Objective—To characterize risk factors, clinical findings,
usefulness of diagnostic tests, and prognosis in
cats with naturally occurring heartworm infection (HWI).
Animals—50 cats with Dirofilaria immitis infection.
Procedure—Medical records, thoracic radiographs,
and echocardiograms were reviewed and findings
compared with appropriate reference populations.
Results—Findings suggested that male cats were
not predisposed to HWI, domestic shorthair cats
were at increased risk, and indoor housing was only
partially protective. Fewer cases of HWI were identified
in the final quarter of the year, compared with
other periods, and prevalence is not apparently
increasing. Signs of respiratory tract disease were
most common, followed by vomiting. Infection was
diagnosed incidentally in > 25% of cats; conversely,
10% of infected cats died suddenly without other
clinical signs. Serologic tests were most useful for
diagnosis, followed by radiography and echocardiography.
Eosinophilia supported the diagnosis.
Overall median survival time was 1.5 years but
exceeded 4 years in cats surviving beyond the day
Conclusions and Clinical Relevance—Sex does not
appear to be a risk factor for HWI in cats, and indoor
housing provides only incomplete protection. Signs
of respiratory tract disease (dyspnea and cough) are
the strongest indicators of HWI in cats, and some
radiographic evidence of infection is detected in
most cases. Antibody screening for HWI in cats is
efficacious, and antigen testing and echocardiography
are most useful for making a definitive antemortem
diagnosis. (J Am Vet Med Assoc 2000;217: