It’s Time to Know Heartworms

In the last VetChat, we learned briefly about heartworms and heartworm disease.  In this column, we’ll spend more time on this parasite that is increasingly coming to the attention of veterinarians and cat guardians.  In fact, concern regarding heartworm disease and the life-threatening diseases that it can cause is becoming so significant that the American Association of Feline Practitioners has joined in a “Know Heartworms” campaign.  Heartworm disease often kills young cats in the prime of their lives.

How much do you know about heartworm disease in cats?

Heartworm disease in cats was first documented in 1921.  However, within the past 10 years, interest in the disease has increased significantly as new laboratory testing now allows us to identify affected cats, veterinarians have become aware of other problems associated with the disease and new preventative medications have become available.

Cats are considered to be “atypical hosts,” for heartworm disease.  As most pet owners know, dogs are more often affected.  In fact, the infection rate in cats is only about 5-20 percent of that in dogs.  Despite this lower incidence of infection, heartworm disease in cats is typically more severe and is different than that seen in dogs.  In several studies, 10-14 percent of all cats presented to shelters had evidence of heartworm disease and in one study conducted at North Carolina State University, nine percent of cats who presented with heart or lung disease had heartworm disease.

What Causes Heartworm Disease in Cats?

Mosquitoes spread heartworm disease, which is caused by the parasite Dirofilia immitis.  In order for a cat to become infected, the mosquito must first feed on the blood of a heartworm-infected dog.  Once the parasite(s) are injected into the cat, it proceeds through several larval (microscopic, pre-adult) stages until it reaches the pulmonary artery of the heart. This is the large blood vessel through which blood leaves the heart and enters the lungs, where it becomes rich in oxygen.  While in the pulmonary artery, the adult worm or worms cause an intense inflammatory reaction that can lead to a narrowing of the blood vessels of the lungs and the formation of blood clots.

Because the pulmonary arteries of cats are much narrower than those of dogs and there are fewer alternative blood vessels ("collateral circulation"), clots of blood or small bundles of worms that dislodge can cause disastrous results and even death for the infected cats.

Recently, a bacteria named Wolbachia has been associated with heartworm disease and is believed to be an important part of the inflammatory response seen in heartworm disease in dogs and cats.

How is Feline Heartworm Disease Different from the Disease in Dogs?

Heartworm disease can be difficult to diagnose in cats.  Unlike dogs, cats often have fewer numbers of adult worms (typically less than six and generally no more than 3) and the worms are often of the same gender (either all males or all females).  Because of this “same sex” infection, no microscopic, juvenile worms (microfilaria) are produced.  In dogs, veterinarians can test for the presence of microfilaria.  This type of testing is not possible in the overwhelming number of cats with heartworm disease.  Additionally, adult heartworms only live about 2-3 years in cats.

What Are the Signs of Heartworm Disease in Cats?

Many heartworm-positive cats are completely free of symptoms, yet may die suddenly.  These sudden deaths are most often caused by a small bundle of dead worms breaking off and resulting in a pulmonary embolism.  Typically, cats with heartworm disease often show respiratory (asthma-like coughing and difficulty breathing) and/or cardiac signs (fluid in the lungs or chest cavity or a very rapid heart rate).  Other cats with heartworm disease have uncommon signs, including vomiting and diarrhea, or show neurological disease (head tilt, difficulty walking, seizures, circling), when the adult worms migrate into the brain.  It is believed that some cats with asthma may have been infected with heartworm disease, but survived the infection with residual lung problems.

How Do Veterinarians Diagnose Heartworm Disease in Cats?

If you live in an area where there are mosquitoes (either year-round or seasonally) and/or live in an area where dogs are infected with heartworms, discuss the benefits of testing your kitty for heartworm disease with your veterinarian.  Most veterinarians will submit a blood test for heartworm antibody or antigen testing.  Cats who have been exposed to Dirofilaria immitis will test positive on the antibody test.  Cats who have heartworm disease will typically test positive on the antigen test.  Cats who are antibody positive are at risk for developing heartworm disease.  The antibody test is the preferred screening test for heartworm disease in cats.  It is not incorrect to begin heartworm preventative in cats without testing, but it is important to know if the cat is positive for heartworm disease, as certain precautions (discussed below) can be initiated.

