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Name
Heartworm Disease, Feline
Short Description
Feline Dirofilaria immitis infection
Affected Animals: Dogs and cats. Overall, cats are infected by
heartworms much less commonly, even in areas in which heavily
infected dogs are present. Male cats tend to be infected more
commonly and have a larger worm burden than female cats, mostly
because male cats spend more time roaming outside and have less
resistance to infection than female cats. Cats that spend
significant amounts of time outside, especially in
mosquito-populated areas, are at increased risk for developing
heartworm infection.
Overview:
Heartworm disease is a serious infection of the
heart by parasitic worms called Dirofilaria immitis. It is not
uncommon for infected cats to develop fatal respiratory and
cardiac-related complications. Other cats, however, will have only
minor symptoms such as vomiting, or no clinical signs at all.
Heartworm disease is spread by mosquitoes, which inject the larvae
from the heartworm parasite into the skin when they bite. Thus,
outdoor cats are at higher risk of infection, as they have an
increased exposure to mosquitoes. Certain sections of the world have
heavier populations of heartworm disease than others; cases of
feline heartworm have been found most frequently in the eastern and
mid-western states and in California.
Because of increased availability of tests to detect feline
heartworms and a greater awareness of the disease, more
veterinarians are able to diagnose the disease. Depending on the
severity of the infection, there are a number of methods for
treatment, including medications to alleviate the symptoms, drugs to
kill the worms, and surgery.
Clinical Signs:
The clinical signs of feline heartworm infection
can be acute or chronic. Pulmonary or central nervous system signs
are seen more often in acute cases. Chronic signs may include
episodic pulmonary disorders, gastrointestinal disorders, lethargy,
or right-sided congestive heart failure. Nonspecific symptoms may
include coughing, dyspnea, vomiting, anorexia, lethargy, and weight
loss.
Some cats may exhibit no signs at all other than
vomiting. Acute pulmonary thromboembolism resulting in severe
dyspnea, weakness, vocalization of distress, and sudden death are
not uncommon manifestations of heartworm disease in cats. Physical
exam findings that could indicate heartworm disease include
respiratory abnormalities, vomiting, heart murmur or gallop heart
rhythms, syncope, and bloodwork suggestive of parasitic infection.
Symptoms: Coughing and trouble breathing are the most
common signs of chronic heartworm disease. Weakness, vomiting, and
acute respiratory distress are also seen.
Description:
Dirofilaria immitis is a parasitic worm that
usually develops within the right side of the heart or the adjoining
large vessels. Infection is diagnosed more commonly in dogs, which
appear to be more susceptible to infection than cats.
Infection is spread to cats when a mosquito bites the skin,
injecting infective heartworm larvae into the bloodstream of the
cat. Some of the heartworm larvae travel to the heart and develop
into the adult stage. Unlike heartworm infections in the dog, cats
have fewer infective offspring that mature into the adult stage, a
lower number of adult worms at one time, and adults that live for a
shorter period.
Heartworms can be a serious disease in cats and can result in sudden
death, if for example, it leads to a blood clot that travels to the
lungs. However, some cats will show minor symptoms, or no clinical
signs at all. Symptoms, if present, are nonspecific for heartworm
disease and require a thorough physical exam and testing to rule out
other possible causes.
Heartworm infections are difficult to suspect,
diagnose and treat in cats due to the relative absence of clinical
signs in most cases, the unreliability of feline heartworm tests,
and the high risk of serious side effects from treatment. A
veterinarian in general practice may refer a cat suspected of having
heartworms to a veterinary cardiologist for specialized testing and
evaluation.
Treatment is controversial and generally reserved for cats suspected
of having large worm infections with recurrent symptoms. Heartworm
disease in some cats may be self-limiting, since the worms
eventually die on their own, and thus the condition goes undetected.
Heartworms should be considered in the list of diseases suspected of
causing signs of respiratory distress, heart disease, and vomiting.
