Reviewed and updated for accuracy on July 8, 2019 by Dr. Hanie Elfenbein, DVM, PhD
Heartworm disease in cats, and dogs, is caused by an infestation of the organism Dirofilaria immitis, a parasitic nematode (roundworm) commonly referred to as the heartworm.
The severity of this disease is directly dependent upon the number of worms present in the body, the duration of the infestation, and how the cat’s body responds to the infection.
You may have heard that cats can’t even get heartworms, and while that is not true, heartworms do infect cats differently. Here’s what you need to know about how cats get infected as well as the symptoms, causes, diagnosis and treatment.
Can Cats Get Heartworms as Easily as Dogs?
The prevalence rate of heartworm disease in unprotected cats is significantly lower than that of unprotected dogs—approximately 5-15 percent the rate of dogs in the same geographic region.
Most infected cats have only a few heartworms present, and the worms are smaller and have a shorter lifespan than those infecting dogs. But heartworm disease is still a serious health condition that can have fatal consequences for cats.
The risk of infection in cats is not known to be affected by age, sex or even indoor/outdoor status. In fact, indoor cats are just as likely to become infected as outdoor cats.
That’s why all cats should be protected. Here is everything you need to know about heartworm disease in cats, including
Causes of Heartworm in Cats
Heartworms are spread through mosquito bites.
Mosquitos can carry infective heartworm larvae that enter a cat’s body when a mosquito feeds. The larvae migrate from the bite wound through the body and mature until they reach the heart and blood vessels of the lungs as adults.
Here, the larvae reproduce, releasing immature heartworms, known as microfilaria, into the cat’s blood. These microfilariae can then infect the next animal through a mosquito bite.
It is important to note that the presence of microfilaria in the blood is in fact uncommon in cats and has been seen in less than 20 percent of infected cats.
Cats have a very robust immune response to heartworm infection, so more than 90 percent of the infective larvae do NOT make it to adulthood.
For those that do, they tend to be single-sex, which means they cannot reproduce. This can make the detection of heartworms in cats very difficult.
One important thing to know about heartworms in cats is that the worms do not need to reach adulthood to start affecting a cat’s health.
Symptoms of Heartworm Disease in Cats
Signs of heartworm infestation in cats include coughing, labored or rapid breathing (known as dyspnea), and vomiting. Weight loss and decreased energy are also common symptoms.
A physical examination may also reveal a heart murmur or otherwise irregular heart rhythm.
The most common signs of heartworm disease in cats deal with the respiratory system—difficulty breathing, coughing and a high respiratory rate—and are often referred to as heartworm-associated respiratory disease (HARD).
HARD is an all-encompassing term used to describe the vascular, airway and interstitial lung lesions caused by the death of immature worms or the death of adult worms.
Many of the respiratory symptoms of heartworm in cats are almost indistinguishable from those of other respiratory diseases, like asthma and allergic bronchitis.
The arrival and death of juvenile worms seems to lead to more noticeable HARD symptoms.
Diagnosing Cat Heartworm Disease
The diagnosis process of heartworms in cats is more complicated than it is in dogs. It requires a combination of clinical and diagnostic testing as opposed to the single blood-screening test seen in dogs.
The reason for these limitations stems from how heartworms develop within a cat’s system.
Heartworm Tests for Cats
The blood tests used to diagnose heartworms in cats are limited to detecting antigens (the parasite itself) and antibodies (body’s response to the parasite). With both of these tests, there are limitations to their validity.
Since feline heartworm antigen tests only detect mature female heartworms, they have a high rate of false-negative test results.
Cats typically only have a few adult heartworms, and those worms tend to be single-sex. So if there are only male heartworms, then the test will show a false negative.
With antibody testing, the accuracy of results can vary widely depending on the stage of larval development at the time blood samples are taken. The results are also difficult to interpret because a positive result doesn’t necessarily mean an infection.
When an antibody result is positive, it simply means that a cat has been exposed to heartworm disease. The test does not confirm whether the infection is current or resolved. A negative result is also not confirmation that a cat is clear from infection, but simply that it is less likely.
Other Diagnostic Methods
If your cat has respiratory symptoms or a positive heartworm antibody test, your veterinarian will want to take X-rays of your cat’s heart and lungs to assess the extent of damage. An echocardiogram may also be useful to diagnose any associated heart disease.
Heartworm Treatment for Cats
Heartworm treatment for cats is currently very limited; there is no approved adulticide therapy (a treatment that kills adult heartworms in the body) for cats. Cats without heartworm symptoms may be able to clear the infection without medical therapy.
Medications can be used to treat heartworm symptoms to help your cat feel more comfortable. These include steroids, bronchodilators and antibiotics that weaken the heartworms.
Continued monitoring of cats with heartworm disease is an essential component of any treatment plan. Diagnostic testing efforts (antibody and antigen measurements, X-rays and echocardiograms) will typically be repeated at 6- to 12-month intervals to determine whether the management strategies have been effective. These tests will also help veterinarians assess a cat’s risk of further complications.
Extracting the adult worms through a surgical procedure is an option for cats with severe infections, but it is not without significant risk and expense.
Living and Management
After treatment, your veterinarian will schedule your cat for follow-up exams in order to test for progress and monitor any side effects.
Often the symptoms will continue despite treating the infection. Your cat may require lifelong medication to help her breathing. This irreversible illness is the reason why prevention is so important.
Preventing Heartworm Disease in Cats
Keeping your cat indoors does not prevent heartworm disease—mosquitos can easily get into any house.
Talk to your veterinarian about prescribing safe feline heartworm prevention that comes in topical treatments and chewables. You should administer these year-round to ensure your cat is protected against heartworm infection.
Many feline heartworm preventatives also protect against other parasites like fleas, ticks and intestinal parasites, so you wouldn’t even have to double up on your monthly treatments.
Featured Image: iStock.com/Jasmin Bauer