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Heartworm Disease in Cats

What is feline heartworm disease, and what causes it?

Heartworm disease is a very serious illness that can be fatal. It affects dogs, cats, and ferrets, as well as other non-domestic species like coyotes, wolves, foxes, etc. In this disease, parasitic worms live inside—and can cause damage to—vital organs such as the heart and pulmonary arteries. Heartworms look like thin, cooked spaghetti when fully grown. Due to the threat heartworms pose, the potential for the disease should be addressed quickly in a young cat’s life.

Here are a few facts about heartworms and the disease they cause in cats:

  • Heartworms are parasites that afflict feline and canine species, both domestic and wild
  • The name “heartworms” comes from their lodging mainly in the blood vessels around the heart and lungs, as well as within the heart itself
  • Heartworms can obstruct the flow of blood, damage vital organs, and lead to death
  • Heartworm disease can only spread to pets through mosquito bites

Dogs are more susceptible to heartworm disease than cats. But cats can still get heartworm disease, and it can be a devastating illness for them. Heartworm disease in cats is caused by the transfer of these parasites in an immature form from an already infected animal to a healthy one. It is not directly contagious; heartworms do not transfer from cat to cat. Rather, the disease can only be transmitted via mosquito bites.

A mosquito becomes the intermediate host of the parasites after biting an infected animal. It picks up baby heartworms from the infected animal. Those baby worms mature inside the mosquito over 10-14 days. The mosquito subsequently transmits the disease when it bites a healthy cat (or another susceptible host).

Diseased cats cannot pass heartworms directly to other animal species or people. When humans themselves are bitten by mosquitos that carry heartworm larvae, they are very rarely infected with the parasites. If they are infected, heartworms do not survive for long in their human hosts.

Adult heartworms can live up to 4 years in cats and can measure between 4 and 12 inches. While dogs can be infected with 15 worms on average, cats typically suffer an infection of just 1 or 2 worms at any given time—and rarely over 6 at a maximum. Even so, only 1 or 2 worms can pose a serious risk to the health of cats.

What risk factors contribute to heartworms in cats?

These factors, explained in more depth below, can increase the chances of your cat contracting heartworm disease:

  • Lapses in heartworm prevention; or no prevention at all
  • Outdoor cats; indoor cats where mosquitoes can come inside (in studies, indoor-only cats are just as likely as indoor-outdoor cats to be infected with heartworms, showing just how good mosquitoes are at being everywhere)
  • Living in, or traveling to, high-risk areas, such as those mentioned next
  • Areas that harbor feral cats, stray dogs, panthers, or other wild felines and canines that may act as a reservoir of baby heartworms
  • Areas with humid, hot, or wet weather that are perfect for mosquito breeding
  • Areas with bodies of water or wetlands; localized areas with standing water (perfect for mosquitoes)

Outdoor cats are especially vulnerable to heartworm disease because outdoors is where mosquitoes breed. Caution is warranted, therefore, in neighborhoods with feral cats or stray dogs—both of whom can be infected with heartworms—if pet owners allow their own cats to go outside. Nevertheless, there is still a risk to indoor cats, as mosquitoes can easily infiltrate homes.

The longer mosquitoes can reproduce each year, the greater the threat of heartworm disease occurring. This is the case in areas where summers are long, such as in the Southern United States. Long rainy seasons can also contribute to the risk. Warm, wet, and humid conditions create ideal environments for mosquitoes to live and breed. Areas with lakes, creeks, rivers, marshes, swamps and coasts are prolific breeding grounds, especially in the hot months.

Mosquitoes can be found in dry and temperate locales, too. So, cat owners shouldn’t let their guard down if they live in a part of the world where heartworm disease seems uncommon. The disease may be more present than you think. Even relatively small geographic areas can harbor parasite-carrying mosquitoes if those areas have the right conditions—either naturally-occurring or man-made (for example, abandoned kiddie pools, dig sites, or any standing water).

The fact is, heartworm disease is documented in every U.S. state, and each year it advances into more localized areas. The spread of heartworm disease can occur through a variety of means. For example, families that move or vacation with their cats can unwittingly take the disease with them or pick it up from their new location. Also, wild animals like coyotes and panthers can contract the disease and carry it into other areas.

Clearly, a favorable climate or location is not a reliable indicator of a heartworm-disease-free zone. The best way to avoid this parasitic disease is to put your cat on heartworm prevention as early as possible. Be that as it may, cat owners should stay vigilant when it comes to heartworm prevention because lapses in administering it can increase the risk and open the door to disease.

How does heartworm disease develop in cats?

Dirofilaria immitis is the technical name of the parasitic worms that define heartworm disease. They are called "heartworms" because they live primarily inside the heart and main arteries around the heart and lungs of their hosts. As mentioned above, heartworm disease is caused by the transfer, by a mosquito, of larval heartworms from an infected cat to a healthy one.

