Heartworm infection in cats - it really is a worm, but it might not be in the heart!

Heartworms are exactly what they sound like – worms in the heart!  They are 9-11 inches long and live either in the heart or in the arteries that supply blood to the lungs.  Although they occur commonly in dogs, most people do not consider them to be a problem in cats.   However, recent studies have shown heartworms to be far more prevalent than previously thought. 

Heartworm Transmission             

Heartworms are transmitted by mosquitoes.  When a mosquito carrying baby heartworms (larvae)

bites a cat, the larvae are deposited under the cat’s skin.  The larvae then begin a long migration through the tissues, eventually reaching the heart.  They mature into adult heartworms about 6 months from the time they enter the cat.  The life span of an adult heartworm in a cat is about 2-3 years (in dogs, the worms may live 7 years or so). Cats are infected with heartworms only when bitten by a mosquito that is carrying heartworm larvae.  Cats that spend time outdoors are more likely to be exposed, but mosquitoes that enter our homes can be just as dangerous to our indoor-only friends. At Exclusively Cats Veterinary Hospital, we have seen several indoor-only cats that have developed severe illness related to heartworm infection.

Clinical Signs of Heartworm Disease

 

One of the difficulties in diagnosing heartworm disease in cats is that there are no specific or consistent signs of disease.  The disease in cats is also very different in cats than in dogs. So much so, in fact, that it has been given the name "Feline Heartworm Associated Respiratory Disease" (or, H.A.R.D.). Immature heartworms in cause significant disease in the small arteries supplying the lungs in about 50% of cats that are infected - before any of the worms mature to adulthood. In some cases, the immature worms die 3-4 months after entering the arteries and cause a severe inflammatory reaction that may result in sudden death of the infected cat. If the worms and the cat survive past this initial phase, the worms act to suppress the cat's immune system.

For many months there may be little or no apparent changes in the cat.  However, as the worms mature, any heartworms that have reached the heart may begin to put strain on the heart muscle and cause a heart murmur, high blood pressure or increased heart rate (tachycardia). Infected cats may cough or have difficulty breathing, and they may be weak or lose weight. These signs of heartworm disease in cats may be easily mistaken for asthma, allergic bronchitis or other respiratory diseases.

Strangely, some cats with heartworms have a history of vomiting with no respiratory signs or signs of heart failure.  Some cats seem to be normal, then die suddenly.  This may occur due to the aforementioned reaction within the lungs to young heartworms or when the pumping action of the heart sends a worm or blood clot (thrombus) into the main arteries leading to the lungs (pulmonary arteries).  If these blood vessels become blocked, the cat will die suddenly.  Additionally, in cats, more so than dogs, the worms may migrate to unusual areas of the body, such as the abdominal cavity or the central nervous system.

The symptoms of heartworm associated respiratory disease are:

• anorexia
• coughing
• difficulty breathing (increased respiratory rate or effort)
• lethargy
• weight loss 
• vomiting
• rapid heart rate
• heart murmur
• blindness
• collapse
• convulsions
• diarrhea
• fainting
• sudden death

Diagnosis of Heartworm Disease      

There are two blood tests that are very helpful in diagnosing heartworm infections in cats, however neither test is conclusive on its own. 

                       

1)      Antibody test.  This test determines whether the cat’s immune system has been exposed to heartworms.  If a cat has ever been infected with heartworm disease in its lifetime, it will test positive, whether or not there are live worms in the cat's body at the time of testing. Therefore, a positive test result may indicate that an active infection is present, OR the cat has had heartworms in the past, but the heartworms have died.  This test is very sensitive, so it is used first. It will give a positive result as soon as 2 months after infection, but once the worm becomes an adult, the test may provide a false negative result, especially in cats with no clinical signs. If this antibody test is positive the next test is performed.

2)      Antigen test.  This test detects the presence of adult female worms.  It is very specific, and is considered the "gold standard" in dogs, but in cats, it is not as sensitive.  This test will not give a positive result in a cat until 5-8 months after infection. A positive antigen test means that heartworms are present, but a negative test does not mean that heartworms are absent.  Because the cat must have at least 2 adult female worms present to make this test positive, a negative antigen test may mean that the cat has only a small number of worms or that all of the worms present are male. A typical feline heartworm infection consists of 6 or fewer worms, with the majority of cats having only one or two worms in their body. This is still considered to be a "heavy" or "significant" infection. Approximately 1/3 of feline heartworm infections consist of only males or only females.

In summary, a diagnosis of heartworms is confirmed if both the antibody and antigen tests are positive.  However, a cat infected with heartworms can have a positive antibody test and a negative antigen test (if the heartworms are all male). 