However, testing for heartworm disease in cats isn’t as straightforward as it would appear.  In one study, heartworm antigen testing was more accurate when there were a greater number of female worms present.  Cats with male worm(s) were not detected as having the disease by antigen testing.  Because of this, a false negative heartworm antigen test in the cat probably occurs frequently (due to the presence of only adult male worms) and makes an accurate diagnosis difficult for some patients.  False positives (tests that say the cat has heartworm disease when he or she does not) are very rare.

Radiographs (x-rays) of the thorax (heart and lungs) can be useful in some cases of feline heartworm disease.  In some affected cats, the pulmonary artery is enlarged and loses its normal straight character, becoming “tortuous” and blunted.  Echocardiograms are often very sensitive for heartworm disease in cats and an experienced veterinarian can often identify the adult worms (which look like white lines) within the pulmonary artery.  However, even the most experienced echocardiographer can miss immature worms.

How Can We Prevent Heartworm Disease?

A better question to ask may be: Is heartworm preventative necessary for all cats?  When discussing feline heartworm preventative medication with a veterinarian, you may consider your cat’s lifestyle (outdoors some or all of the time, living in an urban high rise hundreds of feet in the air), the incidence of heartworm disease in the dog population and the incidence of heartworm disease in cats in your area.  However, living an indoors-only lifestyle doesn’t necessarily protect your cat from heartworm disease.  In the previously cited North Carolina State University study, about 1/3 of cats with heartworm disease were indoors only. 

Of interest is that in some populations, the incidence of heartworm disease is as high as that of feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV).
Also, because heartworm disease can produce drastic consequences for cats and there is no safe and effective treatment for the disease, heartworm preventative medication may be a worthwhile addition to your kitty’s health program.

There are three heartworm preventatives currently available for cats in the US.  None of these is available over-the-counter.  The first is ivermectin, given as a chewable tablet.  Selamectin is a topical spot-on, which also has broad-spectrum coverage against fleas, ear mites and roundworms.  Milbemycin is available as a chewable tablet.  These medications are administered monthly.

In geographic areas with heartworm disease, cats may receive these preventatives either year-round, or from early spring until one month beyond the killing frost.  Your cat’s veterinarian will advise you on the proper length of time for use of these preventatives in your cat.

Treatment for Feline Heartworm Disease

Cats who are heartworm antibody positive are generally started on a preventative and are re-tested, per the particular hospital’s protocol.  Cats who are heartworm antigen positive typically begin a preventative and an anti-inflammatory dose of prednisolone.  The steroid is used to prevent the pulmonary artery inflammation that is associated with the severe clinical signs of heartworm disease.

The use of injectable heartworm treatments, such as those used in heartworm-positive dogs, is problematic in cats.  These drugs pose risks even when administered to normal cats in a laboratory setting.  Injectable heartworm treatment is associated with many (often fatal) outcomes and is not recommended in cats.

Because cats who are positive for heartworm disease are at risk for acute death when the adult worms die, cat guardians should be aware of the signs that may be associated with this event.  Breathing difficulty, lethargy, vomiting and fainting all can be associated with adult worm death in cats.  In cases of severe and acute respiratory clinical signs, oxygen therapy, injectable and rapidly acting glucocorticoids and bronchodilators can be used as emergency treatments.

In a study of 50 cats with naturally occurring heartworm disease, median survival of all heartworm-affected cats after diagnosis was 1,460 days (four years; range 2-4,015 days).  Survival of cats with heartworm disease is comparable to that of other heart-related diseases.  However, since prevention is easy and the disease so disastrous for many cats, giving your kitty a monthly preventative, as recommended by his or her veterinarian, may be a life-saving choice.