The life cycle of heartworms in cats proceeds as follows:

  1. A mosquito bites an already infected pet and ingests microscopic baby heartworms (microfilariae).
  2. In the mosquito, the microfilariae become infective (disease-causing) in about 10 to 14 days.
  3. The mosquito then bites a cat, and the larvae enter its bloodstream through the wound.
  4. In the now-infected cat, the larvae mature into adult heartworms in about 7 to 8 months.
  5. The adult heartworms may mate and release new baby heartworms into the cat’s bloodstream.
  6. A mosquito bites the infected cat, continuing the cycle.

Unlike dogs, cats are not good hosts to heartworms. But cats contract heartworm disease in the same way as other canine and feline animals: through just one bite of a mosquito that carries heartworm larvae.

It is estimated that only 20 percent of cats infected with heartworm larvae will go on to develop heartworm disease, simply because cats are not very hospitable to the parasites. Similarly, some infected cats may host only a few adult heartworms or no adult worms at all. For these reasons, cats may often develop heartworm disease that is never diagnosed.

However, after about 2 months, a few resilient microfilariae can still make a home inside a cat’s heart, lungs, or main arteries. There, they grow and mature into adult heartworms in 7 to 8 months. But even small, immature worms can wreak havoc on a cat’s internal systems, in a condition called HARD (Heartworm Associated Respiratory Disease).

Although most heartworms do not survive into adulthood inside cats, those that do survive can live up to 3 years in their feline host. During that time, they not only reproduce repeatedly but also cause inflammation and further damage to the cat’s circulatory and/or respiratory organs. The result could be lung disease, heart failure, or both. Even if the worms are removed surgically, the damage to the cat’s organs is usually irreversible.

What are the symptoms of cat heartworm disease?

Heartworm disease in cats is not easy to diagnose. In fact, cats may not display any symptoms of the disease. Some cats have been known to eliminate heartworms on their own before symptoms ever appear. When symptoms do appear in cats, they may be mild or severe, and they can appear similar to the symptoms of other health conditions, such as asthma or bronchitis.

Symptoms of cat heartworm disease depend on its severity. They can include:

  • Coughing
  • Fatigue; decreased activity
  • Loss of appetite
  • Loss of weight
  • Labored breathing
  • Rapid breathing rate
  • Vomiting (not necessarily from eating)
  • Difficulty walking
  • Fainting
  • Seizures
  • Bloated stomach
  • Sudden death

Tragically, a cat may collapse due to respiratory failure—or suddenly die—without ever showing any of these signs of heartworm disease. This is because heartworms can obstruct the flow of blood through the arteries. In addition, heart failure may strike abruptly with perhaps no visible warning.

Symptoms are most likely to appear at two stages of the heartworm life cycle. First, symptoms can occur when the larval worms travel to the arteries of the heart and lungs, where many of them die and cause inflammation in the lungs. This is a condition called Heartworm Associated Respiratory Disease (HARD). Second, symptoms can occur after the adult worms die, when they (or even just one of them) release lung-damaging toxins that cause further respiratory complications—or death.

Of all the symptoms of heartworm disease in cats, those related to breathing are the most apparent because heartworms are prone to cause damage to the lungs. Since that is the case, take seriously any coughing, rapid breathing, or labored breathing your cat may display. These may be indicative of heartworm disease.

When heartworms get into a cat’s bloodstream, more than just the circulatory and respiratory systems can become compromised. In addition to generating blood clots and inflammation in the arteries and lungs, heartworms in cats can work their way to other systems and organs—for example, the spinal cord, eyes, or brain—and cause devastating complications there as well.

It is important to note, by way of repetition, that heartworm-infected cats may never display symptoms at all. This makes regular testing even more critical. Cats who test positive for heartworm disease require routine care during the heartworm life cycle to monitor possible damage to internal organs.

How is cat heartworm disease tested for and diagnosed?

There is no single heartworm test that can confirm with complete certainty the existence of heartworm disease in cats. This means that a combination of blood tests and diagnostic images may be needed to diagnose a cat with heartworm disease. Your veterinarian will assess both your cat’s symptoms and the results of the following heartworm screenings to determine whether your pet is heartworm positive:

  • An antigen test can detect the presence of female worms. However, since a cat may be infected with only 1 or 2 heartworms, there is a chance that they are all males. In the case of an all-male infection, this test will result in a false negative reading for worms.
  • An antibody test can detect whether the cat’s body is currently fighting a heartworm infection, or if the cat has fought an infection sometime in the past. However, in the latter case, a positive result does not indicate whether an infection is currently present. In addition, some cats may not produce antibodies, especially if only a single worm is present.
  • Radiographs (X-rays) and echocardiograms are used when a cat tests positive to an antibody test. These screenings can help your veterinarian determine the extent of lung and heart disease.
  • Ultrasounds can give your veterinarian a visual clue to the presence of heartworms in the heart or arteries. These can help the patient’s doctor devise a treatment strategy that will maximize recovery.

