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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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