AUBURN, Ala., Aug. 6 /PRNewswire/ -- A landmark study (1,2) that shows
conclusively that heartworms do not need to reach maturity to cause
pathology in cats has answered many of the questions surrounding feline
heartworm and greatly extended scientific knowledge about the
Recent efforts by researchers at Auburn University bear out a
hypothesis developed through earlier research and confirm fundamental
differences in the way heartworms affect cats and dogs.(3) In so doing,
these reinforce an emerging consensus that feline heartworm is more
insidious than previously thought and underscore the importance of
The study, which was conducted by A. Ray Dillon, DVM, MS, MBA, DACVIM,
and Byron Blagburn, MS, Ph.D., was recently presented at the 2007 American
College of Veterinary Internal Medicine Forum, and was published in a
special Parasitology Supplement to Veterinary Medicine. Dillon and Blagburn
are affiliated with Auburn's College of Veterinary Medicine and are
longtime leaders in the field of heartworm research.
"This is a major study from people who already have contributed a
tremendous amount to our understanding of heartworm disease in dogs and in
cats," says Clarke Atkins, DVM, Professor of Medicine and Cardiology at
North Carolina State University's College of Veterinary Medicine.
"What they've done is show in great detail that the effects of feline
heartworm infection are more extensive than we previously had known. And
because both diagnosis and treatment of the disease are problematic, the
study highlights once again the importance of prevention."
Julie Levy, DVM, Ph.D., Professor of Small Animal Medicine at the
University of Florida, said, "This work has made a major impact in our
understanding of heartworm disease in cats. It has shifted our focus from
adult worms to more immature stages, and solidly defined feline heartworm
disease as a significant pulmonary syndrome, now defined as Heartworm
Associated Respiratory Disease."
Understanding feline heartworm has always been challenging due to the
difficulties in testing for it and its frequent asymptomatic presentation.
As a result, the affliction -- unlike its canine counterpart -- for years
was thought to be infrequent in occurrence and relatively benign in effect.
But conventional wisdom began to shift after the 1998 Heartworm
Symposium, where 60 percent of the papers presented were on feline
heartworm disease. One of the presentations was a study by Dr. Tom Nelson,
past president of the American Heartworm Society, that showed the
prevalence of heartworm infection in a random sample of cats was higher
than the rates of feline leukemia and feline AIDS.(4)
In 2005, researchers at the University of Florida reported pulmonary
arterial lesions in cats that did not have adult heartworms in the heart
and lungs but were antibody positive.(5) This led the scientists to
postulate that the lesions were caused by the death of immature heartworm
larvae. The hypothesis suggested that the disease followed a fundamentally
different course than canine heartworm, since heartworms in dogs typically
do not cause significant pathology until they reach the adult stage.
The recent Dillon-Blagburn study sought to document in detail the
progression of feline heartworm and better understand the origin of lung
lesions. The study also was designed to assess the efficacy of a preventive
For the experiment, three groups of approximately 10 heartworm-free
animals were used. All three groups were subcutaneously infected with L3 D.
immitis larvae. Group 1 received no preventive medication and worms were
allowed to develop naturally. Group 2 was regularly treated with ivermectin
at 150 ug/kg (Ivomec-Merial) beginning on day 84 of the experiment in an
attempt to mimic the natural death of developing heartworms in naturally
infected cats. Group 3 was given the preventive product selamectin
(Revolution-Pfizer Animal Health) beginning 28 days after infection and
continuing through the remaining part of the experiment (240 days).
What the researchers found was that marked lesions and disease do
indeed result from the death of immature heartworms in the lungs of cats.
Specifically, in Group 2 -- the cats in which worm larvae were killed to
simulate natural death of developing heartworms -- lung arterial lesions
were consistent with the findings from the Florida study of naturally
infected cats. Additionally, lesions in the alveoli, bronchioles and
bronchi were observed. The lesions were just as severe as those which
developed in Group 1, the untreated animals, despite the fact that no worm
fragments were recovered from eight of the nine cats in the group. These
findings indicate airway disease, in addition to arterial disease, occurs
in cats infected with heartworm larvae, despite the fact the worms may
never develop to the adult stage.
