Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066

(310)391-6741

www.marvistavet.com

FELINE HEARTWORM INFECTION

FELINE HEARTWORM DISEASE 

Section of normal cat lung.
White areas would be full of air.

Section of a 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. This is the type of lung change typical of Heartworm Associated Respiratory Disease ("HARD")

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

 

CAN CATS BECOME INFECTED WITH HEARTWORMS?

The answer to this question is an unequivocal "yes" but the feline situation is vastly different from the canine situation. In an area where heartworm is common, cats and dogs are infected equally but because heartworm is a natural parasite of the dog and not the cat, the actual disease is not the same. The cat's immune system is much more reactive against the new heartworm invasion so that very few of the young heartworms complete their migration to the heart and pulmonary artery. In the past, a common statistic presented was that within a given geographic area, the feline heartworm infection rate was approximately 10% of the of the canine infection rate. This 10% figure, however, turns out to refer to the incidence of adult heartworms living in the pulmonary arteries and hearts of cats within a regional population and does not reflect the actual infection rate. Most of the invading young worms are killed in a great inflammatory battle in the lung tissue and which makes heartworm disease mostly a lung disease in the cat while it is a heart disease in the dog.

(Photocredit: Morguefile.com)

FELINE HEARTWORM DISEASE
(HEARTWORM-ASSOCIATED RESPIRATORY DISEASE, abbreviated "HARD")

As stated, the cat is not a natural host for the heartworm which means the migrating larval heartworms are not likely to complete their life cycle. To recap from the Heartworm: the Parasite section presented earlier, infection begins with a mosquito bite. Baby heartworms called "microfilaria" are slurped up by the mosquito feeding on an infected dog. The slurped up young heartworms must spend enough time (several weeks) in the mosquito's body to develop into an infective stage at which point it is ready to infect a new host. The infective stage heartworm larva is deposited in a drop of mosquito spit adjacent to a mosquito bite, the larvae crawl into the skin puncture made by the mosquito, gain access to their new host, and continue to develop in the soft tissues of the new host, eventually making their way into the circulation and to the host's pulmonary arteries.

The migrating young worm uses molecular signposts to tell it how to get to its host’s pulmonary arteries where it wants to finish growing up, mate and live out its life. The worm is prepared to read CANINE signposts and does not always migrate correctly trying to understand feline protein signals. The worm may get lost and end up who knows where in the body. If the young worms get to the pulmonary arteries at all they are killed by the especially reactive feline immune response against them. It is this immune reaction that causes heartworm disease in the cat and it can start as soon as 75-90 days after the infecting mosquito bite.

When young heartworms die in the pulmonary arteries, the immune system breaks them into fragments and attempts to remove them. The resulting inflammation leads to lung disease which manifests as coughing, respiratory effort, and vomiting. Not every infected cat shows signs of illness. In one study 28% of cats showed no symptoms but sometimes sudden death is the only symptom.

Heartworm-Associated Respiratory Disease (HARD) mimics feline asthma and the two diseases look identical on radiographs. Cats with HARD will cough, wheeze ( a musical respiratory sign similar to a sigh), and vomit (though it may be hard to tell aggressive productive coughing from vomiting). Breathing may be shallow and rapid and may progress to actual respiratory distress. Heartworm testing is the only way to distinguish these conditions.

VASCULAR DISEASE SEPARATE FROM "HARD"

While in most situations feline heartworm disease is a lung disease and not a vascular disease as it is in the dog, sometimes cats do get adult worms in their pulmonary arteries just as dogs do. These adult worms do not live as long as they do in the canine body and they do not achieve the same length/size. If they do find a mate and give birth to microfilariae, the microfilariae are promptly killed by the feline immune system within the first month. In short, in the feline pulmonary artery, heartworms are much smaller. One would think this would make for milder vascular disease but because cats are so small, even one adult worm takes up a great deal of space in the vasculature. The more usual lung disease is all the worse if the reaction against the immature worms is complicated by the presence of a surviving adult worm in the vasculature. When this parasite finally dies, the subsequent blood clots and inflammation is frequently fatal to the cat. Death can be sudden and unexpected.

