Vet Blog

Heartworms are parasites that, when fully grown, look like long strands of spaghetti and live inside the vessels that carry blood from the heart to the lungs and within the heart itself in severe cases.  I will focus my discussion here on dogs since they are most commonly infected, but keep in mind that cats and ferrets are also at risk.

Dogs get heartworms when they are bitten by a mosquito that has previously fed on another dog infected with the parasites.  Adult worms in the heart and lungs breed and produce tiny offspring called microfilariae that circulate in a dog’s bloodstream.  A mosquito feeding on an infected dog picks up these microfilariae, which then mature for a period of time within the insect.  Later, when the mosquito bites another dog, the immature heartworms are transferred to a new host.  The larval heartworms eventually make their way to this dog’s heart and lungs where they mature and breed to complete their lifecycle.

Adult worms incite a lot of inflammation and block the normal flow of blood from the heart to the lungs.  As the disease progresses, the heart may not be able to pump hard enough to push adequate amounts of blood past the worms, and chambers within the heart may even become filled with the parasites.  Heart failure and death is the result if the disease is not treated in a timely manner.

As horrifying as heartworm disease is, it is thankfully exceptionally easy to prevent.  Heartworm prevention medications are inexpensive, effective, and easy to use – most are either given as an oral treat or applied to the skin once a month.  These drugs work by killing any heartworms that the dog may have picked up in the last month before they can mature and reach the heart and lungs.  Puppies should start on heartworm preventative at the time of their first veterinary visit, usually around eight weeks of age.

Testing a dog for heartworm disease is also very simple – just a few drop of blood are required, and the results are available within only a few minutes.  One thing to keep in mind about the test, however, is that it looks for the presence of female, adult worms only and that it takes roughly six months from the bite of an infected mosquito for larval heartworms to mature into adults.  Therefore, it is possible for a dog to test negative, be placed on preventative, and then test positive for heartworms six months later.  In these cases, heartworm larvae were already in the dog’s body that were too old to be killed by the preventative, but too young to be picked up by the test.  Dogs that are on heartworm prevention should nevertheless be tested every year or two, just in case a dose was missed or was not absorbed well for some reason.

Dogs with heartworm disease frequently cough, are lethargic and tire easily, and may have difficulty breathing, lose weight, and develop a pot-bellied appearance.  Dogs can be treated for heartworm disease, and as long as they are not too severely affected, most will recover.  Prior to beginning their treatment, dogs undergo chest x-rays, blood work and a urinalysis to determine how severely affected they are and what treatment schedule is best for them.  If the veterinarian thinks that a dog has only a few adult worms in its heart and lungs, he or she may give two injections of a medication that kills the parasites deep into the muscles of a dog’s lower back over a two-day period, during which time the pet will be hospitalized for close monitoring.  In more severe cases, the doctor will give one injection first, followed by two injections a month later so that a large number of worms do not die all at once.  All dogs also receive medication to kill any immature heartworms in its body.

Throughout the treatment protocol and for about a month afterwards, dogs need to rest and be watched closely for any problems that may develop as worms die and are absorbed by the pet’s body.  Heartworm disease treatment is not an easy or cheap endeavor.  This is a case where an ounce of prevention is worth far more than a pound of cure.