HEARTWORM DISEASE OVERVIEW

Heartworm disease is as it sounds:  parasitic worms infect animals and take up residence in the heart and pulmonary (lung) arteries.  This is a very serious (and potentially fatal) condition for dogs; cats may also be affected.  Fortunately, heartworm disease is easily preventable.  

Heartworm disease used to be found primarily in the southeastern United States.  Over the past 30 years, the disease has slowly spread northward and westward and is now found in all 50 states, and also in Canada.  The disease affects dogs and cats as well as other mammals: coyotes, foxes, wolves, ferrets, sea lions and even the occasional human have all been reported positive.  Infected wildlife is an important reservoir for infection - that is, the disease can never be fully eradicated from an area where wildlife is involved in the infection cycle.  The heartworm parasite's name is Dirofilaria immitis.  It is a long, thin, round worm that resembles a piece of spaghetti and may grow as long as 14 inches.  

SPREAD OF THE DISEASE

Heartworm disease is spread only by mosquitoes.  When an infected mosquito bites a susceptible animal, the microscopic heartworm larvae are injected into the animal's bloodstream.  Over the next six months, the larvae will migrate to the lungs and heart of the animal and grow into adult worms.  These adults can live as long as 7 years in an animal.  After mating, the female adult worms release microfilariae, or microscopic baby worms, into the animal's bloodstream.  When a mosquito bites the animal, it becomes infected with the microfilariae.  Over about 2 weeks, the microfilariae will mature into larvae in the mosquito and be ready to infect the next animal bitten by the mosquito.   

SIGNS OF INFECTION

When first infected, dogs and cats may not show any signs of the infection.  As the worms grow and numbers increase, the infection slowly becomes apparent and eventually leads to heart failure.  In DOGS, signs of heartworm infection include a persistent mild cough, reluctance to exercise, easily fatiguing, lack of appetite and weight loss.  In very severe infections, a dog may even have difficulty breathing, develop abdominal fluid effusion, collapse, and even die.  In CATS, the signs of heartworm infection can mimic many other diseases.  Cats may vomit, cough, or gag (as with a hairball), have difficulty breathing or rapid breathing (as with feline asthma or allergies), or show lethargy, decreased appetite and weight loss.  

TESTING

DOGS should receive annual testing for heartworm disease, even if they are on consistent heartworm prevention.  The current test is a 4Dx test that detects four diseases:  heartworm, Lyme, ehrlichia, and anaplasma.  The 4Dx is typically run at the time of your pet's annual physical examination or in early spring, depending on the status of monthly heartworm prevention.  PUPPIES should be tested at 6 months of age if they have not been on prevention or at their 1 year annual exam if they are on monthly preventative.

CATS can be tested via blood test or occasionally are diagnosed by ultrasound examination of the heart.  Cats are tested on an individual basis taking into account lifestyle, risks, and current illnesses.  

PREVENTION

It is very easy and inexpensive to prevent heartworm disease in both dogs and cats.  It is difficult and expensive to treat dogs once infected, and there are no approved products on the market to treat cats.  Therefore, it is much better to prevent heartworm than to try to treat it!  There are many products on the market for prevention, and year round prevention is recommended for patients in upstate NY.  The most popular in dogs are Interceptor and Heartgard.  Both are monthly chewables that also prevent and treat many intestinal parasites.  The most popular products for cats are Heartgard and Revolution.  Dogs MUST be tested for heartworm disease prior to starting prevention.  Speak with your veterinarian to decide which product is most appropriate for your pet.

TREATMENT

Most heartworm infections in dogs can be successfully treated.  However, treatment of heartworm disease in dogs is difficult, hard on the dog, and expensive - often running into the thousands of dollars.  The treatment consists of a series of injections given over one to two months, during which the dog must be closely monitored and exercise restricted.  A period of hospitalization is typically required.

There is no approved drug for the treatment of heartworm disease in cats.  Fortunately, cats seem better able to fight off infection than dogs.  In some cases, it may be necessary to surgically remove the worms from the cat's heart and arteries.