Feline Heartworm Disease
Linda Lehmkuhl DVM MS DACVIM – Cardiology
A 7 year old, male castrated, domestic shorthair presented through the
Columbus Veterinary Emergency Service for evaluation of an acute onset of
dyspnea and one episode of either hemoptysis or hematemesis. He was healthy
prior to presentation except for a chronic, intermittent cough suspected to be
asthmatic in origin. He was kept strictly indoors and was current on all
On physical examination, a grade 4/6, right hemithorax, systolic murmur and
increased bronchovesicular sounds associated with tachypnea were detected.
Thoracic radiographs (Figure 1) revealed a focal, well-defined alveolar infiltrate in
the dorsal aspect of the right caudal lung lobe. The left caudal lobar pulmonary
artery was enlarged; the right caudal lobar pulmonary artery was difficult to
evaluate due to the pulmonary infiltrate. There was also a moderate, diffuse
increase in bronchointerstitial pulmonary markings. The heart appeared normal
on the radiograph. A complete blood count and biochemical profile revealed a
lymphopenia (324/ul) and hyperglycemia (180 mg/dl) attributed to stress.
Thyroid hormone level was normal at 2.1 ug/dl. Echocardiography revealed
heartworms in the right atrium, right ventricle and pulmonary artery (Figure 2).
Tricuspid regurgitation was documented by color flow mapping and attributed to
pulmonary hypertension and worms across the tricuspid valve.
Emergency treatment for pulmonary thromboembolism secondary to
heartworm disease consisted of supplemental oxygen; terbutaline (0.01 mg/kg
SQ TID), a beta-2 agonist used for its bronchodilating effects; dexamethasone
(0.2 mg/kg IV) followed by prednisone (1 mg/kg PO BID) to decrease the
inflammatory response; heparin (150 IU/kg SQ TID), to decrease clot
propagation; and cephalothin (22 mg/kg IV TID) to prevent a secondary bacterial
infection in the infarcted lung lobe. The patient improved over the next 36 hours.
On the second day of hospitalization, a median sternotomy was performed.
The cat was pretreated with dexamethasone (0.15 mg/kg IV) and
diphenhydramine (1 mg/kg IM) to prevent an anaphylactic response to traumatic
transection of any worms. Surgical removal of 3 heartworms from the pulmonary
arteries was accomplished through a right ventriculotomy. A right atriotomy was
performed initially, but the heartworms had migrated out into the pulmonary
arteries after anesthetic induction and were not able to be retrieved without the
ventriculotomy. A 10 Fr chest tube was placed intraoperatively and removed 24
hours postoperatively. The cat was released from the hospital 48 hours after
surgery on prednisone (1 mg/kg PO BID), long acting theophylline (25 mg/kg PO
QD), and cefadroxil (22 mg/kg PO QD).
Despite clinical improvement at home, a moderate amount of pleural effusion
was detected 1 week after release from the hospital. The etiology of the effusion
was not determined but it resolved with home furosemide (1.2 mg/kg PO BID)
therapy. Over the next 3 months, furosemide and prednisone therapy were
tapered. The cat continues to do great out 7 months from the initial
thromboembolic event. Feline Heartworm Disease
Feline heartworm disease has received considerable attention in veterinary
professional publications and press releases over the last few years. As the
diagnosis of heartworm disease is being sought after more aggressively and
found more frequently, better diagnostic tests and means of protecting cats from
infection are being developed. However, difficulties remain in the diagnosis and
management of heartworm disease in cats. The significant differences between
feline heartworm disease and its classical canine counterpart will be highlighted
in the following discussion of the prevalence, clinical signs, diagnosis, treatment,
and prevention of feline heartworm disease.
The prevalence of heartworm disease in cats approximates 5 - 20% of the
canine prevalence in a particular geographic area. The worm burden in cats is
usually low, as most have fewer than 4 worms and approximately one third of
infections consist of same sex worms. Circulating microfilariae are found in less
than < 20% of infected cats. Because the cat is an imperfect host for
heartworms, the parasite has a shortened life span of approximately 2-3 years
and a greater tendency for aberrant migration that can lead to central nervous
system signs. Indoor-only cats are also susceptible to infection.
