Feline Heartworm Disease
The first reported case of Dirofilaria
immitis in a cat was in 1922 in Virginia.
This cat had 2 adult worms in the right ventricle. Research
into feline heartworm disease has grown significantly since
then, and is recognized as a potentially life threatening
disease.
Feline heartworm disease is very important
clinically because even a light infection is capable of
producing severe, life threatening disease. The regional
prevalence of heartworm infections in domestic cats tends
to parallel canine heartworm infections but at a lower rate.
Cats are less easily infected by heartworms. It has been
shown that dogs are the preferred host and there are indications
that cats are not the ideal hosts.
The life span of the parasite is shorter in
cats and infections tend to be self-limiting after two to
three years. Cats also have a greater tendency to spontaneously
eliminate the infection or die from the infection. Another
reason cats are proven not to be ideal hosts is the aberrant
migration of fourth stage larvae which is more likely to
occur in cats than in dogs. Large numbers of ectopic heartworms
have been found in the body cavities and central nervous
systems of cats.
Other important differences in feline heartworm
disease exist. Unlike dogs, circulating microfilariae are
seldom found in cats. If found in cats, the average time
needed for the worms to produce the larvae is 8 months post-infection
with microfilariae persisting for one month. Cats generally
cannot tolerate as many adult heartworms as dogs. Dogs
can potentially maintain a burden of several hundred worms
whereas many cats can die from fewer than 10 worms.
The lesions caused by heartworms is slightly
different in cats than in dogs. While cats can develop
interstitial lung disease like dogs, there is also an extensive
alveolar type II cell hyperplasia in cats. These parenchymal
lesions may have an important role in the pathogenesis of
acute respiratory distress 4-9 months post-infection. It
is thought that cats mount a more intense immune and inflammatory
response to the adult heartworms which coincides with the
pathogenesis.
Clinical signs are frequently non-specific
and could correlate with many other disease processes.
The most common chronic signs are tachypnea, coughing, and
anorexia. Abnormal lung sounds may be heard but heart mumurs
are rare. Intermittent vomiting that is not associated
with eating is another common sign. There is also a peracute
syndrome that can be seen with feline heartworm disease.
The signs associated with this may include acute respiratory
distress, ataxia, collapse, seizures, hemoptysis, and sometimes
sudden death.
Diagnostic testing for cats is more difficult
than in dogs. Microfilariae can be detected with the modified
Knott's test, however since cats only have microfilariae
for a short time, a negative test does not necessarily exclude
the diagnosis of heartworms. Reasons why the results of
an ELISA antigen test may produce a false negative result
include: immaturity of parasites, too few female worms
to produce detectable levels of antigen, or infection by
solely male worms.
Thoracic radiographs may help confirm a positive
heartworm test. Radiographic evidence of feline heartworm
disease includes enlargement of lobar and peripheral branches
of pulmonary artery. This sign may be limited to the right
caudal lobar artery where heartworms are most commonly found.
Unlike dogs, right-sided cardiomegaly is seldom seen. There
can be main pulmonary artery enlargement in cats but it
is obscured by the cardiac silhouette. Radiology along
with angiography and echocardiography are primarily used
to confirm a tentative diagnosis of feline heartworm disease
rather than as screening tools.
Treatment of feline heartworm disease must
be based on the clinical signs of the individual cat, not
solely on the basis of a positive Knotts or antigen test.
If the cat displays no overt signs of heartworm disease
it is best to allow time for spontaneous elimination of
the parasites. These cases can be monitored every 6 to
12 months for worsening of radiographic signs.
Infected cats with radiographic signs of pulmonary
interstitial lung disease may benefit from a diminishing
dose of prednisone beginning with a dose of 2 mg/kg per
day and gradually reducing it to 0.5 mg/kg every other day
by two weeks and then discontinuing after an additional
two weeks. At the end of treatment the cat should be assessed
radiographically. This treatment may be repeated periodically
in cats with recurrent respiratory signs.
In cats that are not controlled by the above
regime and are stable clinically, adulticide treatment could
be initiated with this acetarsamide at 2.2 mg/kg intravenously
twice a day for 2 days. Post treatment cats should be in
caged confinement and under close observation for 3 to 4
weeks. Owners should be warned that a possible side effect
of treatment is potentially fatal pulmonary thromboembolism.
As always, prevention is the best method of
controlling this potentially fatal disease in cats. Chemoprophylaxis
with a monthly dose of ivermectin at 24 mg/kg is recommended
to cat owners. Even though the heartworm antigen test sensitivity
is low, it is good medical practice to test for feline heartworm
disease before giving chemoprophylaxis for the first time,
if at least 8 months have passed since there was an opportunity
for infection.
- by Jennifer Keenan, Class of 1999
- edited by Victoria
Owiredu-Laast, DVM
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