Is Your Cat at Risk for Heartworm Disease?

If your cats spend most of their days moving from the couch to the window and back again, it can be easy to assume that a mosquito-borne disease like heartworm isn't a threat.

Since the 20th century, when heartworm disease was first recognized as a disease in cats, the way veterinarians understand heartworm risk and how the disease affects cats has expanded significantly.

What is heartworm disease in cats?

The word “heartworm” conjures up an image of worms that infect and spread through a pet's heart chamber. However, this image describes more of how heartworms affect dogs rather than how they affect cats.

Heartworm disease in dogs is a cardiovascular disease that occurs when adult heartworms, which can be up to a foot long, lodge in the dog's heart and cardiovascular system, causing cardiovascular disease, fluid accumulation, and congestive heart failure.

In cats, most heartworms are killed by the immune system before the worms reach the adult stage, but the presence of dead or dying worms in the small and large blood vessels of the airways and lungs can lead to significant inflammation and respiratory disease.

During this time, heartworms can still survive to the adult stage in cats. When this happens, cats are at significant risk of serious illness and even death. Some cats (up to 10-20%) may suffer from an immune reaction leading to anaphylaxis. Anaphylaxis requires immediate veterinary intervention and can be fatal.

Another potentially life-threatening complication of adult heartworm infection in cats is the migration of the worms to areas outside of the respiratory tract, including the eyes, brain, and spinal cord.

How do cats get heartworm?

Heartworm disease is transmitted through the bite of infected mosquitoes. Many species of mosquitoes can transmit heartworm; but the one characteristic that all mosquitoes must have is their willingness to feed between dogs and cats.

Indeed, dogs, as well as wild canids such as coyotes, foxes and wolves, serve as reservoirs for the disease. For the heartworm life cycle to be completed, a mosquito first feeds on an infected dog with microscopic heartworm “babies” in its bloodstream.

Over the next few weeks, these baby heartworms reach the larval stage in the mosquito's body and can spread to another dog or cat when the infected mosquito feeds on them. Mosquitoes typically feed on sparsely covered areas of a cat, such as the tips of the ears and nose.

About two and a half to three months after infection, immature heartworms reach their target in cats: the largest arteries in the lungs.

At this point, two things can happen:

Why Indoor Cats Are at Risk for Heartworm Disease

Since mosquitoes can enter indoor spaces through open doors or windows, indoor cats are always at risk for heartworms. Cats with limited access to outdoor spaces, such as balconies, catios or terraces, are at greater risk of heartworm disease due to their outdoor exposure.

Due to landscape changes from industrial and residential development, a population of mosquitoes in a given area can survive the harsh winter months. And due to changing climate conditions, early springs and prolonged summer temperatures are also becoming more common.

Mosquitoes breed in standing water, vegetation and artificial objects (e.g. flower pots, birdbaths, etc.) can retain enough water to support the mosquito life cycle in the most arid climates. Together, these factors can ensure that the risk of heartworm disease is never zero.

Meanwhile, cats who travel or move with their parents to areas with increased mosquito numbers are also at increased risk of heartworm disease.

Is my cat at risk of heartworm disease?

Various circumstances should be considered when determining the risk of heartworm infection in cats:

Signs and Symptoms of Heartworms in Cats

While some cats infected with heartworm show no apparent symptoms, others may exhibit a variety of clinical signs. Coughing and vomiting are observed early in the pathological process and are the most common signs of heartworm disease in cats. Cats with heartworms may also have:

Keep in mind that some cats may not show any physical symptoms of heartworm disease. That's why it's important to schedule routine wellness check appointments with your veterinarian to quickly detect any signs of illness.

Can heartworm disease be diagnosed in cats?

Testing for heartworm in cats is much less straightforward than in dogs, which has led to significant underestimation of disease in cats.

Dogs are usually screened for heartworms with antigen blood tests that detect the presence of adult heartworms. Since cats are less likely to harbor adult worms than dogs, antigen testing alone does not provide enough information to veterinarians.

For this reason, the American Heartworm Society recommends that veterinarians regularly test cats with tests called antibody tests as well as antigen tests. Antibody tests detect the cat's immune response to a current or previous heartworm infection.

This is important since most heartworms are eliminated by the cat's immune system before becoming adults. Although antibody tests cannot differentiate between past and present infection, they provide crucial information about the cat's exposure.

In cats suspected of being infected with heartworm, X-rays or ultrasound can help veterinarians detect the presence of worms and possible respiratory changes associated with the disease.

Is there treatment for cats with heartworm disease?

Unfortunately, cats infected with adult heartworm do not have the same treatment options as dogs. The medication used to eliminate adult heartworms in dogs is toxic to cats. Additionally, acute death of an adult worm in a cat can cause anaphylaxis and be fatal to the cat.

Treatment for cats with heartworm disease is generally limited to supportive care. Steroids, which are anti-inflammatory medications, may be used to ease lung inflammation associated with infection and reduce coughing.

In cases where the cat is suffering from acute respiratory distress or anaphylactic shock, oxygen therapy and anti-shock therapy may be administered by the veterinarian. It is also possible to surgically remove adult heartworms in cats, but this is a risky procedure that usually requires referral to a specialist.

Cats diagnosed with heartworm infection should be placed on prevention as soon as possible, as they have been found to be susceptible to the disease.

How to protect your cat from heartworm disease

Since there is no direct treatment to cure cats of heartworm disease, prevention is of paramount importance.

The American Heartworm Society recommends year-round heartworm prevention in cats, a practice that has the added benefit of ensuring cats are protected from other common parasites. Depending on the product prescribed by the veterinarian, these parasites can include internal parasites, such as roundworms and hookworms, and external parasites, such as fleas, ticks and mites. Many of these parasites can be transmitted from cats to humans, so protection is also important for human health.

Options include topical and oral products, all easy to administer. Because products differ in terms of which parasites they prevent, it is important for pet owners to work with their veterinarian to determine which preventative product best fits their cat's needs.

Other ways pet owners can protect their cats:

• Environmental control: Although topical mosquito repellents are not safe to use on cats, pet owners can take steps to reduce the mosquito population in their cat's environment. Getting rid of standing water that can spread the mosquito life cycle, fogging from the outside environment, and ensuring all windows are fitted with screens can help prevent mosquitoes from entering a cat's environment.