Many cats with heartworm disease may be asymptomatic; however, on the
other end of the disease spectrum are cats that die suddenly. The clinical signs
in cats with heartworm disease can be very nonspecific and may mimic other
diseases such as feline asthma. Clinical signs include: chronic cough, shortness
of breath, intermittent vomiting (usually unrelated to eating), anorexia, and weight
loss. Physical examination may be unremarkable or reveal a right-sided systolic
murmur from tricuspid regurgitation, a gallop, or abnormal lung sounds. Right-
sided heart failure (ascites, jugular distension, hepatomegaly) is quite rare in
The diagnosis of heartworm disease in cats is difficult due to the low worm
burden and the usual occult status of infected cats. A recent Florida (Snyder et
al. JAVMA vol 216, 2000) compared heartworm serum antigen and antibody test
results using commercial laboratories and in-house heartworm test kits with
necropsy findings in 330 animal shelter cats (19/330 affected). The antigen tests
(Heska lab, Animal Diagnostics lab, DiroCHEK, and SNAP) were very specific
(over 96%) and also somewhat sensitive (> 68% for all tests with DiroCHEK the
highest at 79% sensitivity). The antigen tests’ lack of better sensitivity is due to
their inability to detect male and immature female worms. A positive antigen (or
microfilaria) test means a cat has heartworm disease but a negative antigen (or
microfilaria) test cannot be used to exclude heartworm disease.
Feline antibody tests (Heska Lab, Animal Diagnostics Lab, ASSURE,
SoloStep) have traditionally been believed to be very sensitive but not very
specific. Prior to the Florida study, a negative feline antibody test was considered
to provide strong evidence a cat was not currently infected; however, this study
showed that a negative heartworm antibody test should not be used to exclude
heartworm disease. Particularly lacking in sensitivity was the Solo Step if
incubated for only 5 or 10 minutes; it detected only 6 of the 19 positive cats (32% sensitivity) after 5 minutes and 9/19 affected cats (47% sensitivity) after 10 minutes. The Heska Lab had the best sensitivity at 90% when using a 5 U/ml positive-cutpoint value. It is critical to remember that a positive feline antibody test reflects exposure and not necessarily current infection. A positive antibody test should be confirmed with an antigen test, radiographic findings, echocardiography, or angiography and should be supported by the presence of appropriate clinical signs. Thoracic radiographs may help confirm the diagnosis and assess disease severity in infected cats. Enlargement of the caudal lobar (especially the right caudal lobar) pulmonary arteries is characteristic but rarely accompanied by a visible main pulmonary artery bulge. Pulmonary arteries with a width greater than 1.7 X the width of the ninth rib at their intersection are of concern (Figure 1). This is quite different than dogs where the pulmonary artery only needs to exceed the width of the ninth rib to raise concern and a main pulmonary artery bulge is often present. It is also important to remember that left-sided congestive heart failure can also cause pulmonary artery enlargement to this degree in cats. If the pulmonary artery enlargement is from left-sided congestive heart failure, one would expect to also appreciate left auricular enlargement on the VD projection and pulmonary infiltrates of cardiogenic pulmonary edema (Figure 3). Other radiographic findings of feline heartworm disease include bronchointerstitial or bronchial changes or alveolar infiltrates (especially of the right caudal lung lobe) characteristic for infarction. The cardiac silhouette is seldom enlarged. Angiography can be used to demonstrate heartworms in the pulmonary arteries; however, it is much more invasive then echocardiography and rarely pursued. Echocardiography may provide visualization of the worms in the pulmonary artery branches (most often), main pulmonary artery, or right heart (Figure 2). Heartworms are seen more readily in cats than dogs due to the small pulmonary artery size in cats. In experienced hands, echocardiography may be the most sensitive test for feline heartworm disease as approximately 1.5 to 2 cms of the right caudal lobar artery and 0.5 cms of the left caudal lobar pulmonary artery can be visualized. Treatment of heartworm disease in cats is controversial. Most cardiologists do not recommend adulticidal therapy with thiacetarsemide (Caparsolate, Rhone Merieux) due to severe toxic (fulminant noncardiogenic pulmonary edema) and pulmonary thromboembolic (associated with worm death) complications. Melarsomine (Immiticide, Rhone Merieux) is not approved for use in cats. Symptomatic cats are treated with prednisone (2 mg/kg/day PO tapering to 0.5 mg/kg QOD over 2-4 weeks) for mild to moderate respiratory signs and treated more aggressively (oxygen, heparin, bronchodilators) for severe thromboembolic complications (illustrated in the case presentation). Aspirin is not of benefit in cats with heartworm disease and may even be of detriment to the pulmonary parenchymal disease. In general, one allows time for spontaneous cure (worm death in <2 years) to occur and treats the complications as they arise. Affected cats should be put on preventative to prevent further infection.
For cats with severe symptoms or symptoms refractory to prednisone
therapy, removal of the adult worms should be considered. This can be done
surgically, as described above, with appropriate perioperative care and surgical
expertise or by forceps or brush removal under fluoroscopic or echocardiographic
guidance following right jugular venotomy. The major limitation of removal via
jugular venotomy is the inability to retrieve worms in the pulmonary arteries and
Monthly chemoprophylaxis can be accomplished with oral ivermectin
(Heartgard for Cats, 24 ug/kg) or topical selamectin (Revolution,
Preventitives can be given to heartworm antibody or antigen positive cats.
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