money and make the decision themselves not to give cats the heartworm test and preventive medicines," Peterson says. "Some think it's not a problem, but many vets across the country believe it is."

Because awareness of the prevalence of feline heartworm disease varies among veterinarians, cat owners may need to be assertive to get heartworm detection tests and preventive medicines for their cats. "Some vets are fearful about alarming people when the incidence is quite small," Peterson says. "My response is that it is the obligation of a veterinarian to make cat owners aware of the potential problem and allow them to make their minds up for themselves."

With the detection and prevention of heartworm larvae being safe and inexpensive, Peterson believes many owners will choose to eliminate the risk of the disease entirely. "The consequences of what may happen if the cat is not protected offset the costs," he states. Stresses Heckler, "Cats -- especially those that go outside -- should be put on the heartworm pill. It's really important."

Statistics

Cats have historically been considered susceptible, but somewhat resistant, to heartworms. Recent scientific studies are showing incidence rates may be higher than previous estimates, which generally placed infection rates somewhere between 1 to 5 percent of the cat population, depending on the abundance of mosquitoes. A postmortem study of cats from two animal shelters in southwest Texas showed a heartworm infection rate of 9 percent. Another study looked at heartworm exposure rates in 21 areas across the country where heartworm has been reported in dogs and found an average rate in cats of 12 percent, with rates as high as 33 percent in some areas. A study in Tulsa, Oklahoma, showed 26 percent of the cats tested to be exposed to heartworm larvae -- a percentage that the study's authors estimate may be close to the prevalence in the dog population. According to the American Heartworm Society, infection rates in dogs run from 5 percent in areas with low mosquito populations to as high as 45 percent in areas along the Atlantic coast from Texas to New Jersey and along the Mississippi River and its major tributaries. There is a need for more research into feline heartworm disease, so that veterinarians can get a clearer sense of its prevalence, says Steve Peterson, DVM, a spokesman for Synbiotics.

Contact the American Heartworm Society

American Heartworm Society
P.O. Box 667
Batavia, IL 60510-0667
Telephone: (630) 844-9676
Fax: (630) 892-0818
E-mail: www.heartwormsociety.org

Heartworm Testing

Two companies provide in-clinic diagnostics for feline heartworm testing. Typically, the veterinarian selects the diagnostic test used, not the cat owner:

Synbiotics Corporation
11011 Via Frontera
San Diego, CA 92127-1702

Heska Corporation
1825 Sharp Point Drive
Ft. Collins, CO 80525