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Heartworm disease has been a major concern for dog owners for
the last few decades. For cat owners, however, heartworm disease barely
registers a blip on our radar screen, with most cat owners wrongly
assuming that cats aren’t susceptible to the disease. In fact, feline
heartworm disease (FHD), though relatively uncommon, is a serious and
potentially fatal disease in cats.
The incidence of FHD is relatively low, and the clinical signs can be
vague and non-specific, often mimicking other conditions, especially
asthma. As a result, many veterinarians are not likely to suspect
heartworms in cats, and the disorder goes undiagnosed or misdiagnosed.
Both cats and dogs can serve as definitive hosts for heartworm disease.
Cats can acquire heartworm disease when heartworm larvae are transmitted
to the cat via the bite of a heartworm-infected mosquito. The larvae
develop into mature adult worms which travel to the heart and lungs.
Cats are biologically less hospitable hosts for heartworms than dogs.
Experimentally, if 100 heartworm larvae are injected into a dog, more
than 70 will survive to adulthood. In cats, however, less than 10 make
it to adulthood. Adult heartworms have a five year lifespan in the dog.
In cats, however, adult worms live for only two years. Cats mount and
intense immune and inflammatory response to the worms. The intense
immune response may explain why adult heartworms have a shorter lifespan
in cats. Although we tend to think of an intense immune response as a
beneficial thing, in the case of heartworm disease, the exuberant immune
response may cause more damage than the parasite itself.
Any age cat of either sex is susceptible to FHD, although middle-aged
males have the highest incidence. Cats housed indoors as well as
outdoors are vulnerable. According to Dr. Gary Norsworthy, owner of
Alamo Feline Health Center in San Antonio and editor of “The Feline
Patient: Essentials of Diagnosis and Treatment”, the most common
findings in cats with FHD are sporadic coughing and labored or difficult
breathing. Poor appetite, lethargy and weight loss are common
non-specific complaints. “Interestingly, vomiting has been reported in a
high percentage of cats with FHD and in some cases may be the only
clinical sign”. Heartworms can cause a sudden obstruction of a vessel in
the lungs, a condition called acute pulmonary thromboembolism. Cats with
this condition can develop sudden respiratory distress and weakness
followed by sudden death. Acute neurologic disturbances such as
incoordination, blindness, and seizures are occasionally seen in
heartworm-infected cats. Congestive heart failure occasionally develops
in chronic cases.
Diagnosis of heartworm disease is much more difficult in cats than in
dogs. Adult heartworms produce microfilariae – little “baby” heartworms.
One method of diagnosis is to look for microfilariae circulating in the
bloodstream. This method is effective in many dogs, but rarely for cats,
because the intense immune response that the cat mounts quickly
eliminates the microfilariae from the bloodstream. Finding these
microfilariae indicates the cat is infected. A negative test is
inconclusive. When the worms reach maturity (approximately 6 months of
age), they shed proteins called antigens into the bloodstream. The cat’s
immune system reacts to these proteins by making antibodies against
them. One method of diagnosis is to look for antigen or antibody or both
in the bloodstream. In dogs, the standard screening test is the antigen
test. In cats, both antibody and antigen tests may be used. Antigen
tests actually detect proteins shed by the adult female heartworms in
the blood of the cat. Antibody tests detect the presence of antibodies
that the cat has made in response to exposure to heartworm larvae and/or
adult heartworms. Each test gives important information, but each test
has its limitations. A positive antigen test reliably indicates that the
cat is infected. A negative test, however, doesn’t necessarily mean the
cat is free of disease. Because the test detects antigens given off by
female worms, infection with male-only heartworms may be missed. While
dogs may be infected with many heartworms, cats may be infected with
only 1 or 2 worms. Low numbers of heartworms means little antigen given
off, and though the tests are highly sensitive, such a small amount of
antigen may be missed by the test. A positive antibody test indicates
that the cat has been exposed to heartworms, but not necessarily the
presence of adult worms in the heart. For example, 50 or 60 days after a
mosquito has transmitted heartworm larvae to the cat, antibodies to
these larvae may be produced. The cat’s immune system, however, might
kill off the larvae before they develop into adults. Antibodies may also
persist after heartworms have died. Cats that have survived heartworm
disease will therefore test positive on an antibody test despite not
having an active infection. The decision of which test to use first
depends on the preference of the veterinarian. Some veterinarians prefer
to antibody test all cats initially, with a negative test generally
eliminating the need for further testing. A positive test indicates
exposure and will require additional testing to see if there are adult
worms in the heart. Additional tests that may be useful include chest
x-rays and cardiac ultrasound.
Unlike dogs, treatment options for FHD are limited. In dogs, drugs are
given, in stages, to kill the adult worms, and then the microfilariae.
Cats don’t have circulating microfilariae, so that stage of treatment
isn’t necessary. Melarsomine, the drug used to kill the adult worms is
generally safe for dogs, but data on its use in cats is limited and
contradictory. Most veterinary cardiologists do not advocate using drugs
to kill the adult worms in cats. Instead, cats are treated
symptomatically, with short-term use of anti-inflammatory drugs to
reduce the intense allergic reaction that develops. “I do not use
melarsomine”, says Dr. Norsworthy. “I treat with the anti-inflammatory
drug prednisone”. Aspirin is often recommended to reduce the risk of
blood clots in the lungs. Although there is still no proven benefit,
giving aspirin is generally harmless and inexpensive. The best method of
dealing with FHD is prevention. The disease is entirely and easily
preventable. There are now three FDA-approved drugs that are marketed
for monthly use in cats. Ivermectin as a chewable treat, milbemycin as a
flavored tablet, and selamectin as a topical formulation. Because the
consequences of FHD are potentially dire and treatment options are
limited, monthly preventatives should be given to cats living in endemic
areas. Dr. Norsworthy is a strong advocate of using heartworm
preventatives, mainly because it can be so difficult to achieve a
diagnosis.
Feline heartworm disease is a serious problem that affects cats
worldwide. Although cats are not the natural hosts for heartworm, they
are at more risk for severe disease once infected. Cat owners and
veterinarians need to be aware of the incidence and risk of FHD. Your
veterinarian can help advise cat owners as to the best prevention
program for cats at high risk.
Updated 2/9/06 |
