AU veterinary professors release landmark study on feline heartworm disease

Ray Dillon (right) and Byron Blagburn examine a cat at the AU College of Veterinary Medicine. The professors’ research has shed new light on feline heartworm disease
Ray Dillon (right) and Byron Blagburn examine a cat at the AU College of Veterinary Medicine. The professors’ research has shed new light on feline heartworm disease

Study finds cats are affected by heartworms

AUBURN – A landmark study led by two Auburn University veterinary professors has proven that immature heartworms cause long-lasting lung disease in cats, a finding that dispels the notion that heartworms only affect dogs.

“This redefines the disease in cats and it emphasizes the need for prevention, ” said Ray Dillon, the Jack O. Rash Chair of Medicine in AU’s College of Veterinary Medicine. “The previous thought was that only adult heartworms were significant, but that is clearly not the case.”

The culprits are immature heartworms that grow to only 1 to 2 1/2 inches long and cause Heartworm Associated Respiratory Disease. Heartworm infection takes place when a mosquito carrying infective, microscopic-size heartworm larvae, bites into a cat or dog. In cats, many heartworms die three to four months after infection and disintegrate in the lungs, leaving the disease and creating lung tissue damage. This occurs even if the cat never gets an adult heartworm, and he says it is a clinical disease that starts three months after the mosquito bite, not the six-month time usually associated with adult heartworms in dogs.

“Previous studies focused on the heart for cats, but we now know that research should have looked more at the lungs for answers,” said Byron Blagburn, a parasitologist and AU Distinguished University Professor. “This was due to thinking that the heartworms would affect cats just like dogs. They are more likely to infect dogs’ hearts, but cats’ lungs.”

Heartworm prevention for dogs has been stressed for years, says Dillon, because heartworms in dogs can live three to five years in the heart and grow to 16 inches long.

“The damage is obvious in dogs, but immature heartworms in cats are like juvenile delinquents and hit-and-run drivers,” Dillon said. “They come in and create lung disease and leave no evidence directly related to heartworms.”

Dillon and Blagburn recently presented the study at the 2007 American College of Veterinary Internal Medicine Forum in Seattle and had it published in a parasitology supplement to the scientific journal Veterinary Medicine.

Immature heartworms that die in the lungs cause severe inflammation. The resulting lesions and the Heartworm Associated Respiratory Disease (the researchers call it “HARD”) remain in the lungs, causing cats to have asthma-like symptoms of coughing, wheezing and breathing difficulty. Some of these cats will have the disease the rest of their lives.

Blagburn says it has been difficult convincing some veterinarians and pet owners that heartworms are a problem for cats. He adds that the respiratory disease is especially a problem in the South because of the long breeding season for mosquitoes.

“It’s a matter of disseminating the information about heartworms, continuing to analyze our data and conducting more studies,” Blagburn said. “Heartworms are very difficult to diagnose in cats and we hope to add new information to help veterinarians spot the disease. Prevention is best, because there is no cure yet, other than treating the symptoms.”

Dillon adds that “many companies have had preventative products on the market for years, but only three percent of cats have been on the medication. Practitioners probably have seen the disease often in cats, but did not recognize it because the testing of blood samples from dogs is specific to adult heartworms only and are not useful in cats. So, cats needing to go on heartworm prevention are not tested prior to the start of medications.”

The next step for the AU researchers is to determine the disease’s long-term effects, find out why some cats get better and heal with minimal lung damage, and learn why others take much longer to heal. “We also need to know how heartworms might interact with other lung diseases, allergies, asthma and other illnesses,” Blagburn said.

Dillon, through his work in AU’s Small Animal Teaching Hospital, suspected for 20 years that the lung disease in cats was somehow related to immature adult heartworms. Ten years ago he did a clinical study of more than 200 cats with coughs at 14 veterinary practices and found signs of a possible connection. Dillon also found that cats kept inside get heartworms as often as outdoor cats in the southeastern United States. In another study he performed in Harvard University’s respiratory biology program, he discovered a unique group of cells in the cat lung that is not present in a dog or human lung, but in cats it controls lung inflammation. In 2003, Pfizer provided $500,000 for Dillon, Blagburn and fellow AU researchers to conduct a definitive study on the connection between heartworms and the disease in cats.

The AU College of Veterinary Medicine funded related studies to collect data beyond the scope of the original study. Dillon said the studies were performed by “a nationally known, all-star cast of people and they are all here at Auburn University.” The research team and their specialties included Michael Tillson, surgery; Bill Brawner, radiology; Calvin Johnson, pathology; Pat Rynders, lab animal health; Betsy Welles, clinical pathology; Dawn Boothe, pharmacology; and Bernhard Kaltenboeck, molecular diagnostics. Numerous technicians and veterinary students also played key roles in the project.

“There is no better example of how beneficial research can be than this study,” Blagburn said. “It was a problem spotted in the clinics, research was conducted and now the findings are being applied back in the clinics.”

(Contributed by Charles Martin.)

Contact: Gary Beard, (334) 844-3699 (, or
Mike Clardy, (334) 844-9999 (