Preventive medication is available and recommended for cats living
in geographic areas where the disease is prevalent, such as the
eastern and Midwestern states, and California. Cats should be tested
for heartworms prior to starting prevention. No adverse reactions to
heartworm preventive treatments have been reported in cats.
Diagnosis:
Diagnosis can be difficult, because several other
diseases can mimic the signs of heartworm disease. Disorders that
exhibit similar symptoms include asthma; cardiomyopathy, or
abnormality of the heart muscle; lungworm infection; lung fluke
infection; and pleural effusion disorders, which cause fluid
build-up in the space surrounding the lungs.
Routine bloodwork results may indicate a parasitic infection, but
cannot positively identify heartworms. Thoracic radiographs, or
chest x-rays, may help support the suspicion of heartworm infection,
although abnormalities in the heart can be difficult to detect
through this method. Definitive diagnosis of heartworm infection
usually depends on the demonstration of adult heartworms by
echocardiography, an ultrasound of the heart and vessels. Other
techniques for positively identifying heartworms include pulmonary
arteriography, a dye injection to visualize the lung arteries
through x-rays, or detection of adult heartworm antigens via blood
serology tests that detect adult heartworm antigens.
Echocardiograms cannot rule out heartworm disease definitively if
there is a low worm count, but these tests are beneficial in ruling
out other possible heart diseases. Cats need to be referred to a
veterinary cardiologist for an echocardiogram or pulmonary
arteriography.
Tests for the presence of heartworm offspring are available, but a
negative test in no way rules out heartworm infection. Most feline
heartworm cases have a low number of microfilaria, or offspring,
that are present for a very limited time.
The most useful blood tests are for adult worm antigens, and
moderately sensitive test kits are now are available in many
veterinary practices. The antigen detected is believed to come from
the reproductive tract of female adult worms. If less than three
females are present in a cat, the test results may be falsely
negative, even though adult worms are present. However, a positive
heartworm antigen test gives the most definitive evidence of feline
heartworm infection.
Some laboratories also can perform an enzyme-linked immunosorbent
assay, or ELISA, test for antibodies in the cat's blood sample. This
test is better at picking up infections, but a positive test result
does not indicate whether the worms are still living.
Prognosis: Complications of the heart and respiratory
system, such as a blood clot traveling to the lungs, carry a poor
prognosis and commonly result in acute death. Other animals will
show mild symptoms or no clinical signs at all. It can be very
difficult to detect abnormalities in cats with heartworm infections,
to accurately diagnose the disease, and to provide any form of
treatment.
Transmission or Cause: Infection with the Dirofilaria immitis parasite
occurs when a mosquito that previously has bitten an infected animal
bites another animal, thereby injecting worm larvae into the new
host. Infections in cats are much less common than in unprotected
dogs in the same locale. Possible reasons include a mosquito
preference to feed from dogs versus cats, a difference in exposure
to infected mosquitoes, and a lower number of infective larvae
developing into adults within the cat.
Treatment: Cats diagnosed with heartworm disease
that lack any clinical signs should not receive any form of
adulticidal heartworm treatment, which kills adult worms, and can
have fatal side effects in cats. Rather, the treatment of heartworm
disease with adulticide medications should be reserved for those
cats with persistent, serious clinical signs in which worm removal
either is not an option, or it has failed to improve the signs of
disease.
Some veterinary cardiologists have performed the surgical removal of
adult worms from the infected cat's heart. This is not a common
feline procedure, though, and it requires referral to an experienced
surgeon with access to the proper equipment. Also, this surgery is
usually attempted only in those animals in which an ultrasound has
revealed a large number of worms.
Severely ill cats can be treated symptomatically to alleviate signs.
Cage confinement, oxygen supplementation, anti-inflammatory drugs,
and fluid therapy are commonly instituted. Once the cat is stable,
heartworm treatment options can be considered.
Prevention: Feline heartworm prevention is now commercially
available through veterinarians as a once-a-month treatment. This
preventive measure is recommended for all cats living in areas with
high heartworm counts.
