What Is Heartworm Disease?

Life History

Pathology

How To Detect Heartworm Infection

Treatment

Heartworm Prevention

     What is Heartworm Disease?  Heartworm disease is usually a slowly developing and chronic disease in which damage to internal organs is quite extensive before the dog acts sick.  On rare occasions it can be an acutely fatal disease.  The disease is caused by large round worms, named Dirofilaria immitis, which live in the right side of the heart and adjacent blood vessels.  The females are up to 14 inches long but the males are no more than half as long.  Commonly there are up to 25 to 50 worms, but as many as 300 to 500 may be found, in a dog.  Heartworm disease is primarily a disease of dogs but occasionally the disease occurs in cats and other animals.

Life History:  Heartworms are only transmitted from one dog to another by any one of more than 70 species of mosquitoes.  Female heartworms discharge active baby worms (called microfilariae) into the blood stream.  The mosquito becomes infected by sucking up these baby worms as it takes a blood meal from an infected dog; during the next 10 to 14 days these microfilariae grow and molt several times to reach the larval stage which enters the dog as the mosquito feeds again.  In the dog it takes about six months for these larvae to grow, move to the heart, mate, and start to produce the new crop of baby worms.  Since transmission is totally dependent upon mosquitoes, the infection is most common and the disease most severe where the dogs are exposed to large numbers of infected mosquitoes.

     Pathology:  Heartworms cause extensive damage to vital organs such as the heart, lungs, liver, and kidney.  Much of the damage occurs before there are any outward signs of the disease.  By the time symptoms occur, the disease is usually well advanced and may be more difficult to treat.  In well advanced cases, there is less prospect of complete recovery even though all the adult worms are killed by treatment.

     A dog which has had heartworm disease for some time may develop a chronic cough, will tire easily on exercise and may collapse from head failure during or after exercise.  Dropsy, the accumulation of fluid in the abdomen causing a "swollen belly," often occurs in advanced cases of the disease.

     On rare occasions a rapidly fatal condition termed liver failure syndrome is encountered in a dog with a massive infection; often this condition is seen in comparatively young dogs.  A great mass of heartworms is found in the large vein which returns the blood to the heart from the liver and kidneys.  Such a dog will suddenly become listless and weak; it refuses food.  The urine is dark brown and the dog is jaundiced and anemic.  The functional damage to liver and kidney is so overwhelming that most of these dogs die within 24 to 72 hours.

     How To Detect Heartworm Infection:  Your veterinarian can usually detect the infection by finding the baby worms (microfilariae) in your dog's blood.  However, about 15 percent of the dogs harbor the adult worms without any microfilariae in the circulating blood.  Tests are available to readily tell if your dog has been infected with heartworms.  Early diagnosis and prompt treatment before the disease is too far advanced is very important.

     Treatment:  Most heartworm infected dogs may be successfully treated.  It has been said that the treatment of heartworm infection is somewhat of an art. There are several strategies that can be used including the option of not treating at all. The important concept to realize is that very harsh arsenic based drugs are necessary to kill adult heartworms and that treating for heartworm infection is neither simple nor safe in itself.  An important basic concept in the treatment of heartworm infection is that there are adult heartworms in the heart and pulmonary arteries to be killed and there are microfilariae circulating in the bloodstream to be killed and one medication cannot be used to do both jobs.

     Should adult worms be treated first or should heartworm preventive medications be started first to clear the microfilariae? There is still a great deal of controversy among experts as to which way is best.

AFTER TREATMENT OF AN ACTIVE HEARTWORM INFECTION, IT IS IMPORTANT TO TEST THE DOG FOUR TO FIVE MONTHS LATER TO INSURE THAT ALL WORMS HAVE BEEN KILLED.  YOUNGER FEMALE WORMS ARE ESPECIALLY RESISTANT AND ONE MAY HAVE TO WAIT FOR THEM TO AGE BEFORE THEY BECOME SUSCEPTIBLE TO THE DRUGS USED.