Heartworms
in Cats
CAPC Recommendations:
Heartworm (Dirofilaria immitis)
Life Cycle and Stages
Cat Life Cycle
The heartworm life cycle is demonstrated in the figure below:
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Adult heartworms are believed to live for 2 to 4 years in cats.
Disease
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Heartworm disease in the may involve some or all of the following:
o Pulmonary arterial, bronchial, and alveolar disease—Heartworm Associated Respiratory
Disease (HARD)—is associated with the death of developing juvenile worms. Cats may
present with cough, dyspnea, and/or wheezing.
o Death of adult heartworms (if present) can potentiate HARD signs. Sudden death
occurs in approximately 10 to 20% of diagnosed cases. Pathogenesis is unclear, but a
condition (similar to acute respiratory distress syndrome [ARDS]) caused by the
release of antigenic moieties from injured or dying adult worms is suspected.
o Vomiting unrelated to eating may be present.
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
Pulmonary thromboemboli (fragments from dead adult worms) may cause acute
vascular and interstitial inflammatory events that lead to dyspnea and death.
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Neurological signs may indicate aberrant migration of the worm to the brain, eye, or
spinal cord.
Prevalence
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Geographic prevalence of feline heartworm infection (adult worms) generally follows canine
infections, but infection occurs at approximately 10% of the prevalence rate for dogs (see map
below for canine prevalence). Cats are infected with juvenile worms at a much higher rate
than with adult worms; estimates based on necropsy and antibody studies suggest that cats
are infected with juvenile worms at an infection rate similar to that of dogs. The greatest
number of cases is seen in the southeastern U.S., the Mississippi River Valley, and Texas.
© American Heartworm Society
The severity of heartworm incidence as shown in this map is based on the average number of cases per reporting clinic.
Some remote regions of the United States lack veterinary clincis, therefore we have no reported cases from these areas.
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Necropsy-based data (percentage infected) for cats with adult heartworms in shelters is shown
in the figure below.
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A study involving more than 2,000 largely asymptomatic cats was conducted in 19 states
comprising 21geographic regions (see figure below). For pet cats (excepting North Carolina,
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which surveyed shelter animals), the nationwide exposure rates (i.e., percentage of cats that
were antibody-positive) were approximately 12%. Many of these cats were from areas not
highly endemic for heartworm infection. Results do not confirm mature heartworm infection
but imply infection with immature worms and increased risk of HARD.
Host Association and Transmission Between Hosts

More than 70 species of mosquito are capable of transmitting D. immitis. However, only about
22 species are common and important vectors.

Differences in biological behavior and host preferences by mosquitoes can affect the
epidemiology of heartworms in a particular location.
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Transmission can occur anytime infected mosquitoes are active and feeding.
Although L3 larvae taken from mosquitoes that had fed on experimentally infected cats were
capable of infecting puppies, the infrequency, low numbers, and transient nature of
microfilaremia in cats support that in many cases, cats are a dead-end host for D. immitis.
Diagnosis

Because of potentially lower worm burden, single-sex infections, and infrequency of
microfilaremia, heartworm infection is more difficult to diagnose in cats than in dogs.

Microfilariae identification
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Hematological testing
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Fewer than 20% of cats with mature heartworm infection are microfilaremic. Because
of the frequent absence of microfilariae, microfilarial testing (e.g., direct smear,
microhematocrit tube, modified Knott test, millipore filter test) is less useful in the cat
than in the dog. Nevertheless, a positive microfilariae test is more likely to confirm
active adult heartworm infection in the cat.
Eosinophilia (and possibly basophilia), although nonspecific, can support a diagnosis of
heartworm infection. Eosinophilia, often transient, is most frequently observed 4 to 7
months after infection.
Antibody testing

Antibody tests (available as both send-off and point-of-care tests) do not require the
presence of circulating antigen produced by mature female worms for a positive test
result. Different antibody tests may detect migrating larvae of different ages. If an
exposed cat eliminates the infection at the juvenile worm stage, HARD may still result.

Antibody testing may not be effective for diagnosis of feline heartworm disease.
Approximately 15 to 25% of cats with adult infection are antibody-negative.
Conversely, 80 to 90% of antibody-positive cats do not harbor mature heartworms.

Depending on the preventive product used, up to 30% of cats on preventives that are
exposed to heartworms will convert to an antibody-positive state without mature
infection or heartworm-related disease.
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In a limited number of experimental studies, 50% of experimentally infected cats with
confirmed pulmonary pathologic lesions (HARD) seroconverted to a negative antibody
status within 8 months after infection. Within 16 months after infection, 100% of
experimentally infected cats (HARD) were negative for detectable antibodies.
However, after initial infection and induction of HARD lesions, these cats were
restricted from further potential exposure to heartworm-infected mosquitoes.
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Positive antibody tests may or may not support heartworms as a cause of respiratory
signs or lesions. Additionally, a negative antibody test does not rule out current or
previous infection.
Antigen testing
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
Antigen testing is available as both send-off and point-of-care tests.
Only mature female worm infections (typically 7 to 8 months post-infection) are
detected. Rarely, in experimental infections, female heartworms may be detected as
early as 6 months after infection. A positive antigen test indicates that the cat harbors
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mature female heartworms or that mature female heartworms were present but died
recently.
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One third of adult heartworm infections in cats consist only of male worms and will not
be detected using available antigen tests.
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Studies of naturally infected cats examined at necropsy have shown that some antigen
tests are capable of detecting a single mature female heartworm.

A negative antigen test does not conclusively rule out adult heartworm infection nor
will it detect juvenile worms often responsible for pulmonary disease (HARD).
Radiography


The most common radiographic findings in feline heartworm disease are enlargement
of the right caudal lobar artery and a bronchointerstitial inflammatory pattern in the
caudal lung lobes.
Echocardiography

Ultrasound in the hands of a skilled ultrasonographer may detect 68% of naturally
infected cats. However, the caudal pulmonary arteries must be examined to their point
of bifurcation within the lung fields.
Prognosis


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The published life expectancy for cats with adult dirofilariasis is 1.5 years (median).
For cats surviving acute disease beyond day 1, the median survival is 4 years.
Current information suggests that more than 80% of cats diagnosed with heartworm infection
survive the infection.
Control and Prevention

All cats with potential exposure to heartworm-infected mosquitoes should be on year-round
heartworm prevention.

Although cats housed indoors have a lower risk for heartworm infection, studies have shown
that 25 to 30% of heartworm-infected cats were characterized by their owners as “strictly”
indoor cats. Furthermore, certain mosquitoes that typically are found indoors will feed on cats.

Available broad-spectrum heartworm preventives also have label claims for other important
feline internal or external parasites.
Public Health Considerations

Because cats typically are amicrofilaremic, heartworm-infected cats pose little to no public
health risk.
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