Feline Heartworm Infection

By Wendy C. Brooks DVM, DABVP Feline Heartworm Disease

Section of normal cat lung.

White areas would be full of air.

Section of lung from a cat with circulating larval

heartworms. The cells of inflammation have

thickened the tissues so that oxygen absorption

is challenged and there is far less room for air.

Thanks to the American Heartworm Society for these pictures. Images courtesy of Dr. Ray

Dillon and Dr. Byron L. Blagburn, Auburn University College of Veterinary Medicine

Do Cats get Heartworm?

The answer is an unequivocal yes but the feline situation is vastly different from the canine

situation. While it is true that the feline infection is not as common as the canine infection,

the feline infection has recently been found to be a much more widespread problem than

previously believed. In the past, a common statistic was that within a given geographic

area, the feline heartworm infection rate was approximately 10% of the canine infection

rate. Recent research indicates this is not so; in heartworm endemic areas, the incidence of

feline heartworm infection rivals or surpasses that of feline leukemia virus or feline

immunodeficiency virus . An incidence of 2% to 14% of all cats has been reported for

endemic areas, making heartworm a concern for any cat living where there are mosquitoes.

The Parasite and its Migration

•

• The cat is not a natural host

for the heartworm, which

means the migrating larval

heartworm is not likely to

complete its life cycle. The

migrating worm uses

molecular sign posts to tell it

how to get to its host’s

pulmonary arteries. The

worm is prepared to read

CANINE directions and may

get lost in the feline body,

ending up who knows where.

Most of the larvae that

actually make it to the

pulmonary artery die soon

afterwards due to the

massive immune attack from

the feline body. Very few

larval heartworms survive to

adulthood in cats.

• Whereas a moderate

heartworm infection in a dog

would involve 25 to 50 adult

heartworms, infected cats

typically have less than six

adult worms. Because the

feline heart and blood

vessels are so small, these

few worms can wreak havoc.

In a dog, six worms or fewer might not be considered worth treating. In a cat, a

single worm could easily represent a lethal infection.

• Whereas worms found in the canine heart can reach lengths up to 14 inches, the

average length of worms found in feline hearts is only 5 to 8 inches long.

• While an adult heartworm can expect to live 5 years in a dog, it will only live 2 to 3 years in a cat, probably due to the cat's strong immune reaction.

Heartworm disease in cats is caused by the inflammatory reaction generated by

the worm’s presence.

In dogs, heartworm disease is mostly about the obstruction of blood flow from the

physical size of the worms.

See an animated depiction of how heartworm infection causes disease in a cat, courtesy of Pfizer Animal Health.

Symptoms of Disease

The cat's immune system is extremely reactive against heartworms. For this reason, it is

virtually impossible to detect microfilariae in an infected cat. (The cat's immune system

removes them too quickly.) Also, symptoms of infection tend to be more immune-related

than heart-failure related. Cats develop more of a lung disease, complete with respiratory

distress, and chronic coughing or vomiting. Feline heartworm disease is often misdiagnosed as feline asthma. Sudden death may occur just as it may occur in infected dogs.

In cats there are two phases where the disease can exert symptoms. The first is when

immature worms reach the lung and pulmonary arteries, as early as 75 to 90 days after

infection. Even small worms are inflammatory and disruptive to the circulation. Cells of

inflammation infiltrate the lung and interfere with the cat’s ability to breathe. The second

phase where problems can occur is when the worm dies. Since cats are not the natural host

for this parasite, most immature worms that make it to the lung are killed. The presence of

the dead worm is extremely inflammatory. (Imagine your body trying to remove or digest the dead body of another animal inside your lung and circulation!)

The effects of this kind of widespread inflammation can reach far beyond the lung and

circulatory system. The kidney can be affected as well as the gastrointestinal tract and even

the nervous system.

Heartworm disease is primarily a lung disease in cats, not a heart disease.

Diagnostic Testing

In dogs, diagnosis is usually not complicated. A blood sample is tested for proteins that can

only be found in the body of the adult female heartworm. In cats, disease is not dependent

on adult worms so this kind of testing has limited applications. Furthermore, there may be

too few adult worms to generate a positive antigen test.

In dogs, testing for microfilariae (off-spring of adult heartworms born in the host’s body) is

also commonly performed. Unfortunately, in cats microfilaria testing is virtually worthless.

First of all, infected cats usually do not have enough adult worms to produce off-spring.

There may be only a few adult worms and single sex infection is common. Microfilariae, if

any, are simply cleared too quickly to be found reliably. As mentioned, in a cat heartworm

disease stems at least in part from migrating immature larvae. No adult worms (and thus

no off-spring) are necessary for disease so testing for microfilariae is not worthwhile in a cat.

Antibody testing may be more sensitive but is not adequate by itself. A negative antibody

test is good evidence that a cat is not infected; however, a positive antibody test may

indicate several things. It could indicate a mature infection. It could indicate immature

worms in the body. It could also indicate a past infection. (Antibody levels will remain

somewhat elevated after the heartworms have long since died of old age.) This means that

a positive antibody test should be accompanied by some kind of signs of heart disease

(either symptoms or radiographic or ultrasonographic evidence) or with a positive antigen

test before making a diagnosis of active heartworm infection in a cat.

So if no single test is reliable, what are we supposed to do for testing? The American

Heartworm Society currently recommends using both an antigen test and an antibody test

for screening apparently healthy cats. If a cat is sick and heartworm disease is suspected,

both these tests are recommended, plus chest radiographs and/or echocardiography to

assess heart and lung disease.

Treatment

Since the major signs of disease in cats are due to inflammation and immune stimulation, a

medication such as prednisone can be used to control symptoms. In general, if the cat does

not appear sick, the American Heartworm Society recommends attempting to wait out the worm's 2 to 3 year life span and simply monitor chest radiographs every 6 months or so.

The same heartworm adulticide therapy used in dogs is best not used in cats as it is

extremely dangerous to do so and is considered a last resort. There may not be a choice,

however, depending on the degree of illness from the heartworm disease. Approximately

one third of cats receiving heartworm adulticide therapy will experience life-threatening

embolic complications when the worms die suddenly (generally an unacceptable statistic).

One month of cage confinement is typically recommended to control circulatory effort after adulticide treatment.

Prevention

The good news is that feline heartworm infection is 100% preventable and there are currently four products on the market that are reliably effective.

The dose of ivermectin (active ingredient of Heartgard) needed to prevent heartworm

infection in cats is about 4 times higher than that in dogs. Heartgard was the first FDA-

approved heartworm prevention medication available for cats. It is a monthly flavored

chewable available by prescription. The American Heartworm Society recommends testing

prior to administration.

Interceptor® also makes a monthly chewable for cats with the same active ingredient

(milbemycin oxime) as Interceptor for dogs. Interceptor for cats also protects against hookworms and roundworms.

Revolution® entered the anti-parasite scene in 1999. This product covers fleas,

roundworms, hookworms, and ear mites in addition to preventing heartworm in cats.

Uniquely, this product is applied topically rather than orally. Read more information direct

from the manufacturer of Revolution.

Advantage Multi® is a new product from Bayer which combines imidacloprid for flea control

and moxidectin for heartworm preventive in one product. Read more information direct from the manufacturer of Advantage Multi®.

The American Heartworm Society recommends monthly preventive for ALL

CATS in heartworm endemic areas. Read their feline guidelines.

Other helpful sites include:

www.knowheartworms.org

www.petsandparasites.org/cat-owners/heartworms.html

Date Published: 1/1/2001

Date Reviewed/Revised: 1/11/2008