Predictably, the cats in Group 1 developed live adult heartworms, with
a mean of 4.3 live worms recovered from the animals. In contrast, the cats
in Group 3, the animals that had been treated with selamectin, harbored
neither adult worms nor evidence of immature heartworms in the lungs.
Matthew W. Miller, Professor of Cardiology in the College of Veterinary
Medicine and Biomedical Science at Texas A&M University, said the Auburn
study argues against long-held conventional wisdom that heartworm does not
pose a significant problem for cats. At the same time, he said, it should
raise awareness among practitioners that asthma-like symptoms can, in fact,
"I think this will prompt us to be more aggressive and proactive with
prevention," he said.
Lynelle Johnson, Associate Professor of Medicine and Epidemiology at
the University of California-Davis and an expert on feline respiratory
illness, agreed that the Auburn study greatly extends the understanding of
feline heartworm's pathophysiology and could help provide greater insight
into feline respiratory problems in the long term.
Because of the study, Johnson said, "We may see changes in the
incidence of lower respiratory disease over time as more cats in
heartworm-affected areas are placed on preventive. But it's important that
we understand how much baseline disease there is now and then use that for
comparison in the future."
Atkins of North Carolina State University said the Auburn experiment
will likely continue to produce new science going forward. "This was a very
broad and ambitious study, and I think we're just scratching the surface in
terms of what we will ultimately gain from it," he said. "I'm sure they
have a lot of data that they haven't had time to look into yet."
Levy added, "I hope these results will raise awareness among
practitioners that heartworms are a threat to cats even if adult worms
never develop. It appears that even transient exposure to immature
parasites can leave cats with substantial lung pathology that may persist
long after any trace of the parasite has been eliminated. Knowing this,
practitioners can take a more aggressive stance in promoting heartworm
preventive use in cats."
Nelson of the American Heartworm Society said the work by Dillon and
Blagburn should help defeat lingering skepticism in the veterinary
community about pathogenicity in infections with juvenile heartworms in
"If you're not able to see something and not able to diagnose it,
you're inclined to believe it isn't there," he said. "But you very quickly
become a believer when you fully understand what has been demonstrated in
these recent studies. It's like smoking. You can't necessarily see it, but
the damage is being done."
For more information please visit http://www.knowheartworms.org
(1) Dillon, AR, Blagburn, BL, Tillson, DM, Brawner, WR, Welles, B,
Johnson, C, Spenser, J, Kaltenboeck, B, Rynders, PE. College of
Veterinary Medicine, Auburn University, Auburn, AL. Immature Heartworm
Infection Produces Pulmonary Parenchymal, Airway, and Vascular Disease
in Cats. ACVIM Research Abstract, June 2007.
(2) Blagburn, BL, Dillon, AR. Feline Heartworm Disease: Solving the
Puzzle. Veterinary Medicine Parasitology Supplement. March 2007;7-14.
(3) Dillon, AR, Warner, AE, Molina, LM. Pulmonary parenchymal changes in
dogs and cats after experimental transplantation of dead adult
Dirofilaria immitis. Proceedings of the Heartworm Symposium '95.
American Heartworm Society, Batavia, IL, 1995;97-111.
(4) Nelson et al. Incidence of Dirofilaria immitis in shelter cats in
southeast Texas. Recent Advances in Heartworm Disease: Symposium '98:
American Heartworm Society, 1998;63-66.
(5) Browne, LE, Carter, TD, Levy, JK, Snyder, PS, Johnson, CM. Pulmonary
arterial disease in cats seropositive for Dirofilaria immitis but
lacking adult heartworms in the heart and lungs.
Available Topic Expert(s): For information on the listed expert(s), click
Dr. Tom Nelson
SOURCE AHS; AAFP; Pfizer Animal Health