CLINICAL MANIFESTATIONS IN CATS
Feline heartworm disease can differ markedly from its canine counterpart and may require several diagnostic tests or procedures
to confirm. Although most cats infected with heartworms develop pathologic changes associated with infection, they remain
asymptomatic. It is impossible to predict when and under what circumstances infected cats will develop clinical heartworm
disease, and some cats will be clinically normal even with significant pulmonary pathology.
Cats with clinical heartworm disease usually present with respiratory signs such as coughing, dyspnea, or both, or intermittent
vomiting not associated with eating. Signs can be limited to weight loss or diarrhea without accompanying respiratory signs.
When present, the respiratory signs are similar to those observed with feline bronchial disease, which is frequently described
as asthma by the owners. A small percentage of cats that develop clinical signs may die suddenly. This peracute presentation
also mimics signs of acute dyspnea associated with feline asthma, cardiomyopathy, pleural diseases, or infections with other
pulmonary parasites.4,6 Many of these cats are clinically normal before the acute event.2-7
Events in the Heartworm Life Cycle in Cats
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Events in the developmental cycle of heartworm in cats are depicted in the boxed text titled "Events in the Heartworm Life
Cycle in Cats." In the past, the signs of feline heartworm disease were usually attributed to the death of adult heartworms
and the cat's unique pulmonary reaction to fragments of dead and dying adult worms. Dr. Dillon has, for years, forwarded the
notion that immature worms may die in the lungs of infected cats before they mature to adults.6 These worms arrive in the heart 70 to 90 days after initial infection, and because of their small size (less than 1 in),
they are carried by the pressure and direction of blood flow to the distal pulmonary arteries. The arrival and death of these
immature worms in the lungs at 90 to 120 days after infection can result in coughing and dyspnea.
These same developmental and pathogenic events are less likely to occur in dogs, since virtually all immature heartworms mature
to adult worms. Early death of immature heartworms at 90 to 180 days in cats and the resulting pulmonary disease have been
referred to as the three-month disease cycle, to distinguish it from disease caused by adult worms (six-month or longer disease cycle).6
As mentioned previously, the acute respiratory signs thought to be associated with the death of immature heartworms in cats
may mimic those of feline asthma. Pulmonary signs may be caused by the arrival of immature worms as early as 80 to 90 days
after infection, which is three months before the heartworms begin to release antigen. Thus, during the initial disease process
in cats, commonly used canine heartworm antigen tests, which test for an adult female heartworm antigen, cannot be used to
confirm a diagnosis of heartworm disease. To further complicate the puzzle, accompanying pulmonary radiographic lesions are
not specific for heartworm disease, and worms or fragments cannot be visualized by using readily available imaging techniques,
especially in the presence of immature worms.6
These factors and the diagnostic problems that they impose served as our stimulus to develop a laboratory model of the three-month
disease cycle and to demonstrate that marked lesions and disease can result from the death of immature heartworms in the lungs
of cats.
SOLVING THE PUZZLE: A STUDY
The study protocol was reviewed and approved by the Auburn University Institutional Animal Care and Use Committee. To create
the laboratory model, we acquired heartworm-naïve cats and separated them into three groups. The number of cats in each group
was different because of the cats' availability. On Day 0, all cats were subcutaneously infected with 100 infective l3 larvae
of D. immitis obtained from laboratory-maintained mosquitoes (Aedes aegypti). At necropsy, 10 cats were evaluated in Group 1, nine in Group 2, and 10 in Group 3.
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