THE CAT AND HEARTWORM DISEASE

 DIFFERENCES IN THE HEARTWORM  LIFE CYCLE IN THE CAT

The cat is not the normal host for the Heartworm parasite.  Since they are not the normal host, it is less common for cats to become infested. When Heartworm in the cat is seen, aberrant migration of the larvae is more common than in the dog and a lower percentage of the worms actually end up in the heart.  It is known that more of the larval worms die before they can mature in the cat than in the dog.  Those worms that do reach adulthood live a shorter time as adults, 2-3 years in the cat versus 7 years in the dog.  Also, for some reason, male cats seem to be more susceptible to getting Heartworm disease than female cats.

Those worms that do complete a normal migration to the heart can cause many problems since the cat’s heart and blood vessels are much smaller than comparative structures in the dog.  The adult worms reach a shorter adult length of 5-8 inches versus approximately 14 inches in the dog.  The other feature not unusual in cats is single sex Heartworm infections.  This means no microfilaria can be produced which makes identification of Heartworm disease more difficult.  These single sex infections are commonly male worms only.  In those cats that do have microfilaria present in the circulation the numbers of microfilaria are fewer and the production much shorter in duration than in the dog.  Cats often will have 1-2 worms when infected.  Larger numbers have been seen and sometimes cats have similar signs as the dog, but this is not common.

Any geographic area where dogs contract Heartworm disease can also be a hazardous area for cats.  The current rate of identification of Heartworm disease in cats is estimated to be from 5-20% that of dogs in the same geographical area. This number will probably become more consistent and possibly increase as we develop more effective methods of Heartworm detection in the cat.

SIGNS OF HEARTWORM DISEASE IN THE CAT

The signs of Heartworm in the cat are usually much different than in the dog.  A cat’s immune system is readily activated against Heartworms.  So the signs of a cat with a heartworm infestation display more immune system responses than a similar infestation in the dog.

Cat owners must be aware that even a small number of Heartworms can cause sudden, serious, and even fatal disease in the cat.  The main effect in cats is inflammation of the arteries (called endarteritis) and the surrounding tissues in the lungs.  The amount of inflammation is much more exaggerated than what commonly occurs in dogs.  The inflammation alone can result in partial to complete blocking of blood vessels.  In cats, when worms die, complete blood vessel blockage often occurs.  Depending upon where these blockages are located and the amount of inflammation, large portions of the lungs can be affected which results in a significant, often sudden breathing difficulty for the cat.    The severity is also affected by the magnitude of the cat’s immune system response.   A single blocked blood vessel with a large inflammatory response can result in an emergency situation for the cat.

The cat’s highly reactive immune response to heartworms may be the reason we see some of the following signs in Feline Heartworm Disease…It is rare to find circulating microfilaria in the cat.

Cats acquire Heartworms at a lower rate than dogs.

Fewer worms cause much more severe reactions.

Often the preferred treatment is controlling the symptoms and allowing the worms to die naturally rather than risking treatment. Severe reactions may still occur when the adult worms die.

Worms live a much shorter time than in the dog… 2 to 3 years in the cat, 7 years in the dog.

One worm can cause a severe medical emergency.

Common signs associated with a Feline Heartworm Disease include lung disease associated with respiratory stress (difficulty breathing or rapid shallow breathing), gagging or vomiting, lethargy, loss of appetite, and weight loss.  Some of the signs are difficult to distinguish from feline asthma.  Sudden death may occur in cats, as well as it does in the dog, however only a few worms in the lungs of a cat can trigger a fatal outcome.  In the average size dog a large number are generally required to cause a sudden fatality.  Other signs indicating lung obstruction are a loss of coordination, seizures, blood from the nose, or collapse.  If collapse does occur the cat may die despite treatment even if you are able to visit your veterinarian for treatment.

 
DIAGNOSING FELINE HEARTWORM
Diagnosing Heartworm in the cat is a bit more challenging than making the diagnosis in the dog.  Since many infestations in cats are single sex  (usually male worms) and therefore sterile with only a few adult worms, the typical antigen Heartworm test is not always successful in diagnosing the presence of adult Heartworms.  The antibody Heartworm test does not distinguish between an old, resolved infestation and a currently active Heartworm case. Since blood tests are very simple and are  test is  positive, you do have your answer because that indicates live worms are present.  Usually these tests are only done when a Heartworm infection needs to be ruled out as a cause of the cat’s abnormal signs.  Since circulating microfilaria are so rare in the cat, the Knotts test and filter tests are not very useful.   Once Heartworm disease in the cat is positively identified, the Knotts test may be done to see if microfilaria are present, but this test method is not performed on a routine basis to make a diagnosis. Most Feline Heartworm infections are diagnosed by presenting signs and history and by using ultrasound instrumentation, x-rays, non-specific angiograms, and other blood work to positively identify the presence of Dirofilaria immitis. 

TREATMENT OF FELINE HEARTWORM DISEASE
In the cat, the treatment of choice may be no treatment at all!  Certainly supportive therapy is utilized if a patient requires help.  Supportive therapy is using medication to reduce the side effects of inflammation in the lungs.  The usual choice is prednisone since aspirin can be toxic to cats.  The need for supportive therapy, length of therapy, and use of other medications varies widely from cat to cat.  Since the need for supportive treatment can be intermittent, and existing parasites may die suddenly creating an emergency, your understanding of what is going on and working closely with your veterinarian is very important to your cat’s survival and long-term health.

 In summary

If the cat does not appear sick and responds well to supportive care, usually no adulticide therapy is given and the worms are left to die on their own.  Supportive therapy is used as needed, and ultrasound and chest X-rays are re-evaluated on a regular basis to monitor the Heartworm Disease.  Re-evaluation once every 6 months is not unusual.  In addition, usually a decision is made to start a monthly Heartworm preventative to preclude another infestation.

PREVENTION OF FELINE HEARTWORM INFECTIONS

There are two Heartworm preventatives approved for use in cats by the FDA.  One is Interceptor ® Flavor Tabs® and the other is Feline HeartGard.  These preventatives are given orally once a month and are quite effective.  In the future, there will be more Heartworm preventatives approved for use in the cat. 

Is the use of Feline Heartworm preventative justified?  The answer to this question may not be determined yet.  It is generally thought that in an area with a high incidence of Canine Heartworm Disease, feline Heartworm prevention is justified.  These areas include most of the southeastern United States and Gulf States along with many other areas where Canine Heartworm infections are common.   In geographical areas where Heartworm occurs only occasionally, it is still under much discussion within the veterinary community as to when it is justifiable to suggest the use of feline Heartworm prevention.

Veterinarians are continuing to learn better ways to diagnose Feline Heartworm Disease.  As more cats are tested, and improved diagnostic techniques are perfected for cats, testing guidelines will be updated and recommendations will be more consistent. Until then, check with your veterinarian for current local recommendations regarding Feline Heartworm Disease.