Additional diagnostics may include the following:

3)      Eosinophil count.  Cats suspected of heartworm disease can be tested for their level of eosinophils.  Eosinophils are normal white blood cells that occur in increased numbers when certain parasites are present.  They are elevated in the presence of heartworms, but this elevation only occurs for a few months.  In addition, cats with intestinal parasites (“worms”) and allergies also commonly have increased eosinophil counts.

4)      Autopsy after sudden death.  Many cats show no clinical signs of illness before sudden death due to heartworm disease.

Radiographs               

Radiographs (x-rays) allow us to view the size and shape of the heart and measure the diameter of the pulmonary arteries.  Many cats with heartworms have an increased size of the pulmonary arteries.  Sometimes these arteries appear to come to a sudden stop (blunted) on their way to the lungs due to worms obstructing them, or the vessels may look twisted or "tortuous".  However, many cats with heartworms have no abnormalities on their radiographs, especially early in the infection. Additionally, cats with certain other parasites, such as Toxocara (roundworms) or Aleurostrongylus (lungworms) may have similar lung patterns.

Ultrasound                 

An ultrasound machine produces an image of internal organs and structures without the use of radiation.  With it, one is able to view the pulmonary arteries and internal structures of the heart.  In some cats the heartworms can be seen, but unfortunately most of the time they are not visible. This method of diagnosis is most accurate about 5 months after infection.

Treatment for Feline Heartworm Disease 

Dogs that have heartworms can be treated with a medication that kills the adult worms in the body.  Unfortunately, there is no such drug for cats.  Another problem is that when the heartworms die, they pass through the pulmonary arteries to the lungs.  This can result in sudden death.  So… we have a dilemma when a cat is diagnosed with heartworms.  One of the following choices must be made:
 

1)      Treat with the drug designed for dogs.  This is not advised as this drug is not approved for cats, the medication is toxic at very low doses and the chance of complications and side effects is high. To date, no study has found a medical treatment to significantly increase the survival rate of cats with heartworm disease.

2)      Treat the symptoms of heartworm disease and hope the cat outlives the worms.  Since heartworms live in a cat for about 2-3 years, prolonged treatment is often needed.  When cats are in crisis, they are treated with oxygen, corticosteroids (“cortisone”) to relieve the reaction occurring in the pulmonary arteries and lungs, and if needed, drugs to remove fluid from the lungs (diuretics).  When cats are stable (not showing any signs of disease), they are treated continuously or periodically with corticosteroids, although the threat of an acute crisis or sudden death always exists. 

3)      Attempt to surgically remove as many worms as possible from the heart itself.  This surgery is very delicate and is quite risky from an anesthetic standpoint, although it has been successful in a number of cats.  Medical therapy is needed in addition to the surgery. 

Prevention                 

It is strongly recommended that all dogs take medications to prevent heartworms, and some of these medications have been formulated for cats.  Prevention of heartworms in cats is safe and easy.  The

reasons a preventative should be considered in your cat are:

1)      Diagnosing heartworms in cats is not as easy as it is in dogs. 

2)      Although heartworms in cats are not as common as they are in dogs, they are probably more common than we realize.  As we look more aggressively for heartworms in cats with better and better tests, we expect to find that the incidence is greater than we thought in the past. 

3)      There is no good treatment for heartworm disease in cats.  Effective drugs are not available, and cats that seem to be doing well may suddenly die.  Heartworm disease in cats is risky, and not treating these cats is just as risky. Even if treatment is successful in relieving symptoms, and even if the worms die in 2-3 years without incident, the cat may experience lifelong respiratory issues that require treatment.

4)      Heartworm prevention medication is safe for cats and kittens as young as 6 weeks.  They only have to be given once each month and they are formulated so that (most) cats will eat them readily. If a cat will not take an oral medication, topical monthly heartworm medications are also available.

5)      Indoor cats can get heartworms!  Because contact with mosquitoes is required for transmission, outdoor cats are more likely to be exposed.  However, about 25% of cats diagnosed with heartworms are reported by their owners to be indoors only.  This simply means that mosquitoes that come in the house are just as dangerous as the ones outdoors.

It is important to continue treating with heartworm preventives for 30-90 days after the end of heartworm season, since prevention works retroactively by flushing the system of any immature heartworms that have been introduced into the cat. For cats in Michigan, this usually means that they should be given preventive medication April through November or December. However, year-round prevention is likely the safest option. April is right around the corner, so make sure you check to see if you need to pick up some heartworm preventive!

Heartworm infection in cats can be a very serious and potentially fatal disease.  Fortunately, prevention is very easy and safe.  Please feel free to discuss any questions or concerns you have about heartworm disease or any other topics with the staff at EXCLUSIVELY CATS.

Thank you for allowing us to help you care for your cat!

Resources:
The American Heartworm Society Feline Guidelines