Important facts About Heartworm

In many areas of North America, April is the time of the year when veterinarians begin to check dogs and cats for exposure to heartworm organisms that may have occurred during the previous mosquito season.  If your pet was infected last mosquito season, evidence of the disease may now begin to be detected.  And like any other pathogenic situation, the earlier a diagnosis is made and treatment is begun, the better the chances are that the patient will recover properly.  

*  Heartworms are a parasitic worm (about the diameter of thin spaghetti) that normally live free floating in the right ventricle of the heart and nearby blood vessels.
*  The parasites are transmitted from one individual to another by mosquitoes.
*  Heartworm is diagnosed with blood tests, and/or X-rays, along with other tests.
*  Heartworms are not detectable with the commonly used antigen blood test until they are sexually mature (about 6 months after entering the patient). Female worms must be present for accurate test results.
*  Heartworm has been diagnosed in all 50 states and also worldwide.  In most areas of the country Heartworm is an important pet health care issue. 
*  In most areas of the USA, veterinarians recommend Heartworm prevention for dogs.  Preventative measures in cats is also becoming more common as veterinarians continue to learn more about the parasite in cats and become skilled at recognizing the disease in cats.  Previously it was thought that felines were not at a significant risk of getting Heartworm and that preventative was not needed.  We are learning this is not necessarily true.  We recommend all cats using some type of heartworm preventative.       
*  In the cat, the presence of Heartworm is difficult to detect with a blood test alone.  Preventative medication is often started without testing unless signs of a Heartworm infection are noticed.
*  In cats, one worm can cause sudden death or sudden (acute) respiratory signs that are indistinguishable from asthma without a medical work up. 
*  Any area where dogs can get Heartworm, cats can get them as well.  The current rate of diagnosis ranges from 5 to 20% that of dogs in the same area.
*  The rate of cat Heartworm diagnosis varies by geographic area.  As veterinarians continue to improve diagnostic techniques in cats, it is suspected there will be less variation from the canine incidence rate in the same area (5-20%). Until the last few years, it was thought that feline Heartworm disease was so rare that prevention was not needed in cats.
*  This view is changing as we learn that many cat Heartworm infections are overlooked since feline Heartworm disease does not commonly present with the same symptoms as dogs and the signs and symptoms look and act like other cat diseases.
*  The detection of adult Heartworms in cats can be difficult and tests are not 100% reliable.  

WHERE DO HEARTWORMS COME FROM?

We are not sure where they evolved originally but we do know they have been around for a long time.  The first written canine documentation was in the "Western Journal of Medicine" in 1847. The first documented cat infection was in the early 1900's.   We have learned much more about them and now know that in areas where dogs contract heartworm, cats can become infected too.  Cats are simply less susceptible to becoming infected when exposed to the larvae deposited by a Heartworm-harboring mosquito.

The education of pet owners about Heartworms has increased significantly over the past twenty years.   The spread of the disease has accelerated due to the trend of pets, and especially dogs, to travel everywhere with their owners.  Pet owners often were unaware they were visiting Heartworm country, their pets became infected and the worm was brought back to their home environment unknowingly.  Thus territories that never experienced Heartworm infections in the past started to see them.   Heartworm disease is now wide spread and has been diagnosed in all 50 states in the USA as well as worldwide.  Actually, cats have been found NOT to be a big factor in the spread of the heartworm disease.

WHAT ANIMALS ACQUIRE HEARTWORMS?

Most parasites have a specific host they prefer to infest. The normal host for Heartworm is the canine.  Heartworms can live in animals other than canines but often have a shorter life, vary in size, or end up in odd locations due to aberrant migration.   This is true of Heartworm infections in the feline.   Some of the other animals that can acquire heartworm include the wolf, coyote, fox, bobcat, jaguar, tiger, muskrat, raccoon, ferret, otter, bear, horse, orangutan, gibbon, sea lion, and man.

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 DiseaseIt 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.  Usually these tests are only done when a Heartworm infection needs to be ruled out as a cause of the cat's abnormal signs.  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.

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

Heartguard- Heartworm and hookworm control in cats.  Meaty pill given once a month and recommended June-November.  Sold individually.

Interceptor- Pill that controls Heartworm, Hookworm, Roundworm, and Whipworm-not Tapeworm in dogs.  Heartworm, Hookworm, and Roundworm control in cats.  Recommended prevention June-November.  Sold individually.

Revolution is our doctor's recommendation for heartworm prevention in cats!