If a cat is diagnosed with heartworm disease, prevention medications should not begin until the current infection is assessed by your veterinarian. This is because some preventatives can be harmful or deadly to cats with existing parasitic infections. For example, microfilariae that are killed by preventive medications can cause blockages within blood vessels, cutting off blood flow to the heart or lungs, or eliminating the oxygen supply to the brain.

If test results are negative, your cat may begin heartworm prevention. However, bear in mind that preventatives do not kill adult heartworms, and heartworm larvae may go undetected in tests. That is why both testing and prevention should be administered on a regular basis.

Given the seriousness of this disease and the fact that there is currently no FDA-approved treatment for heartworm disease in cats, it is vital to acquire a diagnosis as early as possible—with a view to preventing the disease from ever taking hold. Talk to your veterinarian about the best schedule for testing your cat. In general, the following guidelines help your vet determine the timing of testing:

  • In asymptomatic cats, heartworm prevention is generally started using a very safe medication
  • In symptomatic cats, testing may be recommended prior to starting heartworm prevention, or certain medications may be avoided to prevent potential complications

Remember, early symptoms of heartworm disease in cats are either unnoticeable or non-existent. Cats with heartworms may appear healthy on the outside but are infected on the inside. When signs of disease finally appear, your cat is likely already in the clutches of these deadly parasites. So regular testing by your veterinarian is critical.

In addition, regular testing is a commonsense approach to cat preventive care. Treatment is both expensive and hard on your pet. So, your veterinarian will try to make certain that treatment is necessary through testing and confirming a diagnosis. If a diagnosis is confirmed, treatment for your cat’s heartworms can begin. The earlier we can diagnose heartworm disease, the sooner we can treat it—and the more your cat will return to better health.

What is the treatment for cat heartworm disease?

The only FDA-approved method of killing adult heartworms in pets is not an acceptable treatment for cats. The reason is that the feline body cannot well endure the effects of even a few heartworm fragments in its bloodstream—fragments that result when the worms die and break apart. This heartworm debris restricts the flow of blood within the circulatory system. Therefore, the treatment used to kill heartworms can be fatal to cats.

In general, there are two treatment options for cat heartworm disease:

  • Manage the infection with ongoing veterinary care by using symptom-relieving medications such as steroids that reduce inflammation caused by the worms.
  • Remove the heartworms through surgery, in severe cases.

Manage the infection

In most cases, cats diagnosed with heartworm disease will remain infected until the heartworms die of old age. The goals of treatment in cats to manage the infection are:

  • Stabilize your cat’s condition. Stabilizing your cat may require that your veterinarian address indications of respiratory failure, for example.
  • Medicate so your pet can live comfortably. Medications that can help manage your pet’s infection include prednisolone, which may be prescribed to ease inflammation if symptoms are mild.
  • Monitor for changes in the infection and organs. Regular monitoring of your cat will include annual or bi-annual X-rays and/or ultrasound to detect if heartworms are present in the heart or lungs.

Sometimes, heartworm infections have been known to clear up on their own, since cats are not ideal hosts for heartworms. Even then, however, the damage heartworms have caused to a cat’s heart, lungs, or arterial pathways is usually permanent. These conditions must also be managed to help ensure your pet’s quality of life.

Managing your cat’s heartworm infection, if it’s severe enough, may require occasional hospitalization for additional support therapy. In addition to medications to relieve breathing symptoms, such therapy can also include antibiotics and intravenous fluids, together with general healthcare. 

Remove the heartworms

If the disease is severe and ultrasound reveals worms in the heart, surgery may become the recommended treatment, as it may be possible to remove them. However, there are risks involved with surgery. Heartworms must be removed intact; otherwise, broken fragments can lead to anaphylaxis and possibly death. Sadly, almost half of all cats who undergo surgery to remove heartworms do not survive the treatment or die soon after.

Even after successful surgery to eliminate heartworms, damage to heart and lungs may simply be too much for cats to endure. Because of these risks, surgical treatment is usually only recommended for cats with such severe cases that, without surgery, survival prospects are poor. Pet parents must weigh the risks with their veterinarian to determine if surgery is the best option.

Given the risks, early detection and prevention of heartworms are the only effective ways to help cats be free of this disease. Please don't underestimate the value of regular Cat Wellness Exams to help us achieve an early diagnosis. At the same time, heartworm prevention is vital for keeping infections away. Preventatives are available that can be administered either topically or orally. Talk to your vet about which option is best for your cat.

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