Heartworm in Cats: Causes, Symptoms & Treatment Costs

Feline heartworm disease (FHWD) is a rare but potentially fatal condition that has only become better understood over the past couple of decades. Until recently, many mistakenly believed cats never get heartworm, but the American Heartworm Society (AHS) explains that “the true prevalence… is probably understated due to diagnostic limitations, and the greater tendency of cats to exhibit only transient clinical signs or die without confirmation of infection.”¹

We’ll go over everything you need to know about FHWD, including heartworm symptoms in cats, prevalence rates, causes of infection, diagnostic testing, treatment options, and more. Click on a link below or read end-to-end for a complete breakdown of information.

Table of Contents

• What is heartworm in cats?
• How common is heartworm in cats?
• How long do heartworms live in cats?
• How do cats get heartworms?
• How to recognize heartworm symptoms in cats
• How to test for heartworms in cats
• When should my cat get tested for heartworms?
• How to treat heartworm in cats
• How to prevent heartworm in cats
• Frequently Asked Questions
• Key Takeaways

Pro Tip: Some pet insurance plans cover treatment for heartworm infection (HWI). Many providers also offer complimentary wellness plans that can reimburse owners for the cost of heartworm prevention in cats.

What is heartworm in cats?

Heartworm infection (HWI) is a serious and potentially fatal disease that affects cats, dogs, and other mammal species including ferrets, foxes, wolves, coyotes, sea lions, and (rarely) humans.

Heartworm disease is caused by the parasitic species Dirofilaria immitis, commonly known as “heartworms” because these foot-long organisms live in the heart, lungs, or adjacent large blood vessels of the affected host.

The severity of disease caused by heartworm in cats will depend on:

• The number of worms in the cat’s body
• How long the parasite matures and develops
• How the animal’s body responds to the infection

How common is heartworm in cats?

The prevalence of heartworm in cats is less common than in dogs, affecting 5% of felines versus 20% of canines¹. Although heartworm in dogs is more prevalent, the disease behaves quite differently in canine hosts. Infected dogs might have 30 adult worms living and reproducing in the heart and/or lungs for five to seven years.

How long do heartworms live in cats?

Cat heartworms usually die within three to four months after infection. Most feline cases are single-sex parasite infections that can’t reproduce, leading to a lower burden count of worms than in dogs. The majority die as developing adults once they reach the pulmonary vasculature, or the network of blood vessels between the heart and the lungs, but nonetheless cause severe damage to the respiratory system despite their tiny size.

If they survive past this stage in an unfortunate feline host, even one or two mature adults could present a substantial burden due to the smaller size of the feline anatomy. Adult heartworms can live between two and four years in cats, during which time many pets appear asymptomatic.

However, when an adult heartwormworm dies inside a cat’s body, the parasite fragments and releases toxins in the body. This worsens pulmonary inflammation and often results in acute fatal lung injury. Unfortunately, sudden death is sometimes the only sign of heartworm in cats, which can occur with just a single-worm infection.²

How do cats get heartworm?

Cats get heartworm through mosquito bites, but there are several steps that take place before transmission occurs.

1. First, a mosquito bites an infected animal. Dogs are considered natural hosts and commonly have microscopic baby worms circulating in their bloodstream produced by adult females. Foxes and coyotes are also natural hosts that often live near city centers, so they’re another key contributor to heartworm transmission.
2. When the mosquito bites the infected animal, it picks up the baby worms (known as microfilaria) that then develop into “infective stage” larvae over the course of 10 to 14 days.
3. The mosquito transmits the infective larvae by depositing it onto the skin surface of the next cat or susceptible animal it bites, and the larvae enter the bloodstream of the new host through the bite wound.
4. Microscopic worms live in the cat’s bloodstream for four to six months, which incites a severe inflammatory response and typically results in extensive damage to the arteries, bronchioles, and alveoli in the cat’s lungs — a condition called heartworm-associated respiratory disease (HARD).
5. The worms take up their final residence in the heart, lung, or adjacent blood vessel, where a mature adult can live for two to four years.

Keep in mind that heartworm infection can affect all cats, regardless of their sex, age, or living environment. Although outdoor cats are at higher risk of getting bit by an infected mosquito than indoor cats, the Mosquito Control Association says these carriers of disease are adept at getting inside through open doors, window screens, exhaust vents, and so forth.³

How to recognize heartworm symptoms in cats

Some pets will show symptoms immediately, while others display no symptoms at all until they suddenly collapse and die. The clinical signs of heartworm in cats are nonspecific and can vary from subtle to severe. They may include:

• Difficulty breathing (dyspnea)
• Coughing
• Asthma attacks
• Vomiting
• Loss of appetite
• Weight loss
• Difficulty walking
• Diarrhea
• Abdominal fluid accumulation
• Lethargy
• Weakness
• Fainting
• Seizures

Note that some heartworm symptoms may look like other common health issues in cats, such as allergic bronchitis or feline asthma (coughing, difficulty breathing, and an increased respiratory rate). However, these conditions may be secondary, caused by the heartworm infection. Be sure to discuss your pet’s medical history with your veterinarian when testing for cat heartworms so they can better differentiate between HARD and other similar conditions.

