Heartworm Disease 

by Lee Spector, V.M.D. 

Dr. Spector can be reached at Gray Fox Animal Hospital, 207 Glassboro Road, Woodbury Heights, NJ 08097, 609-848-7070, Fax: 609-848-7336

Heartworms are among the most devastating parasites known. These thin worms, 6 to 10 inches long, inhabit the hearts and large blood vessels of dogs, foxes, coyotes and wolves. In large numbers, they can suddenly destroy the function of the heart simply by clogging it. If they are present in numbers too few to clog up the heart, they are still very dangerous. In this case, they sit in the heart and large blood vessels, causing irritation to these tissues. The dog’s immune system tries to attack the worms. This attack may make some of the worms sick, but it also results in collateral damage to the blood vessels themselves.

The damage is not restricted to the heart, but also occurs to a great extent in the lungs, kidneys and liver. As a result, the dog may have great difficulty breathing, processing food, and eliminating waste products properly. It is no wonder so many dogs die of this disease. Those who survive are usually coughing and underweight, often chronically out of breath. Even dogs who appear fairly healthy may be masking a debilitating heartworm disease. If they are working dogs (guard dogs, shepherds, hunting dogs, therapy dogs), they will not do their job as well.

Fortunately, this disease can be treated. If caught early enough, most parasitized dogs can be treated with injections of a drug with arsenic in it. Although arsenic is poisonous for dogs just as it is for humans, the newest formulation of an arsenic drug is relatively safe when handled by an experienced veterinarian. However, it is a very expensive drug. Dogs that have worms clogging the heart severely may require that a veterinarian actually cut into the jugular vein, insert a long instrument down through the neck into the heart and pull the worms out. Because the dog is generally critically ill, general anesthesia cannot be used. Obviously this is a dangerous procedure, but it can save a few dogs’ lives. After treatment, many dogs fully recover, but some experience persistent lung damage from the heartworms.

Treatment of the disease is expense and sometimes dangerous. It sometimes fails to clear all the worms and sometimes does not completely clear the damage done by the worms even if they are all gone. Despite these reservations, it is almost always a good idea to treat any dog with heartworms before he gets sicker. The treatment protocol, the timing of the treatment, the selection of who should be treated is best determined by a veterinarian after he or she is familiar with the patient.

The best way to avoid the danger of heartworm disease, and the risks and expense of treatment, is to prevent it. Avoiding heartworms by avoiding other dogs does not work. The heartworms are transmitted by mosquito bites. The bite of one infected mosquito is enough to cause severe and fatal heartworm disease. In a state like New Jersey where mosquitoes are considered our “state bird”, the chances of a dog being bitten by an infected mosquito are quite high indeed. There is no effective way of avoiding mosquitoes unless a dog is trained to eliminate within the house and never steps outside. Fortunately over the past 20 years a number of tremendously effective preventive medications have become available. Most are medications with extremely good safety records. There are also daily medications which have had an excellent history of success for 40 years. Veterinarians have various testing protocols depending on the type of preventive medication used and, to some extent, the disease incidence in their area.