What causes heartworm disease?
Heartworm disease, or dirofilariasis, is caused by a bloodborne parasite known as Dirofilaria immitis. Adult heartworms live in the heart or nearby large blood vessels of infected animals. Female adult heartworms are 6–14 inches (15–36 cm) long and approximately 1/8 inch (3 mm) wide. Males are about half the size of females.
Heartworm disease is much less common in cats than in dogs. However, recent studies of cats with heart and breathing diseases have found an incidence of heartworms far greater than previously thought. Cats are relatively resistant to adult heartworm infection when compared to dogs, with the infection rate in cats reported to be 5%-20% of the rate in dogs in the same geographic location; however, infection can still occur.
Typically, infected cats have fewer adult worms than dogs — usually less than four. Many pet owners are surprised to learn that up to a third of infected cats live indoors only. Because cats are not natural hosts for heartworms, heartworm disease often affects cats differently than it affects dogs.
How is heartworm disease spread?
Heartworm disease requires mosquitoes to spread from one animal to another. When a mosquito bites an infected animal, often a dog, it ingests microfilariae during its blood meal. These immature heartworms develop inside the mosquito for 10 to 14 days, become infective larvae, and enter the mosquito’s mouthparts.
When the mosquito bites a cat, the infective larvae enter the cat’s body. The larvae migrate through the cat’s tissues as they mature and enter the bloodstream. They eventually reach the pulmonary arteries (the blood vessels that carry blood from the heart to the lungs) and on the right side of the heart. Many immature worms die at this point, as cats are not natural hosts for heartworms. Within seven to eight months, surviving worms may mature into adults and begin producing new microfilariae. Cats are resistant hosts, and few circulating microfilariae are generally found. Adult heartworms live for two to four years in cats.
Heartworm requires the mosquito as an intermediate host, so disease is not spread directly from cat to cat or from a dog to a cat. Disease spread coincides with mosquito season, which can last year-round in many parts of the United States. The risk of infection is greatest when mosquitoes are actively feeding, which typically requires temperatures above 50°F (10°C).
Where is heartworm disease found?
Heartworm disease occurs all over the world. It is most prevalent in the southwestern United States, but can occur in all 50 states. In Canada, the disease is most common in areas where mosquitoes are prevalent, including southern British Columbia, Manitoba, Ontario, Quebec, and New Brunswick.
How does heartworm disease affect cats?
Most heartworm infections in cats don’t develop to the adult stage. Instead, cats most commonly develop heartworm-associated respiratory disease (HARD). HARD results from inflammation caused by dying immature worms in the pulmonary vessels, small airways, and lung tissue.
Clinical signs of HARD can be mistaken for asthma and may include:
- Coughing
- Wheezing
- Difficulty breathing
- Vomiting
- Decreased appetite
In some cats, adult heartworms do develop and cause heartworm disease. Adult heartworms affect the heart and blood vessels that carry blood from the heart to the lungs (pulmonary arteries). They cause chronic inflammation that leads to scarring and narrowing of the pulmonary arteries and thickening (fibrosis) of the surrounding lung tissue.
These irreversible changes cause increased blood pressure in the vessels of the lungs (pulmonary hypertension), which means the heart must work harder to pump blood to the lungs for oxygen. Worms can also interfere with heart valves, further increasing the heart's workload and ultimately leading to heart failure.
Adult heartworms have a more severe effect in cats than in dogs, because cats have much smaller blood vessels. Even a single adult heartworm can cause severe or fatal lung disease.
What are the signs of heartworm disease?
Heartworm disease can cause a wide range of signs in cats, and none are specific to the disease. The most common signs are a sudden onset of coughing and rapid breathing — which can also be caused by several other diseases. Sometimes, a cat that appears healthy may die suddenly, and heartworm disease is diagnosed in an examination after death.
Acute (sudden) clinical signs include:
- Collapse, fainting, shortness of breath
- Convulsions
- Diarrhea
- Vomiting
- Blindness
- Rapid heart rate
- Fainting
- Sudden death
Chronic (long-lasting) clinical signs include:
- Coughing
- Vomiting
- Shortness of breath
- Lethargy
- Lack of appetite, weight loss
- Accumulation of fluid around the lungs
How are heartworms diagnosed?