How to test for heartworms in cats

Testing for heartworm in cats is more complicated in cats than it is in dogs, as it requires a combination of diagnostic and clinical testing. For example, dogs are tested with a blood-screening test called an antigen test. The test can also be done in cats, but it won’t be as accurate since cats tend to have fewer worms than dogs. In some cases, cats may test negative on an antigen test for dogs but actually be infected with heartworms.

Another type of test called antibody test can be used to detect the presence of immature and adult heartworms. Cats who have heartworms often will have antibodies against heartworms that their body has made in attempts to kill the parasite. These antibodies can be detected by a blood test.

If your feline friend has a positive antibody test or respiratory symptoms, your vet will likely order an X-ray or ultrasound of your cat’s lungs and heart to evaluate the extent of the damage. These imaging techniques can visualize potential damage to the heart and/or lungs and can possibly show the actual live worms if they are present.

When should my cat get tested for heartworms?

The FDA recommends that cats get tested annually to detect the presence of heartworms.⁴ If your feline friend is positive, you should re-test your cat for heartworms as many times as your vet deems appropriate (typically, every six to 12 weeks) until the infection is cleared, so they can begin preventative therapy to reduce the risk of recurrence.

Both cats and kittens should always be tested for heartworms before being put on any preventative therapy, as giving preventatives to a feline with heartworms can cause serious harm and is potentially fatal.

Heartworm prevention for cats is crucial, as there is no cat heartworm medicine approved by the FDA to treat the disease.

How to treat heartworms in cats

Although there’s no federally-approved drug to treat heartworm disease in cats, symptoms can be managed with certain medications, including antibiotics, steroids, and bronchodilators. Cats without symptoms might be able to clear the infection without therapy.

All cats with HWI need to be continually monitored. Diagnostic tests (antigen and antibody measurements, echocardiograms, X-rays) will need to be repeated every six to 12 months in order to determine whether the prescribed therapy has been effective.

In cats with more severe infections, adult worms may be extracted through surgery (which is only possible when the parasites can be seen via ultrasound). This procedure is quite risky because if the worms are not removed intact, there could potentially be serious complications, including shock and sudden death.

In many cases, the symptoms will continue even after treatment and your pet might need to take medication to help with breathing for the rest of their life — that’s why heartworm prevention for cats is so imperative.

How to prevent heartworms in cats

Your vet can prescribe adequate heartworm prevention in the form of oral and topical treatments which are administered year-round. Many heartworm preventives for cats also protect against other parasites such as intestinal parasites, ticks, and fleas.

The price of cat heartworm preventatives ranges from $35 to $85 per year, depending on the type of medication you choose.⁵ And since there’s currently no treatment for heartworm in cats, preventative care is crucial. Not only are those veterinary bills significantly more expensive, but the health and vitality of your best fur-riend are priceless.

Pro Tip: Compare pet insurance plans to see how you could get reimbursed for the cost of heartworm prevention in cats, plus many more veterinary services included within policy coverage.

Frequently Asked Questions

How common is heartworm in cats?

The prevalence of heartworm in cats is about 5% compared to 20% of dogs. However, experts believe this figure underestimates the true rate of infection due to complex diagnostic processes and the number of cats who die of heartworm infection without a positive test result.

Can cats get heartworm from dogs?

Technically, yes, although cats cannot get heartworm through direct contact with a dog. For transmission to occur, a mosquito must first bite an infected dog then deposit the infective larvae it picks up onto the skin surface when biting a susceptible cat 10-14 days later.

Do indoor cats need heartworm prevention?

Yes, heartworm prevention for cats living indoors is still essential because mosquitos are known to get inside homes through various entry points, including window screens, open doors, and exhaust vents.

Do cats need heartworm medicine?

There is no FDA-approved heartworm medication for cats, which is why prevention is so important. However, some symptoms of the disease may be managed with certain medications.

Is heartworm in cats fatal?

It can be. Sudden death is possible in cats infected by only one or two heartworms who may otherwise appear asymptomatic until the mature parasite dies and causes fatal acute lung injury.