Most of heartworm disease in the cat is caused by the inflammatory reaction
generated by the worm’s presence.

In the dog, heartworm disease is mostly about
the obstruction of blood flow from the physical size of the worms.

DIAGNOSTIC TESTING

ANTIGEN TESTING?
In the dog, diagnosis is usually not complicated. A blood sample is tested for proteins that can only be found in the uterus and eggs of the adult female heartworm. Most infected dogs have a population of worms in their arteries, so if even one female worm is present an antigen test to show positive. In the cat, disease is caused by immature worms, not adult worms female or otherwise, so this kind of testing has limited applications. There may be no adult worms present at all to generate a positive antigen test, yet the cat is infected.

MICROFILARIA TESTING?
In the dog, testing for microfilariae (offspring of adult heartworms born in the host’s body) is also commonly performed. Unfortunately, in the cat microfilaria testing is virtually worthless.
 First of all, infected cats usually do not have enough adult worms for the production of offspring. There may be only a few adult worms present; single sex infection is common. Further, microfilariae, if any, are simply cleared too quickly by the host's immune system and are not available for detection. As mentioned, in cats heartworm disease in cats stems at least in part from migrating immature larvae and their is no simple test for these.

ANTIBODY TESTING?
Cats produce antibodies to heartworms as soon as they are infected. The idea with antibody testing is that if antibodies are present, the young heartworm larvae must be present, too. Antibody testing may be more sensitive but is not adequate alone. A negative antibody test is good evidence that the cat is probably not infected; however, a positive antibody test may indicate several things. It could indicate a mature infection with one or more adult worms. It could indicate the presence of 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.)

So if no single test is reliable, what are we supposed to do for testing? First of all, unlike dogs where annual screening is the norm, healthy cat screening is probably not necessary. Instead, testing is best done if a cat is sick and heartworm disease is suspected. In this situation, not only are both the antigen and antibody tests are recommended but chest radiographs and/or echocardiography to assess heart and lung disease should be performed as well.

TREATMENT

Since the major signs of disease in the cat are due to inflammation and immune stimulation, a medication such as prednisolone can be used to control symptoms. A bacteria called Wolbachia commonly lives within the heartworm and enhances its ability to generate inflammation. A course of doxycycline is often recommended to address these bacteria. The doxycycline course is short but the prednisolone will be long term. I f the cat does not appear sick, the American Heartworm Society recommends attempting to wait out the adult worm's 2-3 year life span and simply monitor chest radiographs every 6 months or so. Median survival time is 1.5 years for heartworm infection in cats. Radiographs are monitored to check progress.

One might wonder why one cannot use the same treatment as in the dog to kill any adult heartworms the cat might have. Actually, 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 and adulticide therapy should be consider the last resort for an infected cat where symptoms of the disease cannot be controlled with prednisone.

PREVENTION

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

HEARTGARD® FOR CATS
The dose of ivermectin (active ingredient of Heartgard®) needed to prevent heartworm infection in the cat is about 4 times higher than that in the dog. Heartgard was the first FDA approved heartworm prevention medication available for cats. It is a monthly flavored chewable available by prescription.

INTERCEPTOR® AND MILBEMAX®
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. Milbemax is similar but adds praziquantel for regular tapeworm removal.

REVOLUTION®
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.

 


ADVANTAGE MULTI®
Advantage Multi® is a topical product from Bayer which combines imidocloprid for flea control and moxidectin for heartworm preventive all in one product. It covers roundworms, hookworms and ear mites as well as heartworm.

(Photo Credit: CDC Public Health Image Library)


In studies of infected cats, 25% of infected cats were considered INDOOR ONLY CATS. Because of this and the disastrous effect of even one heartworm to a cat, the American Heartworm Society recommends monthly prevention for ALL CATS living in heartworm endemic areas. To view their feline guidelines visit:

https://heartwormsociety.org/images/pdf/Feline-Guidelines-Summary.pdf

Other helpful sites include:

www.knowheartworms.org

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


(Graphic Courtesy of the American Heartworm Society)

Page last updated: 9/15/2018