There are several tests used to diagnose heartworms in cats, but none are 100% reliable. Your veterinarian will diagnose heartworms based on a combination of clinical signs and diagnostic tests. Because the signs of heartworm can resemble other diseases, such as asthma and heart disease, further testing is usually needed. Recommended tests may include:
- Routine bloodwork, which may show non-specific changes such as increased eosinophils (a type of white blood cell that responds to parasitic infections) or globulins (due to increased inflammatory proteins)
- Heartworm antibody testing to detect early stages of heartworm infection
- Heartworm antigen testing to identify the presence of adult female worms
- Testing for microfilariae, although results are often negative in cats
- Chest X-rays (radiographs) to evaluate the size and shape of the heart and any lung changes, such as enlarged pulmonary arteries (although many cats with heartworms have no abnormal findings on their X-rays)
- Echocardiography (heart ultrasound) to assess heart size and function, and to look for adult heartworms (few may be seen in cats)
No single test is completely reliable in cats, so results from several tests are often interpreted together.
How is heartworm disease treated?
There are no drugs approved for treating heartworms in cats. Melarsomine (Immiticide®) is used to treat heartworm in dogs, and has been used in cats, but it causes significant side effects and is considered toxic to cats. In addition, this treatment causes adult heartworms to die and then pass through the pulmonary arteries to the lungs, where the reaction to the dead and dying worms can cause sudden death in cats.
Monitoring Mild Cases: If a cat is diagnosed with heartworm but is not showing clinical signs, all that may be needed is to monitor antibody and antigen testing, along with regular chest X-rays, while the heartworm infection is naturally eliminated. This can take several years, and the hope is that the cat outlives the worms.
Medical Management: Medical options are available to manage the clinical signs associated with heartworm disease and eliminate the worms more quickly. These may include:
- Prednisolone and other corticosteroids can reduce inflammation and scarring in the pulmonary arteries and lungs.
- An antibiotic called doxycycline can eliminate the Wolbachia bacteria that seem to help worms survive.
- A monthly dose of a heartworm preventive may weaken the worms and eliminate any microfilariae that are present.
- For a cat in crisis, oxygen support, bronchodilators (medications to open airways), and diuretics (medications to remove fluids from the lungs) may be needed.
When stable, some cats continue to receive corticosteroids periodically or long-term. In many cats, this treatment will reduce clinical signs and improve quality of life. However, there is always a threat of an acute crisis or sudden death.
Surgical Treatment: Surgical removal of heartworms may be attempted in cats with severe heartworm disease. This procedure must be performed by a specialist, often at a college of veterinary medicine. Studies have shown that up to 40% of cats die during or after this procedure, so surgery is generally reserved for cats with severe disease and a poor prognosis without surgery.
How can I prevent my cat from getting heartworms?
Veterinarians now strongly recommend year-round monthly heartworm prevention for all cats. Heartworm preventives available for cats include NexGard® Combo, Revolution®, Revolution® Plus, Advantage Multi®, Interceptor®, Heartgard®, Bravecto Plus®, and Centragard®. Preventives are recommended for all cats for several reasons:
- Diagnosis is difficult: Diagnosing heartworms is not as easy in cats as in dogs.
- Heartworms may be more common than thought: Heartworms are less common in cats than in dogs, but studies suggest they may be more common than previously believed. Up to 15% of all cats in certain locations have been exposed to heartworms.
- No good treatment: There is no consistently effective treatment for heartworm-infected cats. Some infected cats that may die suddenly even when they appear healthy. Treating heartworm infections in cats is risky at best, and not treating these cats is just as risky. It takes about two years for heartworm to be eliminated in the cat, and serious clinical signs can suddenly appear at any time.
- Prevention is safe and easy: Heartworm preventives have an excellent safety record and can be used in kittens as young as six weeks of age.
- Indoor cats get heartworms too: Cats do not need direct contact with infected animals to become infected. An infected mosquito can enter the home and transmit heartworms to an indoor cat.
With the safe and effective preventives available today, heartworm disease is largely preventable. Talk with your veterinarian about the best heartworm prevention program for your cat.
