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Heartworm Disease
Key points
• Feline heartworm disease causes more morbidity than previously believed and definitive diagnosis is often elusive.
• Extensive lesions can occur secondary to circulating larvae, even if the cat successfully clears the infection and no adult worms
develop.
• Feline heartworm disease appears to be a primary factor in a subset of feline asthma cases.
• The acronym HARD (heartworm-associated respiratory disease) was coined to describe the pulmonary pathology resulting
from the death of immature heartworms or the presence of adult heartworms in the lungs.
INTRODUCTION definitive host, the life cycle of the parasite can be com-
pleted in cats, and feline infection rates in endemic
Heartworm disease is a particularly common problem regions are usually 10% to 20% of that of dogs within
in tropical and subtropical regions; however, the disease the same enzootic region. If the disease is present in a
has spread throughout most areas of the United States. local canine population, it is occurring in local cats as
Although the prevalence is still low at high elevations well (Nelson 2008a). The infection rate is lower in cats
and in most northern states, evidence suggests that the compared to dogs for several reasons. Some mosquito
disease is expanding throughout the United States species do not prefer to feed on cats. Additionally, cats
(Nelson et al. 2007). Endemic foci frequently occur in are relatively resistant to infection due to immune clear-
regions with otherwise low prevalence and it is difficult ance of infective larvae, so infected cats usually have
to eliminate heartworms from a region once they have fewer adult worms than infected dogs (usually six or
been established. Wild animal reservoirs include wolves, less), and the lifespan of the worms is shorter in cats
coyotes, foxes, California gray seals, sea lions, raccoons, than dogs (2–3 years compared with 5–7). However, it
and ferrets. Heartworm infection is an increasingly diag- is likely that the morbidity in cats associated with heart-
nosed entity in feline practice due to increased aware- worm disease is underestimated for reasons that will be
ness of the disease in cats and improved diagnostic discussed later in this chapter. Additional evidence that
methods. In spite of these recent developments, diagno- cats are not the definitive host for the parasite include
sis remains a challenge in cats and few cats are treated their typical low and transient microfilarial counts, and
with heartworm preventative. a longer average prepatent period (usually 1–2 months
longer) in cats than in dogs. Finally, aberrant migration
ETIOLOGY AND LIFE CYCLE
of the fourth-stage larvae (L 4 ) occurs more frequently in
Heartworm infection is produced by the parasite cats than in dogs (Nelson et al. 2005).
Dirofilaria immitis and is transmitted to dogs mostly by Female mosquitoes are the intermediate hosts and
10–15 species of mosquitos. Although dogs are the usual acquire the first-stage larvae (microfilariae) by feeding
Feline Cardiology, First Edition. Etienne Côté, Kristin A. MacDonald, Kathryn M. Meurs, Meg M. Sleeper.
© 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
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